Skip to content

Instantly share code, notes, and snippets.

@corporatepiyush

corporatepiyush/datastar.md

Last active June 16, 2026 10:39
Show Gist options

  • Select an option

    Learn more about clone URLs
  • Save corporatepiyush/eae80a45f355ef04aec211f7392eb67a to your computer and use it in GitHub Desktop.

Select an option

Learn more about clone URLs
Save corporatepiyush/eae80a45f355ef04aec211f7392eb67a to your computer and use it in GitHub Desktop.
Download ZIP
The Ultimate Guide - Datastar framework for building complex UI with SSE, PostgreSQL and MongoDB
Raw
datastar.md

Datastar + Tailwind CSS — Complex UI Development Guide

Version: Datastar v1.0.x (latest stable) | Composable CSS: Tailwind CSS v4.x
Philosophy: Server-driven reactivity with declarative data-* attributes. No build step, no virtual DOM, ~10.7KB client.

Table of Contents

▶ Quick Reference Cheatsheet — fast lookup for attributes, actions, SSE events & backend patterns

  1. Core Architecture
  2. Signal & Expression Deep Dive
  3. Multi-Screen Navigation (SPA Routing)
  4. Complex UI Scenarios
  5. Complex Chat UI with Rich Widgets
  6. Server-First State Management
  7. PostgreSQL + LISTEN/NOTIFY + SSE
  8. MongoDB 8.x + Change Streams + SSE
  9. Mobile & Admin Dashboard Components
  10. Plugin Development & Extensions
  11. Tailwind CSS Integration
  12. Performance & Anti-Patterns
  13. Server-Side Patterns
  1. Testing & Debugging

Quick Reference Cheatsheet

Jump table: Sections 1–14 above. This cheatsheet is the fast lookup; sections are the deep dives.

Attributes (data-*)

<!-- ── Signals: declare reactive state ── -->
<div data-signals:count="0"></div>                  <!-- single signal -->
<div data-signals="{user: {name: '', age: 0}}"></div> <!-- nested object -->
<div data-signals:total="$a + $b"></div>            <!-- initial expression -->
<div data-signals__ifmissing:count="0"></div>       <!-- only set if not already defined -->

<!-- ── Two-way binding ── -->
<input data-bind="email" />                          <!-- text input -->
<input type="checkbox" data-bind="agree" />          <!-- boolean -->
<select data-bind="country"></select>                <!-- select -->
<input data-bind:email__case.kebab="emailAddr" />    <!-- transform signal name -->

<!-- ── Display / text / attributes ── -->
<div data-show="$isOpen">Toggles display:none</div>
<span data-text="$user.name"></span>                 <!-- textContent -->
<a data-attr:href="$url" data-attr:disabled="$busy"></a>
<div data-attr="{title: $tip, 'aria-label': $label}"></div>
<div data-class="{'bg-red-500': $error, 'opacity-50': $busy}"></div>
<div data-style:width="$pct + '%'"></div>
<div data-style="{opacity: $fade ? 1 : 0}"></div>

<!-- ── Events ── -->
<button data-on:click="$count++">click</button>
<form   data-on:submit__prevent="@post('/save')"></form>
<input  data-on:input__debounce.300ms="@get('/search')" />
<div    data-on:scroll__throttle.200ms="…"></div>
<div    data-on-signal-patch="…"></div>              <!-- any signal patched (filter w/ data-on-signal-patch-filter) -->
<div    data-on:interval__duration.5s="@get('/poll')"></div>
<div    data-on:load="@get('/init')"></div>          <!-- alias of data-init use -->
<div    data-on:keydown__window="if(evt.key==='Escape') $modal=false"></div>

<!-- ── Computed (read-only derived) ── -->
<div data-computed:fullName="$first + ' ' + $last"></div>
<div data-computed:cartTotal="$items.reduce((s,i)=>s+i.price,0)"></div>

<!-- ── Refs & lifecycle ── -->
<input data-ref="search" />
<button data-on:click="$search.focus()">focus</button>
<div data-init="@get('/data')"></div>                <!-- run once on insert -->
<div data-init__delay.500ms="$animate()"></div>
<div data-effect="console.log($count)"></div>        <!-- re-run on dep change -->
<div data-on-intersect="@get('/more')"></div>        <!-- lazy load on visible -->

<!-- ── Indicators & persistence ── -->
<button data-indicator="saving">…</button>           <!-- sets $saving while in flight -->
<div data-persist="theme cart"></div>                <!-- ⚠ PRO: localStorage-backed signals -->
<div data-persist__session="draft"></div>            <!-- ⚠ PRO: sessionStorage -->
<pre data-json-signals></pre>                         <!-- FREE: live signal inspector (filterable) -->

<!-- ── Modifiers (chain after __) ── -->
__debounce.300ms  __throttle.200ms  __delay.500ms
__prevent  __stop  __once  __passive  __capture
__window   __outside   __case.kebab|camel|snake
__duration.5s (interval)   __viewtransition

Actions (inside expressions, prefixed @)

@get('/api/data')            // GET   (signals → query string)
@post('/api/save')           // POST  (signals → JSON body)
@put('/api/update')          // PUT
@patch('/api/partial')       // PATCH
@delete('/api/item/4')       // DELETE
// Options (2nd arg) — all FREE:
@get('/x', {contentType:'form'})          // send closest <form>, not signals
@get('/x', {headers:{'X-CSRF': $tok}})    // custom headers
@get('/x', {filterSignals:{include:/^foo\./}})  // send only some signals
@get('/x', {openWhenHidden:true})         // keep SSE open in background tab (dashboards)
@get('/x', {retry:'error'})               // retry on 4xx/5xx (default 'auto' = network only)
@get('/x', {requestCancellation:'disabled'})    // allow concurrent (default auto-cancels same URL+method)
@setAll(true, {include:/^ui\./})          // set all matching signals to a value
@toggleAll({include:/^is/})               // flip all matching booleans
@peek(() => $count)                        // read a signal WITHOUT subscribing
@clipboard('text')           // ⚠ PRO    @fit(v,0,100,0,255) ⚠ PRO    @intl('number',n,…) ⚠ PRO

Free vs Pro. Everything in this guide uses the free core. Pro (paid) adds: attributes data-persist, data-query-string, data-replace-url, data-on-raf, data-on-resize, data-animate, data-match-media, data-custom-validity, data-scroll-into-view, data-view-transition; and actions @clipboard, @fit, @intl. Where this guide hand-rolls something Pro automates (e.g. history.pushState instead of data-query-string__history in §6.10), that's deliberate — it keeps the patterns dependency-free.

SSE Events (server → client)

event: datastar-patch-signals
data: signals {"count": 5, "user": {"name": "Ann"}}   ← JSON Merge Patch (RFC 7386)
                                                         null removes a key

event: datastar-patch-elements
data: selector #target        ← omit to match by element id
data: mode outer              ← outer|inner|replace|append|prepend|before|after|remove
data: useViewTransition true
data: elements <div id="target">…COMPLETE element with id…</div>

event: datastar-execute-script
data: script window.location='/login'

Patch modes at a glance:

mode effect
outer (default) morph entire element by id
inner replace children, preserves element state
replace replace element, resets state
append / prepend add inside target, end / start
before / after add as sibling
remove delete the target

Server response: HTTP/2 SSE (Node.js, §13.5)

function sseHead(stream) {
  stream.respond({ ':status': 200,
    'content-type': 'text/event-stream',
    'cache-control': 'no-cache' });        // NO Connection header in h2!
}
const patchSignals  = (s,o)   => s.write(`event: datastar-patch-signals\ndata: signals ${JSON.stringify(o)}\n\n`);
const patchElements = (s,h,{selector,mode}={}) => { /* §13.5 */ };
stream.on('close', cleanup);               // h2 disconnect (not req.aborted)

Request payload shape

GET/DELETE  →  ?datastar={"signals":{…}}     (query param)
POST/PUT/PATCH  →  body: {"datastar":{"signals":{…}}}
Reconnect   →  header  Last-Event-ID: <token>   (resume: PG offset / Mongo resume token)
Detect DS request  →  header  datastar-request: true

Signal namespace conventions (§6.4)

Critical: Datastar sends every signal in each request except those prefixed with _. The namespaces below are a discipline, not enforcement — ui.* is still serialized into every request unless you prefix it (_ui.*) or pass filterSignals. For genuinely client-only state, use a leading underscore so it's auto-excluded.

Prefix Owner Sent to server? Example
form.* / filter.* client draft yes, on submit/debounce $form.email
_ui.* (underscore!) client only never (auto-excluded) $_ui.sidebarOpen
ui.* (no underscore) client ⚠ yes — sent every request prefer _ui.* for pure chrome
server.* server read-only display $server.user.name
domain (count, items) server server owns truth patched via SSE

Real-time backend cheat (§7 / §8)

-- PostgreSQL: LISTEN/NOTIFY (wake-up only, payload in table — §7.9)
LISTEN orders;   SELECT pg_notify('orders', 'wake');
CREATE INDEX … WHERE NOT processed;          -- partial index (§7.8), predicate must be IMMUTABLE
… PARTITION BY RANGE (created_at);           -- time partitioning (§7.7), not NOW()-based indexes
uuidv7()  RETURNING OLD.*/NEW.*              -- PG 18 (§7.6)
// MongoDB 8.x: Change Streams (auto-emitted, resumable — §8.3)
db.coll.watch([{ $match: { 'fullDocument.userId': id } }],
              { fullDocument: 'updateLookup', resumeAfter: token });
{ partialFilterExpression: { readAt: null } }    // partial index (§8.4)
{ expireAfterSeconds: 86400 }                    // TTL = retention (§8.4)
// ESR index order: Equality → Sort → Range

Max-performance levers (§7.10 / §8.7):

Lever PostgreSQL MongoDB
Fan-out one LISTEN conn → N in-memory streams one watch() cursor → N streams
Pooling listener direct; queries via PgBouncer transaction mode (LISTEN breaks in tx mode, NOTIFY is fine) driver maxPoolSize sized to ops, not users
Payload NOTIFY = wake-up id only (§7.9) drop updateLookup; $project in pipeline
Batching coalesce patches on ~16–50ms timer; catch-up WHERE id = ANY($1) batchSize + maxAwaitTimeMS; same coalesce timer
Read offload read replicas for catch-up/initial reads readPreference: 'secondaryPreferred'
Pre-compute materialized rollup table $merge rollup collection, watch that
Hot reads Index-Only Scan (INCLUDE), BRIN on time covered query (_id: 0, indexed fields only)
Write churn synchronous_commit=off (scoped), fillfactor+HOT, aggressive autovacuum atomic $inc/$push, bulkWrite, w to taste

Common patterns (one-liners)

<!-- Single-flight button (no double submit, §6.9) -->
<button data-on:click="$meta.busy=true; @post('/x')" data-attr:disabled="$meta.busy">Go</button>

<!-- Debounced live search -->
<input data-bind="q" data-on:input__debounce.300ms="@get('/search')" />

<!-- Modal driven by one signal -->
<div data-show="$ui.modal" data-on:click="$ui.modal=false"></div>

<!-- Infinite scroll sentinel -->
<div data-on-intersect="@get('/page/'+$nextPage)"></div>

<!-- Optimistic toggle then confirm (§6.5) -->
<button data-on:click="$liked=!$liked; @post('/like')">♥</button>

<!-- Auth: token NEVER in a signal; cookie rides every request (§4.11) -->
<a href="/auth/oauth/google">Continue with Google</a>

<!-- URL-synced state: pointer in the URL, data on the server (§6.10) -->
<button data-on:click="$ui.step++; history.pushState(null,'','?step='+$ui.step); @post('/next')">Next</button>
<body data-on:popstate__window="@get('/resume?step='+(new URLSearchParams(location.search).get('step')||1))"></body>

Decision: client vs server (§6.9)

Does anyone else / audit care?            → server
Would editing it in DevTools matter?      → server
Derived from domain data?                 → server computes, client displays
Pure presentation, per-session?           → client (ui.*), don't tell server
Uncommitted draft input?                  → client (form.*) → flush on submit

Gotchas

  • Patched elements must be complete tags with an id — never bare fragments.
  • In expressions the event object is evt, not event (evt.clientX, evt.key). event is the deprecated global and breaks in some browsers.
  • Refs are signals: data-ref:thing creates $thing (the element). There is no $$refs.
  • data-effect auto-tracks the signals in its expression — there's no :deps; use @peek(() => $x) to exclude a signal from triggering it.
  • React to specific signal changes with data-on-signal-patch (+ data-on-signal-patch-filter), not a signals-change event.
  • ui.* signals are sent to the server every request — prefix with _ (_ui.*) to exclude them, or use filterSignals.
  • HTTP/2 forbids Connection/Keep-Alive headers; SSE needs proxy_buffering off.
  • Partial-index predicates must be IMMUTABLE — no NOW(); partition by time instead.
  • Change Streams require a replica set, even single-node in dev.
  • Never put tokens/secrets in signals, localStorage, or data-persist.
  • data-effect should not fire server actions (hidden request loops).
  • Intercept/transform SSE bytes by monkey-patching fetch before Datastar loads (§13.10) — unofficial, guard by content-type.

Power features worth using (all FREE core)

These ship with open-source Datastar and replace a lot of hand-rolled code:

<!-- 1. Automatic single-flight: a new request cancels the in-flight one to the same
        URL+method on that element. You rarely need a manual "isLoading" race guard. -->
<button data-on:click="@post('/save')">Save</button>   <!-- double-click ≠ double POST -->

<!-- 2. _ prefix = never sent to the server (great for drag/hover/scroll chrome) -->
<div data-signals="{_drag: {dx: 0}}"></div>

<!-- 3. Built-in reactive signal inspector — delete your custom debug panel -->
<pre data-json-signals="{exclude: /password/}"></pre>

<!-- 4. React to a SPECIFIC signal changing, with a regex filter (no effect loops) -->
<div data-on-signal-patch__debounce.300ms="@get('/search')"
     data-on-signal-patch-filter="{include: /^query$/}"></div>

<!-- 5. Preserve user/DOM state across morphs: keep <details open>, scroll, etc. -->
<details open data-preserve-attr="open"><summary>…</summary>…</details>

<!-- 6. Protect a third-party widget (chart/map/editor) from Datastar morphing -->
<div id="chart" data-ignore-morph><!-- Chart.js owns this subtree --></div>

<!-- 7. File uploads with zero plumbing: type=file binds to base64 {name,contents,mime}[] -->
<input type="file" data-bind:files multiple />

<!-- 8. Smooth patches via the View Transition API -->
<button data-on:click__viewtransition="@get('/next')">Next</button>

<!-- 9. Bulk signal ops with regex filters -->
<button data-on:click="@setAll(false, {include: /^panel\./})">Collapse all</button>
<button data-on:click="@toggleAll({include: /^is/})">Toggle flags</button>

<!-- 10. Custom cancellation via an AbortController signal -->
<div data-signals="{_ctrl: new AbortController()}">
  <button data-on:click="@get('/x', {requestCancellation: $_ctrl})">Start</button>
  <button data-on:click="$_ctrl.abort()">Cancel</button>
</div>

Server-side: skip SSE framing for one-shot patches. A backend action accepts a plain text/html body with response headers datastar-selector and datastar-mode (or application/json for signals) — lighter than the SSE event:/data: framing when you're sending a single patch rather than a stream:

res.writeHead(200, {
  'content-type': 'text/html',
  'datastar-selector': '#cart-count',
  'datastar-mode': 'inner',        // outer|inner|replace|append|prepend|before|after|remove
});
res.end('<span id="cart-count">3</span>');

Use the SSE event stream (§13) when pushing multiple patches or a long-lived feed; use plain-response headers for a simple one-and-done reply.

1. Core Architecture

1.1 How Datastar Works

Datastar is a hypermedia-driven framework. State lives primarily on the server; the client is a thin reactive layer (~10.7KB).

┌─────────────────┐      SSE / HTTP       ┌─────────────────┐
│   Browser       │ ◄──────────────────►  │   Server        │
│  (Datastar JS)  │   HTML elements       │  (Your Backend) │
│  ~10.7KB        │   Signal patches      │                 │
└─────────────────┘                       └─────────────────┘

Key Concepts:

Concept Description Example
Signals Reactive state variables prefixed with $ $user, $cart.items
Bindings Two-way sync between DOM and signals data-bind="email"
Actions Server communication triggered by events @post('/api/save')
Elements HTML chunks patched into the DOM via SSE datastar-patch-elements
Computed Derived reactive values data-computed:fullName
Effects Side effects that run when signals change data-effect

1.2 Attribute Syntax (v1.0+)

Datastar v1.0 uses colon-delimited attributes (HTML5 standard compliant):

<!-- OLD (pre-v1.0) -->
<div data-signals-foo="bar" data-on-click="@get('/api')">

<!-- NEW (v1.0+) -->
<div data-signals:foo="bar" data-on:click="@get('/api')">

Modifier syntax: Double-underscore for modifiers, dot for arguments.

<button data-on:click__debounce.300ms="@post('/search')">
<button data-on:click__throttle.500ms.leading="@get('/track')">
<button data-on:click__prevent="$count++">
<button data-on:click__stop="@post('/api')">

1.3 Expression Context

Variables available inside every Datastar expression:

$signalName     // Access any signal (reading subscribes to changes)
el              // The element the attribute is on
evt             // The event object (in data-on handlers) — NOT `event`
@peek(() => $x) // Read a signal WITHOUT subscribing to it

Refs are signals, not a separate namespace: data-ref:search (or data-ref="search") creates $search, which is the element — call $search.focus(), $search.scrollIntoView(), etc. There is no $$refs or $$signals; to inspect all signals use the built-in data-json-signals attribute.

1.4 Action Plugins

Action HTTP Method Description
@get(url) GET Fetches from server, processes SSE response
@post(url) POST Sends data to server via POST
@put(url) PUT Sends data to server via PUT
@patch(url) PATCH Sends partial updates via PATCH
@delete(url) DELETE Sends delete request
@setAll(obj) — Merges an object into the signal store
@toggleAll(...paths) — Toggles boolean signals at given paths
@peek(expr) — Reads signals without creating reactive dependencies

2. Signal & Expression Deep Dive

2.1 Namespacing & Hierarchical State

For complex UIs, organize signals hierarchically using dot notation:

<body data-signals="{
  ui: {
    sidebarOpen: false,
    activeModal: null,
    toastQueue: [],
    screen: 'home',
    screenHistory: []
  },
  data: {
    users: [],
    selectedUserId: null,
    filters: { role: 'all', status: 'active' }
  },
  meta: {
    isLoading: false,
    error: null,
    lastSync: null
  }
}">

Best Practice: Keep UI state (ui.*), domain data (data.*), and metadata (meta.*) separated.

2.2 Computed Signals for Derived State

Use data-computed to avoid duplicating state:

<div data-signals="{items: [], searchQuery: '', minPrice: 0, maxPrice: 1000}"
     data-computed:filteredItems="$items.filter(i => 
       i.name.toLowerCase().includes($searchQuery.toLowerCase()) && 
       i.price >= $minPrice && 
       i.price <= $maxPrice
     )"
     data-computed:hasResults="$filteredItems.length > 0"
     data-computed:resultCount="$filteredItems.length"
     data-computed:totalValue="$filteredItems.reduce((sum, i) => sum + i.price, 0)">

  <p data-text="$resultCount + ' results found ($' + $totalValue + ' total value)'"></p>
  <div data-show="$hasResults">
    <!-- Render filtered items -->
  </div>
</div>

2.3 Deep Signal Binding

Bind to nested object properties directly:

<div data-signals="{user: {profile: {name: '', email: '', settings: {theme: 'light'}}}}">
  <input data-bind="user.profile.name" placeholder="Name" />
  <input data-bind="user.profile.email" placeholder="Email" />
  <select data-bind="user.profile.settings.theme">
    <option value="light">Light</option>
    <option value="dark">Dark</option>
  </select>
</div>

2.4 Signal Initialization with Delay

<!-- Delayed initialization -->
<div data-init__delay.500ms="$loadData()">
  Loading...
</div>

<!-- Conditional initialization -->
<div data-init="if(!$dataLoaded) { @get('/api/data') }">
</div>

2.5 Effects for Side Effects

<!-- Log when signal changes -->
<div data-effect="console.log('User changed:', $user.name)">
</div>

<!-- Auto-focus input when editing -->
<div data-effect="if($editingId) { setTimeout(() => $editInput?.focus(), 50) }">
  <input data-ref="editInput" data-show="$editingId" data-bind="editValue" />
</div>

3. Multi-Screen Navigation (SPA Routing)

Datastar enables full SPA-like navigation without page reloads. The server patches the main content area while preserving layout, navigation state, and signal context.

3.1 Screen Router Pattern

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Datastar App</title>
  <script type="module" src="https://cdn.jsdelivr.net/gh/starfederation/datastar@v1.0.2/bundles/datastar.js"></script>
  <script src="https://cdn.tailwindcss.com"></script>
</head>
<body class="bg-gray-50 text-gray-900"
      data-signals="{
        ui: {
          screen: 'dashboard',
          screenHistory: [],
          sidebarOpen: true,
          isNavigating: false
        },
        data: {
          user: {name: 'John Doe', avatar: '/avatar.jpg', role: 'admin'},
          notifications: [],
          unreadCount: 0
        }
      }">

  <!-- ═══════════════════════════════════════════════════════════════ -->
  <!--  APP SHELL  —  Persistent layout; screen content patched by server -->
  <!-- ═══════════════════════════════════════════════════════════════ -->
  <div class="flex h-screen overflow-hidden">

    <!-- ═══════════════════════════════════════════════════════════════ -->
    <!--  SIDEBAR  —  Collapsible; navigation triggers screen changes  -->
    <!-- ═══════════════════════════════════════════════════════════════ -->
    <aside class="bg-gray-900 text-white transition-all duration-300 flex flex-col"
           data-class="{'w-64': $ui.sidebarOpen, 'w-16': !$ui.sidebarOpen}">
    <!-- DS:CLASS — Width animates based on sidebarOpen signal -->

      <div class="p-4 flex items-center gap-3">
        <div class="w-8 h-8 bg-blue-600 rounded-lg flex items-center justify-center font-bold flex-shrink-0">A</div>
        <span class="font-bold text-lg transition-opacity" data-class="{'opacity-0 hidden': !$ui.sidebarOpen}">AppName</span>
      </div>

      <nav class="flex-1 px-2 space-y-1 mt-4">
        <!-- Navigation Items -->
        <button data-on:click="$ui.screenHistory = [...$ui.screenHistory, $ui.screen]; $ui.screen = 'dashboard'; $ui.isNavigating = true; @get('/api/screens/dashboard')"
                data-class="{'bg-gray-800 text-white': $ui.screen === 'dashboard', 'text-gray-400 hover:bg-gray-800 hover:text-white': $ui.screen !== 'dashboard'}"
                class="w-full flex items-center gap-3 px-3 py-2 rounded-lg transition-colors">
          <svg class="w-5 h-5 flex-shrink-0" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M4 6a2 2 0 012-2h2a2 2 0 012 2v2a2 2 0 01-2 2H6a2 2 0 01-2-2V6zM14 6a2 2 0 012-2h2a2 2 0 012 2v2a2 2 0 01-2 2h-2a2 2 0 01-2-2V6zM4 16a2 2 0 012-2h2a2 2 0 012 2v2a2 2 0 01-2 2H6a2 2 0 01-2-2v-2zM14 16a2 2 0 012-2h2a2 2 0 012 2v2a2 2 0 01-2 2h-2a2 2 0 01-2-2v-2z"/></svg>
          <span class="transition-opacity" data-class="{'opacity-0 hidden': !$ui.sidebarOpen}">Dashboard</span>
        </button>

        <button data-on:click="$ui.screenHistory = [...$ui.screenHistory, $ui.screen]; $ui.screen = 'projects'; $ui.isNavigating = true; @get('/api/screens/projects')"
                data-class="{'bg-gray-800 text-white': $ui.screen === 'projects', 'text-gray-400 hover:bg-gray-800 hover:text-white': $ui.screen !== 'projects'}"
                class="w-full flex items-center gap-3 px-3 py-2 rounded-lg transition-colors">
          <svg class="w-5 h-5 flex-shrink-0" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M3 7v10a2 2 0 002 2h14a2 2 0 002-2V9a2 2 0 00-2-2h-6l-2-2H5a2 2 0 00-2 2z"/></svg>
          <span class="transition-opacity" data-class="{'opacity-0 hidden': !$ui.sidebarOpen}">Projects</span>
        </button>

        <button data-on:click="$ui.screenHistory = [...$ui.screenHistory, $ui.screen]; $ui.screen = 'chat'; $ui.isNavigating = true; @get('/api/screens/chat')"
                data-class="{'bg-gray-800 text-white': $ui.screen === 'chat', 'text-gray-400 hover:bg-gray-800 hover:text-white': $ui.screen !== 'chat'}"
                class="w-full flex items-center gap-3 px-3 py-2 rounded-lg transition-colors relative">
          <svg class="w-5 h-5 flex-shrink-0" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M8 12h.01M12 12h.01M16 12h.01M21 12c0 4.418-4.03 8-9 8a9.863 9.863 0 01-4.255-.949L3 20l1.395-3.72C3.512 15.042 3 13.574 3 12c0-4.418 4.03-8 9-8s9 3.582 9 8z"/></svg>
          <span class="transition-opacity" data-class="{'opacity-0 hidden': !$ui.sidebarOpen}">Chat</span>
          <span data-show="$data.unreadCount > 0" 
                class="absolute right-2 bg-red-500 text-white text-xs rounded-full w-5 h-5 flex items-center justify-center"
                data-text="$data.unreadCount"></span>
        </button>

        <button data-on:click="$ui.screenHistory = [...$ui.screenHistory, $ui.screen]; $ui.screen = 'settings'; $ui.isNavigating = true; @get('/api/screens/settings')"
                data-class="{'bg-gray-800 text-white': $ui.screen === 'settings', 'text-gray-400 hover:bg-gray-800 hover:text-white': $ui.screen !== 'settings'}"
                class="w-full flex items-center gap-3 px-3 py-2 rounded-lg transition-colors">
          <svg class="w-5 h-5 flex-shrink-0" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M10.325 4.317c.426-1.756 2.924-1.756 3.35 0a1.724 1.724 0 002.573 1.066c1.543-.94 3.31.826 2.37 2.37a1.724 1.724 0 001.065 2.572c1.756.426 1.756 2.924 0 3.35a1.724 1.724 0 00-1.066 2.573c.94 1.543-.826 3.31-2.37 2.37a1.724 1.724 0 00-2.572 1.065c-.426 1.756-2.924 1.756-3.35 0a1.724 1.724 0 00-2.573-1.066c-1.543.94-3.31-.826-2.37-2.37a1.724 1.724 0 00-1.065-2.572c-1.756-.426-1.756-2.924 0-3.35a1.724 1.724 0 001.066-2.573c-.94-1.543.826-3.31 2.37-2.37.996.608 2.296.07 2.572-1.065z"/><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M15 12a3 3 0 11-6 0 3 3 0 016 0z"/></svg>
          <span class="transition-opacity" data-class="{'opacity-0 hidden': !$ui.sidebarOpen}">Settings</span>
        </button>
      </nav>

      <!-- User Profile (bottom of sidebar) -->
      <div class="p-4 border-t border-gray-800">
        <div class="flex items-center gap-3">
          <img data-attr:src="$data.user.avatar" class="w-8 h-8 rounded-full bg-gray-700" />
          <div class="transition-opacity" data-class="{'opacity-0 hidden': !$ui.sidebarOpen}">
            <p class="text-sm font-medium" data-text="$data.user.name"></p>
            <p class="text-xs text-gray-500" data-text="$data.user.role"></p>
          </div>
        </div>
      </div>
    </aside>

    <!-- MAIN CONTENT AREA -->
    <main class="flex-1 flex flex-col overflow-hidden">

      <!-- TOP BAR (persistent) -->
      <header class="h-16 bg-white border-b border-gray-200 flex items-center justify-between px-6 flex-shrink-0">
        <div class="flex items-center gap-4">
          <button data-on:click="$ui.sidebarOpen = !$ui.sidebarOpen"
                  class="p-2 hover:bg-gray-100 rounded-lg">
            <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M4 6h16M4 12h16M4 18h16"/></svg>
          </button>

          <!-- Breadcrumbs -->
          <nav class="flex items-center gap-2 text-sm">
            <button data-on:click="if($ui.screenHistory.length > 0) { const prev = $ui.screenHistory.pop(); $ui.screen = prev; $ui.isNavigating = true; @get('/api/screens/' + prev) }"
                    data-show="$ui.screenHistory.length > 0"
                    class="text-gray-500 hover:text-gray-700 flex items-center gap-1">
              <svg class="w-4 h-4" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M10 19l-7-7m0 0l7-7m-7 7h18"/></svg>
              Back
            </button>
            <span data-show="$ui.screenHistory.length > 0" class="text-gray-300">/</span>
            <span class="font-medium capitalize" data-text="$ui.screen"></span>
          </nav>
        </div>

        <div class="flex items-center gap-3">
          <!-- Global Search -->
          <div class="relative">
            <input type="text" 
                   data-bind="ui.searchQuery"
                   data-on:input__debounce.300ms="@get('/api/search')"
                   placeholder="Search..."
                   class="w-64 pl-9 pr-4 py-2 bg-gray-100 border-transparent rounded-lg text-sm focus:bg-white focus:ring-2 focus:ring-blue-500" />
            <svg class="w-4 h-4 text-gray-400 absolute left-3 top-2.5" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M21 21l-6-6m2-5a7 7 0 11-14 0 7 7 0 0114 0z"/></svg>
          </div>

          <!-- Notifications -->
          <button data-on:click="$ui.activeModal = 'notifications'; @get('/api/notifications')"
                  class="relative p-2 hover:bg-gray-100 rounded-lg">
            <svg class="w-5 h-5 text-gray-600" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M15 17h5l-1.405-1.405A2.032 2.032 0 0118 14.158V11a6.002 6.002 0 00-4-5.659V5a2 2 0 10-4 0v.341C7.67 6.165 6 8.388 6 11v3.159c0 .538-.214 1.055-.595 1.436L4 17h5m6 0v1a3 3 0 11-6 0v-1m6 0H9"/></svg>
            <span data-show="$data.unreadCount > 0" 
                  class="absolute top-1 right-1 w-2 h-2 bg-red-500 rounded-full"></span>
          </button>
        </div>
      </header>

      <!-- SCREEN CONTENT (server-patched) -->
      <div id="screen-content" class="flex-1 overflow-y-auto p-6 relative">
        <!-- Loading overlay -->
        <div data-show="$ui.isNavigating" class="absolute inset-0 bg-white/80 z-10 flex items-center justify-center">
          <div class="w-8 h-8 border-4 border-blue-600 border-t-transparent rounded-full animate-spin"></div>
        </div>

        <!-- Initial screen content -->
        <div data-show="!$ui.isNavigating && $ui.screen === 'dashboard'">
          <h1 class="text-2xl font-bold mb-6">Dashboard</h1>
          <!-- Dashboard content loaded via initial server render -->
        </div>
      </div>

    </main>
  </div>

</body>
</html>

3.2 Screen Transition with View Transitions API

<!-- Add to screen content wrapper for smooth transitions -->
<div id="screen-content" 
     class="flex-1 overflow-y-auto p-6"
     data-class="{'view-transition': $ui.useTransitions}">
</div>

<!-- Server sends with view transition enabled -->

Server SSE Response:

event: datastar-patch-elements
data: selector #screen-content
data: useViewTransition true
data: elements <div id="screen-content" class="flex-1 overflow-y-auto p-6"><h1>Projects</h1>...</div>

3.3 Deep Linking & URL Sync (Manual)

Since Datastar core doesn't include query-string sync, implement it manually:

<body data-signals="{ui: {screen: 'dashboard'}}"
      data-init="const params = new URLSearchParams(window.location.search); if(params.has('screen')) { $ui.screen = params.get('screen'); @get('/api/screens/' + $ui.screen) }"
      data-effect="const url = new URL(window.location); url.searchParams.set('screen', $ui.screen); window.history.pushState({}, '', url);">

  <!-- Navigation -->
  <button data-on:click="$ui.screen = 'projects'; @get('/api/screens/projects')">
    Projects
  </button>
</body>

3.4 Screen-Specific State Isolation

<!-- Each screen gets its own signal namespace -->
<div data-show="$ui.screen === 'projects'"
     data-signals="{projects: {list: [], filters: {}, selectedId: null}}">
  <!-- Projects screen content -->
</div>

<div data-show="$ui.screen === 'chat'"
     data-signals="{chat: {rooms: [], activeRoom: null, messages: [], draft: ''}}">
  <!-- Chat screen content -->
</div>

3.5 Nested Sub-Screens

<div data-show="$ui.screen === 'settings'">
  <div class="flex gap-6">
    <!-- Sub-navigation -->
    <nav class="w-48 space-y-1">
      <button data-on:click="$ui.subScreen = 'profile'; @get('/api/settings/profile')"
              data-class="{'bg-blue-50 text-blue-700': $ui.subScreen === 'profile'}"
              class="w-full text-left px-3 py-2 rounded-lg">Profile</button>
      <button data-on:click="$ui.subScreen = 'security'; @get('/api/settings/security')"
              data-class="{'bg-blue-50 text-blue-700': $ui.subScreen === 'security'}"
              class="w-full text-left px-3 py-2 rounded-lg">Security</button>
      <button data-on:click="$ui.subScreen = 'billing'; @get('/api/settings/billing')"
              data-class="{'bg-blue-50 text-blue-700': $ui.subScreen === 'billing'}"
              class="w-full text-left px-3 py-2 rounded-lg">Billing</button>
    </nav>

    <!-- Sub-screen content -->
    <div id="settings-content" class="flex-1">
      <!-- Server-patched -->
    </div>
  </div>
</div>

4. Complex UI Scenarios

4.1 Master-Detail / Split-Pane Layouts

Pattern: A split-pane layout where the left side (master) presents a scrollable list or grid of items, and the right side (detail) displays the full content, metadata, and actions for the currently selected item. The key architectural principle is that selecting an item does not trigger a full page navigation or server-side render of the entire layout. Instead, only the detail pane's content is fetched from the server (as an HTML fragment via SSE or a targeted GET) and patched into the DOM. The master list remains mounted in the DOM with its scroll position, filter state, and selection preserved. Active selection is visualized using data-class for conditional styling (highlighting the selected row) without re-rendering the list. Skeleton loaders provide immediate visual feedback during the fetch, and the server returns both signal updates (e.g., $isLoadingDetail = false) and element patches (the actual detail HTML) in a single SSE stream. This pattern is ideal for email clients, file explorers, CRM contact lists, admin dashboards, and any interface where users need to rapidly scan and inspect many records without losing context.

<div class="flex h-screen" data-signals="{selectedId: null, detailHtml: '', isLoadingDetail: false}">

  <!-- ═══════════════════════════════════════════════════════════════ -->
  <!--  MASTER PANE  —  List + Filter + Selection                    -->
  <!-- ═══════════════════════════════════════════════════════════════ -->
  <aside class="w-80 border-r border-gray-200 overflow-y-auto bg-gray-50">
    <div class="p-4">
      <div class="flex justify-between items-center mb-4">
        <h2 class="text-lg font-bold">Items</h2>
        <!-- DS:ON — Action triggers server POST to create new item -->
        <button data-on:click="@post('/api/items')"
                class="text-sm bg-blue-600 text-white px-3 py-1 rounded-lg hover:bg-blue-700">
          + New
        </button>
        <!-- /DS:ON -->
      </div>

      <!-- DS:BIND + DS:ON — Two-way filter input with debounced server fetch -->
      <input type="text"
             data-bind="listFilter"
             data-on:input__debounce.200ms="@get('/api/items?filter=' + $listFilter)"
             placeholder="Filter items..."
             class="w-full px-3 py-2 border rounded-lg mb-3 text-sm" />
      <!-- /DS:BIND + /DS:ON -->

      <div id="item-list" class="space-y-1">
        <!-- Server-rendered list -->
        <!-- DS:ON + DS:CLASS — Click sets selection + triggers detail fetch; class highlights active row -->
        <button class="w-full text-left p-3 rounded-lg hover:bg-gray-100 transition-all border border-transparent"
                data-on:click="$selectedId = 1; $isLoadingDetail = true; @get('/api/items/1/detail')"
                data-class="{'bg-blue-50 border-blue-200 shadow-sm': $selectedId === 1}">
          <div class="flex justify-between items-start">
            <div>
              <p class="font-medium">Item 1</p>
              <p class="text-sm text-gray-500">Category A</p>
            </div>
            <span class="text-xs px-2 py-1 bg-green-100 text-green-700 rounded-full">Active</span>
          </div>
        </button>
        <!-- /DS:ON + /DS:CLASS -->
      </div>
    </div>
  </aside>

  <!-- ═══════════════════════════════════════════════════════════════ -->
  <!--  DETAIL PANE  —  Skeleton → Server-Patched Content            -->
  <!-- ═══════════════════════════════════════════════════════════════ -->
  <main class="flex-1 overflow-y-auto bg-white">
    <!-- DS:SHOW — Empty state when nothing selected -->
    <div data-show="!$selectedId" class="h-full flex items-center justify-center text-gray-400">
      <div class="text-center">
        <svg class="w-12 h-12 mx-auto mb-3 text-gray-300" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M9 12h6m-6 4h6m2 5H7a2 2 0 01-2-2V5a2 2 0 012-2h5.586a1 1 0 01.707.293l5.414 5.414a1 1 0 01.293.707V19a2 2 0 01-2 2z"/></svg>
        <p>Select an item to view details</p>
      </div>
    </div>
    <!-- /DS:SHOW -->

    <!-- DS:SHOW — Detail view wrapper (hidden when no selection) -->
    <div data-show="$selectedId" class="p-8">
      <!-- DS:SHOW — Skeleton loader visible during server fetch -->
      <div data-show="$isLoadingDetail" class="space-y-4">
        <div class="h-8 bg-gray-200 rounded w-1/3 animate-pulse"></div>
        <div class="h-4 bg-gray-200 rounded w-full animate-pulse"></div>
        <div class="h-4 bg-gray-200 rounded w-5/6 animate-pulse"></div>
        <div class="h-32 bg-gray-200 rounded w-full animate-pulse mt-4"></div>
      </div>
      <!-- /DS:SHOW -->

      <!-- Server patches this div via SSE with actual detail HTML -->
      <div id="detail-content" data-show="!$isLoadingDetail">
        <!-- Patched by server via SSE -->
      </div>
    </div>
    <!-- /DS:SHOW -->
  </main>

</div>

Server Response (SSE):

event: datastar-patch-signals
data: signals {"isLoadingDetail": false}

event: datastar-patch-elements
data: selector #detail-content
data: elements <div id="detail-content"><h1 class="text-2xl font-bold mb-2">Item 1</h1><p class="text-gray-600">...</p></div>

4.2 Multi-Step Wizards & Forms

Pattern: A linear multi-step workflow (wizard) that breaks a complex form or process into digestible stages, each with its own fields, validation rules, and UI. The client maintains a $step signal (1 to N) that controls which stage is visible via data-show, ensuring that form data in hidden steps is preserved in the DOM (not destroyed) so users can navigate back and forth without losing input. Each step has independent validation triggered on blur or debounced input, with errors stored in a $errors signal keyed by step (e.g., $errors.step1.name). Progress is visualized through a computed signal ($progress) that drives a CSS width transition on a progress bar and step indicator dots. The server validates each step individually via POST, returning error signals or allowing progression. The final step presents a review summary computed from the accumulated form data, and submission sends the complete payload. This pattern is essential for user onboarding, checkout flows, configuration wizards, survey builders, and any process where cognitive load must be managed through progressive disclosure.

<div class="max-w-2xl mx-auto p-6"
     <!-- DS:SIGNALS — Wizard state: step index, form data, validation, submission -->
     data-signals="{
       step: 1,
       maxSteps: 4,
       form: { name: '', email: '', plan: 'basic', payment: {}, terms: false },
       errors: {},
       touched: {},
       isSubmitting: false,
       isValidating: false
     }"
     <!-- /DS:SIGNALS -->
     <!-- DS:COMPUTED — Derived values: progress %, step validity, can proceed -->
     data-computed:progress="Math.round(($step / $maxSteps) * 100)"
     data-computed:canProceed="Object.keys($errors).filter(k => k.startsWith('step' + $step)).length === 0"
     data-computed:stepValid="!Object.keys($errors).some(k => k.startsWith('step' + $step))">
     <!-- /DS:COMPUTED -->

  <!-- Progress Bar -->
  <div class="mb-8">
    <div class="flex justify-between mb-2">
      <span class="text-sm font-medium" data-text="'Step ' + $step + ' of ' + $maxSteps"></span>
      <span class="text-sm text-gray-500" data-text="$progress + '%'"></span>
    </div>
    <div class="w-full bg-gray-200 rounded-full h-2">
      <div class="bg-blue-600 h-2 rounded-full transition-all duration-500 ease-out"
           data-style:width="$progress + '%'"></div>
    </div>
    <!-- Step indicators -->
    <div class="flex justify-between mt-2">
      <template data-for="s in [1,2,3,4]">
        <div class="flex flex-col items-center">
          <div class="w-8 h-8 rounded-full flex items-center justify-center text-sm font-bold transition-colors"
               data-class="{
                 'bg-blue-600 text-white': s <= $step,
                 'bg-gray-200 text-gray-500': s > $step
               }">
            <span data-text="s"></span>
          </div>
        </div>
      </template>
    </div>
  </div>

  <!-- Step 1: Basic Info -->
  <div data-show="$step === 1" class="space-y-4">
    <h2 class="text-xl font-bold">Personal Information</h2>
    <div>
      <label class="block text-sm font-medium mb-1">Full Name <span class="text-red-500">*</span></label>
      <input type="text" data-bind="form.name" 
             data-on:blur="$touched.name = true; @post('/api/validate', {field: 'name', value: $form.name})"
             class="w-full px-3 py-2 border rounded-lg focus:ring-2 focus:ring-blue-500 transition-all"
             data-class="{'border-red-500 ring-1 ring-red-500': $errors.name && $touched.name}" />
      <p data-show="$errors.name && $touched.name" class="text-red-500 text-sm mt-1" data-text="$errors.name"></p>
    </div>
    <div>
      <label class="block text-sm font-medium mb-1">Email <span class="text-red-500">*</span></label>
      <input type="email" data-bind="form.email"
             data-on:blur="$touched.email = true; @post('/api/validate', {field: 'email', value: $form.email})"
             class="w-full px-3 py-2 border rounded-lg focus:ring-2 focus:ring-blue-500 transition-all"
             data-class="{'border-red-500 ring-1 ring-red-500': $errors.email && $touched.email}" />
      <p data-show="$errors.email && $touched.email" class="text-red-500 text-sm mt-1" data-text="$errors.email"></p>
    </div>
  </div>

  <!-- Step 2: Plan Selection -->
  <div data-show="$step === 2" class="space-y-4">
    <h2 class="text-xl font-bold">Choose Your Plan</h2>
    <div class="grid grid-cols-1 md:grid-cols-3 gap-4">
      <label class="cursor-pointer relative">
        <input type="radio" name="plan" value="basic" class="sr-only" data-bind="form.plan" />
        <div class="p-4 border-2 rounded-lg hover:shadow-md transition-all h-full"
             data-class="{'border-blue-500 bg-blue-50': $form.plan === 'basic', 'border-gray-200': $form.plan !== 'basic'}">
          <h3 class="font-bold text-lg">Basic</h3>
          <p class="text-2xl font-bold my-2">$9<span class="text-sm font-normal text-gray-500">/mo</span></p>
          <ul class="text-sm text-gray-600 space-y-1">
            <li>✓ 5 Projects</li>
            <li>✓ Basic Analytics</li>
            <li>✗ Priority Support</li>
          </ul>
        </div>
      </label>
      <label class="cursor-pointer relative">
        <input type="radio" name="plan" value="pro" class="sr-only" data-bind="form.plan" />
        <div class="p-4 border-2 rounded-lg hover:shadow-md transition-all h-full"
             data-class="{'border-blue-500 bg-blue-50': $form.plan === 'pro', 'border-gray-200': $form.plan !== 'pro'}">
          <div class="absolute -top-3 left-1/2 -translate-x-1/2 bg-blue-600 text-white text-xs px-2 py-0.5 rounded-full">Popular</div>
          <h3 class="font-bold text-lg">Pro</h3>
          <p class="text-2xl font-bold my-2">$29<span class="text-sm font-normal text-gray-500">/mo</span></p>
          <ul class="text-sm text-gray-600 space-y-1">
            <li>✓ Unlimited Projects</li>
            <li>✓ Advanced Analytics</li>
            <li>✓ Priority Support</li>
          </ul>
        </div>
      </label>
      <label class="cursor-pointer relative">
        <input type="radio" name="plan" value="enterprise" class="sr-only" data-bind="form.plan" />
        <div class="p-4 border-2 rounded-lg hover:shadow-md transition-all h-full"
             data-class="{'border-blue-500 bg-blue-50': $form.plan === 'enterprise', 'border-gray-200': $form.plan !== 'enterprise'}">
          <h3 class="font-bold text-lg">Enterprise</h3>
          <p class="text-2xl font-bold my-2">$99<span class="text-sm font-normal text-gray-500">/mo</span></p>
          <ul class="text-sm text-gray-600 space-y-1">
            <li>✓ Everything in Pro</li>
            <li>✓ Custom Integrations</li>
            <li>✓ Dedicated Support</li>
          </ul>
        </div>
      </label>
    </div>
  </div>

  <!-- Step 3: Payment -->
  <div data-show="$step === 3" class="space-y-4">
    <h2 class="text-xl font-bold">Payment Details</h2>
    <div class="bg-gray-50 p-4 rounded-lg border border-gray-200">
      <p class="text-sm text-gray-600 mb-4">Selected plan: <span class="font-bold" data-text="$form.plan.toUpperCase()"></span></p>
      <div id="payment-form">
        <!-- Server-rendered payment form (Stripe/PayPal integration) -->
      </div>
    </div>
  </div>

  <!-- Step 4: Review & Confirm -->
  <div data-show="$step === 4" class="space-y-4">
    <h2 class="text-xl font-bold">Review Your Order</h2>
    <div class="bg-gray-50 rounded-lg border border-gray-200 overflow-hidden">
      <div class="p-4 border-b border-gray-200">
        <h3 class="font-medium mb-2">Account Details</h3>
        <div class="grid grid-cols-2 gap-2 text-sm">
          <span class="text-gray-500">Name:</span> <span class="font-medium" data-text="$form.name"></span>
          <span class="text-gray-500">Email:</span> <span class="font-medium" data-text="$form.email"></span>
        </div>
      </div>
      <div class="p-4 border-b border-gray-200">
        <h3 class="font-medium mb-2">Plan</h3>
        <div class="flex justify-between items-center">
          <span class="font-medium" data-text="$form.plan.charAt(0).toUpperCase() + $form.plan.slice(1)"></span>
          <span class="font-bold" data-text="$form.plan === 'basic' ? '$9/mo' : $form.plan === 'pro' ? '$29/mo' : '$99/mo'"></span>
        </div>
      </div>
      <div class="p-4 bg-gray-100">
        <div class="flex items-start gap-3">
          <input type="checkbox" data-bind="form.terms" class="mt-1" />
          <label class="text-sm">I agree to the <a href="/terms" class="text-blue-600 hover:underline">Terms of Service</a> and <a href="/privacy" class="text-blue-600 hover:underline">Privacy Policy</a></label>
        </div>
        <p data-show="$errors.terms" class="text-red-500 text-sm mt-1" data-text="$errors.terms"></p>
      </div>
    </div>
  </div>

  <!-- Navigation -->
  <div class="flex justify-between mt-8 pt-4 border-t">
    <button data-show="$step > 1"
            data-on:click="$step--"
            class="px-4 py-2 border rounded-lg hover:bg-gray-50 transition-colors">
      ← Previous
    </button>
    <div class="flex gap-2">
      <button data-show="$step < $maxSteps"
              data-on:click="if($stepValid) { $step++ } else { @post('/api/validate/step', {step: $step}) }"
              class="px-4 py-2 bg-blue-600 text-white rounded-lg hover:bg-blue-700 transition-colors">
        Next →
      </button>
      <button data-show="$step === $maxSteps"
              data-on:click="@post('/api/submit')"
              data-attr:disabled="!$form.terms || $isSubmitting"
              class="px-6 py-2 bg-green-600 text-white rounded-lg hover:bg-green-700 disabled:opacity-50 disabled:cursor-not-allowed transition-colors">
        <span data-show="!$isSubmitting">Complete Signup</span>
        <span data-show="$isSubmitting" class="flex items-center gap-2">
          <svg class="w-4 h-4 animate-spin" fill="none" viewBox="0 0 24 24"><circle class="opacity-25" cx="12" cy="12" r="10" stroke="currentColor" stroke-width="4"></circle><path class="opacity-75" fill="currentColor" d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4zm2 5.291A7.962 7.962 0 014 12H0c0 3.042 1.135 5.824 3 7.938l3-2.647z"></path></svg>
          Processing...
        </span>
      </button>
    </div>
  </div>

</div>

4.3 Real-Time Dashboards with SSE

Pattern: A real-time dashboard where the server acts as the single source of truth, continuously pushing data updates to the client through Server-Sent Events (SSE). The client does not poll; instead, it maintains a persistent SSE connection (with __openWhenHidden to survive tab backgrounding) that receives two types of patches: signal patches (datastar-patch-signals) for scalar values like KPI numbers, timestamps, and boolean flags, and element patches (datastar-patch-elements) for complex visualizations like SVG charts, maps, and lists. The client layer is extremely thin — it applies these patches reactively without business logic. The server controls update frequency, batching, and prioritization (e.g., critical alerts use prepend mode to appear at the top of a feed). Time-range selectors and filter buttons on the client trigger new SSE streams or parameterized GETs, but the server decides what to push. This pattern is the foundation for live monitoring dashboards, trading platforms, IoT control panels, sports scoreboards, and any system where stale data is unacceptable and server authority is paramount.

<div class="min-h-screen bg-gray-900 text-white p-6"
     data-signals="{metrics: {}, alerts: [], lastUpdate: null, chartData: []}">

  <!-- DS:INIT — Auto-establishes persistent SSE stream; survives tab backgrounding -->
  <div data-init="@get('/api/dashboard/stream')__openWhenHidden.true"
       class="hidden"></div>
  <!-- /DS:INIT -->

  <!-- Header -->
  <header class="flex justify-between items-center mb-8">
    <div>
      <h1 class="text-2xl font-bold">Live Dashboard</h1>
      <p class="text-sm text-gray-400 mt-1">Real-time system monitoring</p>
    </div>
    <div class="flex items-center gap-4">
      <div class="flex items-center gap-2 bg-gray-800 px-3 py-1.5 rounded-full">
        <span class="w-2 h-2 bg-green-500 rounded-full animate-pulse"></span>
        <span class="text-sm text-gray-300">Connected</span>
      </div>
      <span class="text-sm text-gray-400 font-mono" data-text="$lastUpdate ? new Date($lastUpdate).toLocaleTimeString() : '--:--:--'"></span>
    </div>
  </header>

  <!-- KPI Cards with trend indicators -->
  <div class="grid grid-cols-1 md:grid-cols-2 lg:grid-cols-4 gap-4 mb-8">
    <div class="bg-gray-800 p-5 rounded-xl border border-gray-700 hover:border-gray-600 transition-colors">
      <div class="flex justify-between items-start mb-2">
        <h3 class="text-gray-400 text-sm font-medium">Revenue</h3>
        <span class="text-xs px-2 py-0.5 rounded-full"
              data-class="{'bg-green-500/20 text-green-400': ($metrics.revenueChange || 0) > 0, 'bg-red-500/20 text-red-400': ($metrics.revenueChange || 0) < 0}"
              data-text="($metrics.revenueChange > 0 ? '↑ ' : '↓ ') + Math.abs($metrics.revenueChange || 0) + '%'"></span>
      </div>
      <p class="text-3xl font-bold" data-text="'$' + ($metrics.revenue || 0).toLocaleString()"></p>
      <p class="text-xs text-gray-500 mt-1">vs last month</p>
    </div>

    <div class="bg-gray-800 p-5 rounded-xl border border-gray-700 hover:border-gray-600 transition-colors">
      <div class="flex justify-between items-start mb-2">
        <h3 class="text-gray-400 text-sm font-medium">Active Users</h3>
        <span class="w-2 h-2 bg-green-500 rounded-full animate-pulse"></span>
      </div>
      <p class="text-3xl font-bold" data-text="($metrics.activeUsers || 0).toLocaleString()"></p>
      <p class="text-xs text-gray-500 mt-1">Currently online</p>
    </div>

    <div class="bg-gray-800 p-5 rounded-xl border border-gray-700 hover:border-gray-600 transition-colors">
      <div class="flex justify-between items-start mb-2">
        <h3 class="text-gray-400 text-sm font-medium">Avg. Latency</h3>
        <span class="text-xs px-2 py-0.5 rounded-full bg-yellow-500/20 text-yellow-400">P95</span>
      </div>
      <p class="text-3xl font-bold" data-text="($metrics.latency || 0) + 'ms'"></p>
      <p class="text-xs text-gray-500 mt-1">Request response time</p>
    </div>

    <div class="bg-gray-800 p-5 rounded-xl border border-gray-700 hover:border-gray-600 transition-colors">
      <div class="flex justify-between items-start mb-2">
        <h3 class="text-gray-400 text-sm font-medium">Error Rate</h3>
        <span class="text-xs px-2 py-0.5 rounded-full"
              data-class="{'bg-green-500/20 text-green-400': ($metrics.errorRate || 0) < 1, 'bg-red-500/20 text-red-400': ($metrics.errorRate || 0) >= 1}"
              data-text="($metrics.errorRate || 0) < 1 ? 'Healthy' : 'Critical'"></span>
      </div>
      <p class="text-3xl font-bold" data-text="($metrics.errorRate || 0) + '%'"></p>
      <p class="text-xs text-gray-500 mt-1">Last 5 minutes</p>
    </div>
  </div>

  <!-- Charts Area (server-patched SVG) -->
  <div class="grid grid-cols-1 lg:grid-cols-3 gap-4 mb-8">
    <div class="lg:col-span-2 bg-gray-800 p-5 rounded-xl border border-gray-700">
      <div class="flex justify-between items-center mb-4">
        <h3 class="text-lg font-bold">Traffic Over Time</h3>
        <div class="flex gap-2">
          <button data-on:click="$timeRange = '1h'; @get('/api/dashboard/chart?range=1h')"
                  data-class="{'bg-blue-600 text-white': $timeRange === '1h', 'bg-gray-700 text-gray-300': $timeRange !== '1h'}"
                  class="px-3 py-1 rounded text-sm transition-colors">1H</button>
          <button data-on:click="$timeRange = '24h'; @get('/api/dashboard/chart?range=24h')"
                  data-class="{'bg-blue-600 text-white': $timeRange === '24h', 'bg-gray-700 text-gray-300': $timeRange !== '24h'}"
                  class="px-3 py-1 rounded text-sm transition-colors">24H</button>
          <button data-on:click="$timeRange = '7d'; @get('/api/dashboard/chart?range=7d')"
                  data-class="{'bg-blue-600 text-white': $timeRange === '7d', 'bg-gray-700 text-gray-300': $timeRange !== '7d'}"
                  class="px-3 py-1 rounded text-sm transition-colors">7D</button>
        </div>
      </div>
      <div id="traffic-chart" class="h-64">
        <!-- Server patches SVG chart here -->
      </div>
    </div>

    <div class="bg-gray-800 p-5 rounded-xl border border-gray-700">
      <h3 class="text-lg font-bold mb-4">Top Endpoints</h3>
      <div id="endpoints-list" class="space-y-3">
        <!-- Server-patched list -->
      </div>
    </div>
  </div>

  <!-- Live Alert Feed with severity colors -->
  <div class="bg-gray-800 rounded-xl border border-gray-700 overflow-hidden">
    <div class="p-4 border-b border-gray-700 flex justify-between items-center">
      <div class="flex items-center gap-2">
        <h3 class="text-lg font-bold">Live Alerts</h3>
        <span class="bg-red-500/20 text-red-400 text-xs px-2 py-0.5 rounded-full" data-text="$alerts.filter(a => a.severity === 'critical').length + ' critical'"></span>
      </div>
      <div class="flex gap-2">
        <button data-on:click="$alertFilter = 'all'; @get('/api/alerts?filter=all')"
                data-class="{'text-white': $alertFilter === 'all', 'text-gray-400': $alertFilter !== 'all'}"
                class="text-sm hover:text-white transition-colors">All</button>
        <button data-on:click="$alertFilter = 'critical'; @get('/api/alerts?filter=critical')"
                data-class="{'text-white': $alertFilter === 'critical', 'text-gray-400': $alertFilter !== 'critical'}"
                class="text-sm hover:text-white transition-colors">Critical</button>
        <button data-on:click="$alerts = []" class="text-sm text-gray-400 hover:text-white transition-colors ml-4">
          Clear All
        </button>
      </div>
    </div>
    <div id="alert-feed" class="max-h-80 overflow-y-auto divide-y divide-gray-700">
      <div data-show="$alerts.length === 0" class="p-8 text-gray-500 text-center">
        <svg class="w-12 h-12 mx-auto mb-2 text-gray-600" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M9 12l2 2 4-4m6 2a9 9 0 11-18 0 9 9 0 0118 0z"/></svg>
        <p>No active alerts</p>
      </div>
      <!-- Alerts patched here by server -->
    </div>
  </div>

</div>

Server-Side SSE Stream:

event: datastar-patch-signals
data: signals {"metrics": {"revenue": 125000, "revenueChange": 12.5, "activeUsers": 4523, "latency": 45, "errorRate": 0.2}, "lastUpdate": "2026-06-09T15:20:00Z"}

event: datastar-patch-elements
data: selector #traffic-chart
data: elements <svg id="traffic-chart" class="h-64">...</svg>

event: datastar-patch-elements
data: selector #alert-feed
data: mode prepend
data: elements <div class="p-3 flex items-start gap-3 bg-red-900/20"><span class="text-red-400">●</span><div><p class="font-medium text-red-300">High CPU Usage</p><p class="text-sm text-gray-400">Server-03 at 95% capacity</p></div><span class="text-xs text-gray-500 ml-auto">2m ago</span></div>

4.4 Nested Data Tables with Inline Editing

Pattern: A data-dense table that behaves like a mini-application within the page. Each row supports three interaction modes: read (default), expanded (revealing child detail panels with additional metadata and actions), and edit (inline form fields replacing static text). Only one row can be in edit mode at a time ($editingId), preventing conflicting state. Sorting is handled by clicking column headers, which toggles $sortBy and $sortDir signals and triggers a server fetch with the new parameters. Inline editing pre-populates an $editForm signal when entering edit mode, and changes are sent to the server via POST on save. The server responds by patching either the single row (for inline edits) or the entire table body (for sorts/filters). Bulk selection via checkboxes supports header-level "select all" with indeterminate state, and the toolbar adapts dynamically to show bulk action options only when items are selected. This pattern is the workhorse of admin panels, inventory systems, user management interfaces, and any CRUD-heavy data view.

<div class="p-6" data-signals="{editingId: null, expandedIds: [], sortBy: 'name', sortDir: 'asc', selectAll: false, selectedIds: []}">

  <!-- ═══════════════════════════════════════════════════════════════ -->
  <!--  TOOLBAR  —  Filter Inputs + Bulk Actions                     -->
  <!-- ═══════════════════════════════════════════════════════════════ -->
  <div class="flex justify-between items-center mb-4">
    <div class="flex items-center gap-3">
      <!-- DS:BIND + DS:ON — Search input with debounced server fetch -->
      <input type="text" data-bind="tableFilter"
             data-on:input__debounce.300ms="@get('/api/users?filter=' + $tableFilter)"
             placeholder="Search users..."
             class="px-3 py-2 border rounded-lg text-sm w-64" />
      <!-- /DS:BIND + /DS:ON -->
      <!-- DS:BIND + DS:ON — Role filter dropdown triggers immediate refetch -->
      <select data-bind="roleFilter" data-on:change="@get('/api/users?role=' + $roleFilter)"
              class="px-3 py-2 border rounded-lg text-sm">
        <option value="">All Roles</option>
        <option value="admin">Admin</option>
        <option value="user">User</option>
      </select>
      <!-- /DS:BIND + /DS:ON -->
    </div>
    <div class="flex gap-2">
      <button data-show="$selectedIds.length > 0"
              data-on:click="@post('/api/users/bulk-delete', {ids: $selectedIds})"
              class="px-3 py-2 bg-red-600 text-white rounded-lg text-sm hover:bg-red-700">
        Delete Selected (<span data-text="$selectedIds.length"></span>)
      </button>
      <button data-on:click="@get('/api/users/export')"
              class="px-3 py-2 border rounded-lg text-sm hover:bg-gray-50">
        Export CSV
      </button>
    </div>
  </div>

  <div class="border rounded-lg overflow-hidden">
    <table class="w-full text-sm">
      <thead class="bg-gray-50 border-b">
        <tr>
          <th class="p-3 w-10">
            <input type="checkbox" 
                   data-on:change="$selectedIds = evt.target.checked ? [1,2,3,4,5] : []"
                   data-attr:checked="$selectedIds.length === 5" />
          </th>
          <th class="p-3 text-left cursor-pointer hover:bg-gray-100 transition-colors"
              data-on:click="$sortBy = 'name'; $sortDir = $sortDir === 'asc' ? 'desc' : 'asc'; @get('/api/users?sort=' + $sortBy + '&dir=' + $sortDir)">
            <div class="flex items-center gap-1">
              Name
              <span data-show="$sortBy === 'name'" data-text="$sortDir === 'asc' ? '↑' : '↓'"></span>
            </div>
          </th>
          <th class="p-3 text-left">Email</th>
          <th class="p-3 text-left">Role</th>
          <th class="p-3 text-left">Status</th>
          <th class="p-3 text-left">Last Active</th>
          <th class="p-3 text-right">Actions</th>
        </tr>
      </thead>
      <tbody class="divide-y">
        <!-- Row 1 -->
        <tr class="hover:bg-gray-50 transition-colors"
            data-on:click="$expandedIds = $expandedIds.includes(1) ? $expandedIds.filter(id => id !== 1) : [...$expandedIds, 1]">
          <td class="p-3" data-on:click__stop>
            <input type="checkbox" 
                   data-on:change__stop="$selectedIds = evt.target.checked ? [...$selectedIds, 1] : $selectedIds.filter(id => id !== 1)"
                   data-attr:checked="$selectedIds.includes(1)" />
          </td>
          <td class="p-3">
            <div class="flex items-center gap-2">
              <span class="transform transition-transform duration-200"
                    data-class="{'rotate-90': $expandedIds.includes(1)}">▶</span>
              <img src="/avatar1.jpg" class="w-8 h-8 rounded-full bg-gray-200" />
              <div>
                <p data-show="$editingId !== 1" class="font-medium">John Doe</p>
                <input data-show="$editingId === 1" data-bind="editForm.name" 
                       class="border rounded px-2 py-1 text-sm w-32" />
              </div>
            </div>
          </td>
          <td class="p-3 text-gray-600">john@example.com</td>
          <td class="p-3">
            <span data-show="$editingId !== 1" class="px-2 py-1 bg-blue-100 text-blue-800 rounded-full text-xs font-medium">Admin</span>
            <select data-show="$editingId === 1" data-bind="editForm.role" class="border rounded px-2 py-1 text-sm">
              <option>Admin</option><option>User</option><option>Guest</option>
            </select>
          </td>
          <td class="p-3">
            <span class="inline-flex items-center gap-1.5 px-2 py-1 rounded-full text-xs font-medium"
                  data-class="{'bg-green-100 text-green-800': true, 'bg-gray-100 text-gray-600': false}">
              <span class="w-1.5 h-1.5 rounded-full bg-green-500"></span>
              Active
            </span>
          </td>
          <td class="p-3 text-gray-500">2 min ago</td>
          <td class="p-3 text-right">
            <button data-show="$editingId !== 1"
                    data-on:click__stop="$editingId = 1; $editForm = {name: 'John Doe', role: 'Admin'}"
                    class="text-blue-600 hover:text-blue-800 text-sm mr-3">Edit</button>
            <button data-show="$editingId === 1"
                    data-on:click__stop="@post('/api/users/1/update')"
                    class="text-green-600 hover:text-green-800 text-sm mr-2">Save</button>
            <button data-show="$editingId === 1"
                    data-on:click__stop="$editingId = null"
                    class="text-gray-600 hover:text-gray-800 text-sm">Cancel</button>
          </td>
        </tr>

        <!-- Expanded Detail Row -->
        <tr data-show="$expandedIds.includes(1)">
          <td colspan="7" class="p-0">
            <div class="bg-gray-50 p-4 border-t border-gray-100">
              <div class="grid grid-cols-4 gap-4 text-sm">
                <div><span class="text-gray-500 block text-xs mb-1">Department</span> <span class="font-medium">Engineering</span></div>
                <div><span class="text-gray-500 block text-xs mb-1">Joined</span> <span class="font-medium">Jan 15, 2024</span></div>
                <div><span class="text-gray-500 block text-xs mb-1">Last Login</span> <span class="font-medium">2 hours ago</span></div>
                <div><span class="text-gray-500 block text-xs mb-1">2FA</span> <span class="font-medium text-green-600">Enabled</span></div>
              </div>
              <div class="mt-3 flex gap-2">
                <button data-on:click__stop="@post('/api/users/1/reset-password')"
                        class="text-xs px-3 py-1.5 border rounded hover:bg-white transition-colors">Reset Password</button>
                <button data-on:click__stop="@post('/api/users/1/disable')"
                        class="text-xs px-3 py-1.5 border border-red-200 text-red-600 rounded hover:bg-red-50 transition-colors">Disable Account</button>
              </div>
            </div>
          </td>
        </tr>
      </tbody>
    </table>
  </div>

  <!-- Pagination -->
  <div class="flex justify-between items-center mt-4 text-sm">
    <span class="text-gray-500">Showing 1-5 of 42 results</span>
    <div class="flex gap-1">
      <button data-attr:disabled="true" class="px-3 py-1 border rounded hover:bg-gray-50 disabled:opacity-50">Previous</button>
      <button data-on:click="@get('/api/users?page=2')" class="px-3 py-1 border rounded hover:bg-gray-50">1</button>
      <button data-on:click="@get('/api/users?page=2')" class="px-3 py-1 border rounded hover:bg-gray-50">2</button>
      <button data-on:click="@get('/api/users?page=3')" class="px-3 py-1 border rounded hover:bg-gray-50">3</button>
      <span class="px-2 py-1">...</span>
      <button data-on:click="@get('/api/users?page=2')" class="px-3 py-1 border rounded hover:bg-gray-50">Next</button>
    </div>
  </div>

</div>

4.5 Kanban Boards & Drag-Drop

Pattern: A visual task board (Kanban) composed of vertical columns representing workflow stages (e.g., To Do, In Progress, Done), each containing draggable cards representing individual work items. The client uses HTML5 Drag and Drop API events (dragstart, dragover, drop) mapped to Datastar signals ($draggingId, $dragOverColumn) to provide immediate visual feedback (highlighting the target column, dimming the dragged card) without waiting for the server. The actual move operation is only sent to the server on drop, where the server validates the transition (e.g., checking permissions, enforcing workflow rules) and returns the updated board state. Cards support rich metadata (tags with color coding, priority indicators, assignee avatars), inline editing, and expandable detail views. New tasks can be created directly in columns via an inline form. This pattern is essential for project management tools, issue trackers, editorial calendars, recruitment pipelines, and any workflow that benefits from visual status tracking and manual prioritization.

<div class="p-6 bg-gray-100 min-h-screen"
     data-signals="{
       columns: [
         {id: 'todo', title: 'To Do', color: 'gray', items: [{id: 1, text: 'Design mockups', tags: ['design'], priority: 'high'}, {id: 2, text: 'Setup repo', tags: ['dev'], priority: 'medium'}]},
         {id: 'doing', title: 'In Progress', color: 'blue', items: [{id: 3, text: 'Implement auth', tags: ['dev', 'security'], priority: 'high'}]},
         {id: 'review', title: 'Review', color: 'yellow', items: []},
         {id: 'done', title: 'Done', color: 'green', items: [{id: 4, text: 'Project setup', tags: ['dev'], priority: 'low'}]}
       ],
       draggingId: null,
       dragOverColumn: null,
       newTaskColumn: null,
       newTaskText: ''
     }">

  <!-- ═══════════════════════════════════════════════════════════════ -->
  <!--  KANBAN BOARD  —  Columns with DnD + Inline Create/Edit      -->
  <!-- ═══════════════════════════════════════════════════════════════ -->
  <div class="flex gap-4 overflow-x-auto pb-4 items-start">
    <template data-for="col in $columns">
      <!-- DS:CLASS + DS:ON — Column highlights on drag-over; drop sends move to server -->
      <div class="w-80 flex-shrink-0 bg-gray-200 rounded-xl p-3"
           data-class="{'ring-2 ring-blue-400 ring-offset-2': $dragOverColumn === col.id}"
           data-on:dragover__prevent="$dragOverColumn = col.id"
           data-on:dragleave="$dragOverColumn = null"
           data-on:drop__prevent="$dragOverColumn = null; @post('/api/tasks/move', {columnId: col.id, taskId: $draggingId})">
      <!-- /DS:CLASS + /DS:ON -->

        <!-- Column Header -->
        <div class="flex justify-between items-center mb-3 px-1">
          <div class="flex items-center gap-2">
            <div class="w-3 h-3 rounded-full"
                 data-class="{
                   'bg-gray-400': col.color === 'gray',
                   'bg-blue-500': col.color === 'blue',
                   'bg-yellow-500': col.color === 'yellow',
                   'bg-green-500': col.color === 'green'
                 }"></div>
            <h3 class="font-bold text-sm" data-text="col.title"></h3>
            <span class="bg-gray-300 text-gray-600 text-xs px-2 py-0.5 rounded-full font-medium" 
                  data-text="col.items.length"></span>
          </div>
          <button data-on:click="$newTaskColumn = col.id"
                  class="text-gray-400 hover:text-gray-600 p-1 rounded hover:bg-gray-300 transition-colors">
            <svg class="w-4 h-4" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M12 4v16m8-8H4"/></svg>
          </button>
        </div>

        <!-- Add Task Form -->
        <div data-show="$newTaskColumn === col.id" class="mb-2 bg-white p-2 rounded-lg shadow-sm">
          <input type="text" data-bind="newTaskText" 
                 placeholder="Enter task title..."
                 class="w-full text-sm px-2 py-1 border rounded mb-2"
                 data-ref="newTaskInput"
                 data-effect="if($newTaskColumn === col.id) { setTimeout(() => $newTaskInput?.focus(), 50) }" />
          <div class="flex gap-2">
            <button data-on:click="@post('/api/tasks', {columnId: col.id, text: $newTaskText}); $newTaskColumn = null; $newTaskText = ''"
                    class="px-3 py-1 bg-blue-600 text-white text-xs rounded hover:bg-blue-700">Add</button>
            <button data-on:click="$newTaskColumn = null; $newTaskText = ''"
                    class="px-3 py-1 text-gray-500 text-xs hover:text-gray-700">Cancel</button>
          </div>
        </div>

        <!-- Cards -->
        <div class="space-y-2 min-h-[100px]">
          <template data-for="item in col.items">
            <div class="bg-white p-3 rounded-lg shadow-sm cursor-move hover:shadow-md transition-all group"
                 draggable="true"
                 data-attr:draggable="$editingId !== item.id"
                 data-on:dragstart="$draggingId = item.id"
                 data-on:dragend="$draggingId = null"
                 data-on:click="$expandedTask = $expandedTask === item.id ? null : item.id">

              <div class="flex justify-between items-start mb-2">
                <p data-text="item.text" data-show="$editingId !== item.id" class="text-sm font-medium"></p>
                <input data-show="$editingId === item.id" data-bind="editText" 
                       class="w-full text-sm border rounded px-2 py-1" />

                <div class="flex gap-1 ml-2 opacity-0 group-hover:opacity-100 transition-opacity">
                  <button data-show="$editingId !== item.id"
                          data-on:click__stop="$editingId = item.id; $editText = item.text"
                          class="text-gray-400 hover:text-blue-600 p-0.5">✎</button>
                  <button data-show="$editingId === item.id"
                          data-on:click__stop="@post('/api/tasks/' + item.id + '/update', {text: $editText})"
                          class="text-green-600 hover:text-green-800 p-0.5">✓</button>
                </div>
              </div>

              <!-- Tags & Priority -->
              <div class="flex items-center justify-between">
                <div class="flex gap-1">
                  <template data-for="tag in item.tags">
                    <span class="text-[10px] px-1.5 py-0.5 rounded font-medium"
                          data-class="{
                            'bg-purple-100 text-purple-700': tag === 'design',
                            'bg-blue-100 text-blue-700': tag === 'dev',
                            'bg-red-100 text-red-700': tag === 'security'
                          }"
                          data-text="tag"></span>
                  </template>
                </div>
                <span class="text-xs"
                      data-class="{
                        'text-red-500': item.priority === 'high',
                        'text-yellow-500': item.priority === 'medium',
                        'text-gray-400': item.priority === 'low'
                      }">●</span>
              </div>

              <!-- Expanded details -->
              <div data-show="$expandedTask === item.id" class="mt-2 pt-2 border-t text-xs text-gray-500 space-y-1">
                <p>Created: 2 days ago by John Doe</p>
                <p>Assignee: <span class="text-gray-700">Jane Smith</span></p>
                <div class="flex gap-2 mt-2">
                  <button data-on:click__stop="@post('/api/tasks/' + item.id + '/assign')"
                          class="text-blue-600 hover:underline">Assign</button>
                  <button data-on:click__stop="@post('/api/tasks/' + item.id + '/archive')"
                          class="text-gray-600 hover:underline">Archive</button>
                </div>
              </div>

            </div>
          </template>
        </div>

      </div>
    </template>
  </div>

</div>

4.6 Modal, Drawer & Toast Systems

Pattern: A layered overlay system that manages multiple types of transient UI surfaces through a unified signal-based state machine. Modals are stored in a stack array ($modals) supporting nested dialogs (e.g., "Are you sure?" inside "Delete Item"), with backdrop clicks popping the top modal. Drawers slide in from edges (right, left, bottom) using CSS transforms driven by boolean signals, with a shared backdrop that closes all open drawers on click. Toasts are ephemeral notifications stored in a queue ($toasts) with auto-dismiss timers managed via data-init + setTimeout, supporting multiple types (success, error, warning, info) with distinct colors and icons. All three systems coexist: a drawer can trigger a modal, which can trigger a toast, without z-index conflicts or state corruption. The server can push modal content via SSE (e.g., "Session Expiring" warnings), and the client can open overlays in response to user actions or server events. This pattern is fundamental to any modern application requiring confirmations, detail inspection, progress feedback, and system alerts.

<div data-signals="{
  modals: [],
  drawers: {right: false, left: false, bottom: false},
  toasts: [],
  toastIdCounter: 0
}">

  <!-- ═══════════════════════════════════════════════════════════════ -->
  <!--  MODAL SYSTEM  —  Stackable dialogs with backdrop dismissal   -->
  <!-- ═══════════════════════════════════════════════════════════════ -->
  <!-- DS:SHOW + DS:ON — Modal stack visible when non-empty; backdrop click pops top modal -->
  <div data-show="$modals.length > 0" 
       class="fixed inset-0 z-50 flex items-center justify-center bg-black/50 backdrop-blur-sm transition-opacity"
       data-on:click="$modals = $modals.slice(0, -1)">
  <!-- /DS:SHOW + /DS:ON -->

    <div class="bg-white rounded-xl shadow-2xl max-w-lg w-full mx-4 max-h-[90vh] overflow-hidden flex flex-col"
         data-on:click__stop
         data-class="{'scale-95 opacity-0': $modals.length === 0, 'scale-100 opacity-100': $modals.length > 0}">

      <!-- Modal Header -->
      <div class="flex justify-between items-center p-4 border-b">
        <h2 class="text-lg font-bold" data-text="$modals[$modals.length - 1]?.title || ''"></h2>
        <button data-on:click="$modals = $modals.slice(0, -1)" 
                class="text-gray-400 hover:text-gray-600 p-1 rounded-lg hover:bg-gray-100 transition-colors">✕</button>
      </div>

      <!-- Modal Body (server-patched) -->
      <div id="modal-content" class="p-4 overflow-y-auto flex-1">
        <!-- Content patched by server based on modal type -->
      </div>

      <!-- Modal Footer -->
      <div class="flex justify-end gap-2 p-4 border-t bg-gray-50">
        <button data-on:click="$modals = $modals.slice(0, -1)" 
                class="px-4 py-2 border rounded-lg hover:bg-gray-100 transition-colors">Cancel</button>
        <button data-on:click="@post('/api/modal/confirm')"
                class="px-4 py-2 bg-blue-600 text-white rounded-lg hover:bg-blue-700 transition-colors">Confirm</button>
      </div>
    </div>
  </div>

  <!-- RIGHT DRAWER -->
  <div data-show="$drawers.right"
       class="fixed inset-y-0 right-0 z-40 w-[480px] bg-white shadow-2xl transform transition-transform duration-300 flex flex-col"
       data-class="{'translate-x-0': $drawers.right, 'translate-x-full': !$drawers.right}">
    <div class="flex justify-between items-center p-4 border-b">
      <h2 class="text-lg font-bold">Details</h2>
      <button data-on:click="$drawers.right = false" class="text-gray-400 hover:text-gray-600 p-1 rounded-lg hover:bg-gray-100">✕</button>
    </div>
    <div id="drawer-content" class="flex-1 overflow-y-auto p-4">
      <!-- Server-patched content -->
    </div>
    <div class="p-4 border-t bg-gray-50 flex justify-end gap-2">
      <button data-on:click="$drawers.right = false" class="px-4 py-2 border rounded-lg">Close</button>
    </div>
  </div>

  <!-- Drawer Backdrop -->
  <div data-show="$drawers.right || $drawers.left || $drawers.bottom" 
       data-on:click="$drawers = {right: false, left: false, bottom: false}"
       class="fixed inset-0 z-30 bg-black/30 transition-opacity"></div>

  <!-- BOTTOM SHEET (mobile-friendly) -->
  <div data-show="$drawers.bottom"
       class="fixed inset-x-0 bottom-0 z-40 bg-white rounded-t-xl shadow-2xl transform transition-transform duration-300 max-h-[80vh] overflow-y-auto"
       data-class="{'translate-y-0': $drawers.bottom, 'translate-y-full': !$drawers.bottom}">
    <div class="p-4">
      <div class="w-12 h-1 bg-gray-300 rounded-full mx-auto mb-4"></div>
      <div id="bottom-sheet-content">
        <!-- Server-patched -->
      </div>
    </div>
  </div>

  <!-- TOAST NOTIFICATIONS -->
  <div class="fixed bottom-4 right-4 z-50 flex flex-col gap-2 pointer-events-none">
    <template data-for="toast in $toasts">
      <div class="p-4 rounded-lg shadow-lg transform transition-all duration-300 max-w-sm pointer-events-auto flex items-start gap-3"
           data-class="{
             'bg-green-500 text-white': toast.type === 'success',
             'bg-red-500 text-white': toast.type === 'error',
             'bg-blue-500 text-white': toast.type === 'info',
             'bg-yellow-500 text-white': toast.type === 'warning'
           }"
           data-init="setTimeout(() => { $toasts = $toasts.filter(t => t.id !== toast.id) }, toast.duration || 5000)">
        <span class="text-xl">
          <span data-show="toast.type === 'success'">✓</span>
          <span data-show="toast.type === 'error'">✕</span>
          <span data-show="toast.type === 'info'">ℹ</span>
          <span data-show="toast.type === 'warning'">⚠</span>
        </span>
        <div class="flex-1">
          <p class="font-medium text-sm" data-text="toast.title"></p>
          <p class="text-sm opacity-90" data-text="toast.message"></p>
        </div>
        <button data-on:click="$toasts = $toasts.filter(t => t.id !== toast.id)" 
                class="opacity-75 hover:opacity-100 ml-2">✕</button>
      </div>
    </template>
  </div>

  <!-- TRIGGER BUTTONS -->
  <button data-on:click="$modals = [...$modals, {title: 'Create User', type: 'create-user'}]; $toastIdCounter++; @get('/api/modals/create-user')"
          class="px-4 py-2 bg-blue-600 text-white rounded-lg hover:bg-blue-700 transition-colors">
    Open Modal
  </button>

  <button data-on:click="$drawers.right = true; @get('/api/drawers/profile')"
          class="px-4 py-2 border rounded-lg hover:bg-gray-50 transition-colors">
    Open Drawer
  </button>

  <!-- Toast trigger example -->
  <button data-on:click="$toastIdCounter++; $toasts = [...$toasts, {id: $toastIdCounter, type: 'success', title: 'Saved!', message: 'Your changes have been saved.', duration: 3000}]"
          class="px-4 py-2 bg-green-600 text-white rounded-lg hover:bg-green-700 transition-colors">
    Show Toast
  </button>

</div>

4.7 Infinite Scroll & Virtual Lists

Pattern: A content feed that loads data incrementally as the user scrolls toward the bottom, eliminating the need for pagination controls and providing a seamless browsing experience. An invisible "sentinel" element at the bottom of the list is observed via IntersectionObserver (initialized in data-init) with a root margin to trigger loading before the user reaches the absolute bottom. When the sentinel enters the viewport, $isLoading is set to true (showing a spinner), and a GET request fetches the next page. The server returns new items which are appended to the $items signal, and $page is incremented. The existing items remain in the DOM — they are not re-rendered — preserving scroll position and any per-item state (like expanded comments or liked status). A "Refresh" button allows resetting the feed to page 1. This pattern is used in social media feeds, news aggregators, search results, activity logs, and any long list where users expect to browse continuously without explicit page boundaries.

<div class="h-screen overflow-y-auto" id="scroll-container"
     data-signals="{items: [], page: 1, hasMore: true, isLoading: false, scrollPos: 0, totalCount: 0}">

  <div class="max-w-2xl mx-auto p-4">
    <!-- Header with count -->
    <div class="flex justify-between items-center mb-4">
      <h1 class="text-xl font-bold">Feed</h1>
      <span class="text-sm text-gray-500" data-text="$totalCount + ' items total'"></span>
    </div>

    <!-- Items -->
    <div id="feed-items" class="space-y-4">
      <template data-for="item in $items">
        <div class="p-4 border rounded-lg hover:shadow-md transition-shadow bg-white">
          <div class="flex items-start gap-3">
            <img data-attr:src="item.avatar" class="w-10 h-10 rounded-full bg-gray-200 flex-shrink-0" />
            <div class="flex-1">
              <div class="flex justify-between items-start">
                <div>
                  <p class="font-medium" data-text="item.author"></p>
                  <p class="text-xs text-gray-500" data-text="item.timeAgo"></p>
                </div>
                <button data-on:click="@post('/api/items/' + item.id + '/bookmark')"
                        class="text-gray-400 hover:text-yellow-500 transition-colors">
                  <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M5 5a2 2 0 012-2h10a2 2 0 012 2v16l-7-3.5L5 21V5z"/></svg>
                </button>
              </div>
              <p class="mt-2 text-gray-700" data-text="item.content"></p>
              <div class="flex gap-4 mt-3 text-sm text-gray-500">
                <button data-on:click="@post('/api/items/' + item.id + '/like')" class="flex items-center gap-1 hover:text-red-500 transition-colors">
                  <svg class="w-4 h-4" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M4.318 6.318a4.5 4.5 0 000 6.364L12 20.364l7.682-7.682a4.5 4.5 0 00-6.364-6.364L12 7.636l-1.318-1.318a4.5 4.5 0 00-6.364 0z"/></svg>
                  <span data-text="item.likes"></span>
                </button>
                <button class="flex items-center gap-1 hover:text-blue-500 transition-colors">
                  <svg class="w-4 h-4" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M8 12h.01M12 12h.01M16 12h.01M21 12c0 4.418-4.03 8-9 8a9.863 9.863 0 01-4.255-.949L3 20l1.395-3.72C3.512 15.042 3 13.574 3 12c0-4.418 4.03-8 9-8s9 3.582 9 8z"/></svg>
                  <span data-text="item.comments"></span>
                </button>
              </div>
            </div>
          </div>
        </div>
      </template>
    </div>

    <!-- Loading State -->
    <div data-show="$isLoading" class="flex justify-center py-6">
      <div class="w-8 h-8 border-4 border-blue-600 border-t-transparent rounded-full animate-spin"></div>
    </div>

    <!-- ═══════════════════════════════════════════════════════════════ -->
    <!--  SENTINEL  —  IntersectionObserver triggers next page load    -->
    <!-- ═══════════════════════════════════════════════════════════════ -->
    <!-- DS:SHOW + DS:INIT — Sentinel visible when more pages exist; IO watches for viewport entry -->
    <div data-show="$hasMore && !$isLoading"
         data-init="const obs = new IntersectionObserver((entries) => { if(entries[0].isIntersecting && $hasMore) { $isLoading = true; @get('/api/items?page=' + $page) } }, {rootMargin: '100px'}); obs.observe(el);"
         class="h-20 flex items-center justify-center text-gray-400">
    <!-- /DS:SHOW + /DS:INIT -->
      <div class="flex items-center gap-2">
        <div class="w-2 h-2 bg-gray-300 rounded-full animate-bounce"></div>
        <div class="w-2 h-2 bg-gray-300 rounded-full animate-bounce" style="animation-delay: 0.1s"></div>
        <div class="w-2 h-2 bg-gray-300 rounded-full animate-bounce" style="animation-delay: 0.2s"></div>
      </div>
    </div>

    <!-- End State -->
    <div data-show="!$hasMore && $items.length > 0" class="text-center text-gray-400 py-8">
      <p>You've reached the end</p>
      <button data-on:click="$page = 1; $items = []; $hasMore = true; @get('/api/items?page=1')"
              class="mt-2 text-blue-600 hover:underline text-sm">Refresh feed</button>
    </div>
  </div>

</div>

4.8 Multi-Select & Bulk Actions

Pattern: A selection interface that allows users to mark multiple records for a collective operation, with safeguards against accidental destructive actions. Selection state is maintained as an array of IDs ($selectedIds) that grows or shrinks as individual checkboxes are toggled. The header checkbox supports three states: unchecked (none selected), checked (all visible selected), and indeterminate (some selected). When selections exist, a contextual action bar slides into view offering bulk operations (delete, archive, export, tag). Destructive actions (like delete) trigger an intermediate confirmation modal rather than executing immediately, displaying the exact count of affected items. The server receives the full ID array and processes the batch atomically, returning a success signal that clears the selection and refreshes the table. This pattern is critical for email clients, file managers, product catalogs, user directories, and any interface where users routinely manage groups of records.

<div class="p-6" data-signals="{selectedIds: [], selectAll: false, bulkAction: '', showBulkConfirm: false}">

  <!-- ═══════════════════════════════════════════════════════════════ -->
  <!--  BULK ACTION BAR  —  Appears when items are selected          -->
  <!-- ═══════════════════════════════════════════════════════════════ -->
  <!-- DS:SHOW — Bar slides in only when selection array is non-empty -->
  <div data-show="$selectedIds.length > 0" 
       class="mb-4 p-3 bg-blue-50 border border-blue-200 rounded-lg flex items-center justify-between animate-slide-in">
  <!-- /DS:SHOW -->
    <div class="flex items-center gap-3">
      <input type="checkbox" checked disabled class="rounded" />
      <span class="text-blue-800 font-medium" data-text="$selectedIds.length + ' selected'"></span>
    </div>
    <div class="flex gap-2">
      <select data-bind="bulkAction" class="border rounded px-3 py-1.5 text-sm bg-white">
        <option value="">Bulk Actions</option>
        <option value="delete">Delete</option>
        <option value="archive">Archive</option>
        <option value="export">Export CSV</option>
        <option value="tag">Add Tag</option>
      </select>
      <button data-on:click="if($bulkAction === 'delete') { $showBulkConfirm = true } else { @post('/api/bulk-action', {ids: $selectedIds, action: $bulkAction}) }"
              data-attr:disabled="!$bulkAction"
              class="px-4 py-1.5 bg-blue-600 text-white rounded-lg text-sm hover:bg-blue-700 disabled:opacity-50 disabled:cursor-not-allowed transition-colors">
        Apply
      </button>
      <button data-on:click="$selectedIds = []" class="text-gray-500 hover:text-gray-700 text-sm px-2">Cancel</button>
    </div>
  </div>

  <!-- Bulk Delete Confirmation Modal -->
  <div data-show="$showBulkConfirm" class="fixed inset-0 z-50 flex items-center justify-center bg-black/50">
    <div class="bg-white rounded-lg shadow-xl p-6 max-w-md w-full mx-4">
      <div class="flex items-center gap-3 mb-4">
        <div class="w-10 h-10 bg-red-100 rounded-full flex items-center justify-center">
          <svg class="w-5 h-5 text-red-600" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M12 9v2m0 4h.01m-6.938 4h13.856c1.54 0 2.502-1.667 1.732-3L13.732 4c-.77-1.333-2.694-1.333-3.464 0L3.34 16c-.77 1.333.192 3 1.732 3z"/></svg>
        </div>
        <h3 class="text-lg font-bold">Delete Items?</h3>
      </div>
      <p class="text-gray-600 mb-6">Are you sure you want to delete <span class="font-bold" data-text="$selectedIds.length"></span> selected items? This action cannot be undone.</p>
      <div class="flex justify-end gap-2">
        <button data-on:click="$showBulkConfirm = false" class="px-4 py-2 border rounded-lg hover:bg-gray-50">Cancel</button>
        <button data-on:click="@post('/api/bulk-action', {ids: $selectedIds, action: 'delete'}); $showBulkConfirm = false; $selectedIds = []"
                class="px-4 py-2 bg-red-600 text-white rounded-lg hover:bg-red-700">Delete</button>
      </div>
    </div>
  </div>

  <!-- Table -->
  <div class="border rounded-lg overflow-hidden">
    <table class="w-full text-sm">
      <thead class="bg-gray-50 border-b">
        <tr>
          <th class="p-3 w-10">
            <input type="checkbox" 
                   data-on:change="$selectedIds = evt.target.checked ? $allIds || [] : []"
                   data-attr:checked="$selectedIds.length > 0 && $selectedIds.length === ($allIds || []).length"
                   data-attr:indeterminate="$selectedIds.length > 0 && $selectedIds.length < ($allIds || []).length" />
          </th>
          <th class="p-3 text-left">Item</th>
          <th class="p-3 text-left">Category</th>
          <th class="p-3 text-left">Status</th>
          <th class="p-3 text-left">Date</th>
          <th class="p-3 text-right">Actions</th>
        </tr>
      </thead>
      <tbody class="divide-y">
        <tr class="hover:bg-gray-50 transition-colors">
          <td class="p-3">
            <input type="checkbox" 
                   data-on:change__stop="$selectedIds = evt.target.checked ? [...$selectedIds, 1] : $selectedIds.filter(id => id !== 1)"
                   data-attr:checked="$selectedIds.includes(1)" />
          </td>
          <td class="p-3">
            <div class="flex items-center gap-3">
              <div class="w-8 h-8 bg-blue-100 rounded-lg flex items-center justify-center text-blue-600 font-bold">A</div>
              <div>
                <p class="font-medium">Project Alpha</p>
                <p class="text-xs text-gray-500">ID: #1001</p>
              </div>
            </div>
          </td>
          <td class="p-3"><span class="px-2 py-1 bg-purple-100 text-purple-700 rounded-full text-xs">Development</span></td>
          <td class="p-3"><span class="px-2 py-1 bg-green-100 text-green-700 rounded-full text-xs">Active</span></td>
          <td class="p-3 text-gray-500">Jan 15, 2024</td>
          <td class="p-3 text-right">
            <button data-on:click="@get('/api/items/1/edit')" class="text-blue-600 hover:text-blue-800 text-sm mr-2">Edit</button>
            <button data-on:click="@post('/api/items/1/delete')" class="text-red-600 hover:text-red-800 text-sm">Delete</button>
          </td>
        </tr>
      </tbody>
    </table>
  </div>

</div>

4.9 Collaborative Editing & Presence

Pattern: A collaborative editing environment where multiple users interact with the same document simultaneously, with their presence visualized through real-time cursors, text selections, and activity indicators. Each user's cursor position (x, y coordinates) and selection bounds (left, top, width, height) are broadcast to all connected clients via SSE at a throttled rate (e.g., every 100ms during active editing). The client renders these as absolutely positioned overlay elements using data-style for dynamic positioning, with each user assigned a persistent color. An activity feed logs operations (joins, edits, saves) in chronological order. The local user's typing state is broadcast via a debounced input event to show "is typing..." indicators to others. The document content itself can be edited via contenteditable with changes synced through the same SSE channel. This pattern powers Google Docs-style editors, Figma-like design tools, live coding environments, whiteboards, and any shared workspace requiring awareness of remote participants.

<div class="relative h-screen flex flex-col"
     data-signals="{cursors: {}, selections: {}, users: {}, myColor: '#3B82F6', showPresence: true}">

  <!-- ═══════════════════════════════════════════════════════════════ -->
  <!--  PRESENCE BAR  —  Live user avatars + typing indicators        -->
  <!-- ═══════════════════════════════════════════════════════════════ -->
  <div class="h-12 border-b flex items-center justify-between px-4 bg-white">
    <div class="flex items-center gap-2">
      <span class="text-sm text-gray-500">Active now:</span>
      <div class="flex -space-x-2">
        <template data-for="[userId, user] in Object.entries($users)">
          <!-- DS:STYLE + DS:TEXT + DS:SHOW — Avatar color from signal; typing dot conditional -->
          <div class="w-8 h-8 rounded-full border-2 border-white flex items-center justify-center text-xs font-bold text-white relative"
               data-style:background-color="user.color"
               data-text="user.initials"
               title="user.name">
            <span data-show="user.isTyping" class="absolute -bottom-1 -right-1 w-3 h-3 bg-green-400 rounded-full border-2 border-white animate-pulse"></span>
          </div>
          <!-- /DS:STYLE + /DS:TEXT + /DS:SHOW -->
        </template>
      </div>
      <span class="text-xs text-gray-400" data-text="Object.keys($users).length + ' users'"></span>
    </div>
    <button data-on:click="$showPresence = !$showPresence"
            data-class="{'text-blue-600': $showPresence, 'text-gray-400': !$showPresence}"
            class="text-sm hover:text-gray-600 transition-colors">
      <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M15 12a3 3 0 11-6 0 3 3 0 016 0z"/><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M2.458 12C3.732 7.943 7.523 5 12 5c4.478 0 8.268 2.943 9.542 7-1.274 4.057-5.064 7-9.542 7-4.477 0-8.268-2.943-9.542-7z"/></svg>
    </button>
  </div>

  <!-- Editor Area -->
  <div class="flex-1 relative overflow-hidden bg-white">
    <div id="editor" class="h-full p-8 overflow-y-auto relative"
         contenteditable="true"
         data-on:input__throttle.100ms="@post('/api/doc/sync')"
         data-on:selectionchange__throttle.200ms="@post('/api/doc/cursor')">
      <h1 class="text-3xl font-bold mb-4 outline-none" placeholder="Document Title">Project Spec</h1>
      <div class="prose max-w-none outline-none" placeholder="Start typing...">
        <p>This is a collaborative document...</p>
      </div>
    </div>

    <!-- Remote Cursors -->
    <div data-show="$showPresence">
      <template data-for="[userId, cursor] in Object.entries($cursors)">
        <div class="absolute pointer-events-none transition-all duration-150 z-20"
             data-style:left="cursor.x + 'px'"
             data-style:top="cursor.y + 'px'">
          <svg class="w-4 h-5" viewBox="0 0 24 24" fill="none" data-style:color="cursor.color">
            <path d="M5.5 3.21V20.8c0 .45.54.67.85.35l4.86-4.86a.5.5 0 01.35-.15h6.87a.5.5 0 00.35-.85L6.35 2.85a.5.5 0 00-.85.35z" fill="currentColor" stroke="white" stroke-width="1.5"/>
          </svg>
          <div class="text-xs text-white px-1.5 py-0.5 rounded -mt-1 ml-3 whitespace-nowrap"
               data-style:background-color="cursor.color"
               data-text="cursor.name"></div>
        </div>
      </template>
    </div>

    <!-- Remote Selections -->
    <div data-show="$showPresence">
      <template data-for="[userId, sel] in Object.entries($selections)">
        <div class="absolute pointer-events-none z-10"
             data-style:left="sel.left + 'px'"
             data-style:top="sel.top + 'px'"
             data-style:width="sel.width + 'px'"
             data-style:height="sel.height + 'px'"
             data-style:background-color="sel.color + '30'"
             data-style:border-color="sel.color"
             class="border"></div>
      </template>
    </div>
  </div>

  <!-- Activity Feed -->
  <div class="h-48 border-t bg-gray-50 overflow-y-auto p-4">
    <h4 class="text-sm font-bold text-gray-500 mb-2 uppercase tracking-wide">Activity</h4>
    <div id="activity-feed" class="space-y-2 text-sm">
      <!-- Server-patched activity items -->
    </div>
  </div>

</div>

4.10 Tabbed Interfaces with Persistent State

Pattern: A tabbed interface where each tab functions as an independent workspace with its own form fields, validation state, and scroll position, all preserved when the user switches between tabs. Unlike simple tab systems that destroy hidden content, this pattern uses data-show to keep all tab DOM mounted but visually hidden, ensuring that unsaved form input, expanded sections, and scroll positions survive tab switches. Each tab maintains its state in a nested signal object ($tabState.{tabName}) including scroll position (captured before switching and restored via data-init on the content container), form data, and validation errors. Tab navigation buttons save the current tab's scroll position to its state object before activating the new tab. Error indicators (red dots) on tab headers show which tabs contain invalid fields. The save button submits the aggregated state of all tabs in a single request. This pattern is essential for complex settings pages, multi-part forms, product configurators, and any interface where users need to cross-reference information across sections without losing their place.

<div class="max-w-4xl mx-auto"
     data-signals="{
       activeTab: 'general',
       tabState: {
         general: {scroll: 0, data: {}, valid: false},
         security: {scroll: 0, data: {}, valid: false},
         notifications: {scroll: 0, data: {}, valid: false}
       },
       tabErrors: {}
     }">

  <!-- ═══════════════════════════════════════════════════════════════ -->
  <!--  TAB NAVIGATION  —  Saves scroll + switches active tab         -->
  <!-- ═══════════════════════════════════════════════════════════════ -->
  <div class="border-b mb-6">
    <nav class="flex gap-1">
      <!-- DS:ON + DS:CLASS — Click saves current scroll, switches tab, clears errors; class shows active state -->
      <button data-on:click="$tabState[$activeTab].scroll = document.getElementById('tab-content').scrollTop; $activeTab = 'general'; $tabErrors = {}"
              data-class="{'border-b-2 border-blue-600 text-blue-600': $activeTab === 'general', 'text-gray-500 hover:text-gray-700': $activeTab !== 'general'}"
              class="px-4 py-3 font-medium transition-colors relative">
        General
        <!-- DS:SHOW — Red dot appears when this tab has validation errors -->
        <span data-show="$tabErrors.general" class="absolute top-1 right-1 w-2 h-2 bg-red-500 rounded-full"></span>
        <!-- /DS:SHOW -->
      </button>
      <!-- /DS:ON + /DS:CLASS -->
      <button data-on:click="$tabState[$activeTab].scroll = document.getElementById('tab-content').scrollTop; $activeTab = 'security'; $tabErrors = {}"
              data-class="{'border-b-2 border-blue-600 text-blue-600': $activeTab === 'security', 'text-gray-500 hover:text-gray-700': $activeTab !== 'security'}"
              class="px-4 py-3 font-medium transition-colors relative">
        Security
        <span data-show="$tabErrors.security" class="absolute top-1 right-1 w-2 h-2 bg-red-500 rounded-full"></span>
      </button>
      <button data-on:click="$tabState[$activeTab].scroll = document.getElementById('tab-content').scrollTop; $activeTab = 'notifications'; $tabErrors = {}"
              data-class="{'border-b-2 border-blue-600 text-blue-600': $activeTab === 'notifications', 'text-gray-500 hover:text-gray-700': $activeTab !== 'notifications'}"
              class="px-4 py-3 font-medium transition-colors relative">
        Notifications
      </button>
    </nav>
  </div>

  <!-- Tab Content -->
  <div id="tab-content" class="h-[600px] overflow-y-auto"
       data-init="el.scrollTop = $tabState[$activeTab].scroll || 0">

    <!-- General Tab -->
    <div data-show="$activeTab === 'general'" class="space-y-6 p-2">
      <div class="grid grid-cols-2 gap-4">
        <div>
          <label class="block font-medium mb-1">Display Name</label>
          <input type="text" data-bind="tabState.general.data.displayName"
                 class="w-full border rounded-lg px-3 py-2 focus:ring-2 focus:ring-blue-500" />
        </div>
        <div>
          <label class="block font-medium mb-1">Username</label>
          <input type="text" data-bind="tabState.general.data.username"
                 class="w-full border rounded-lg px-3 py-2 focus:ring-2 focus:ring-blue-500" />
        </div>
      </div>
      <div>
        <label class="block font-medium mb-1">Bio</label>
        <textarea data-bind="tabState.general.data.bio" rows="4"
                  class="w-full border rounded-lg px-3 py-2 focus:ring-2 focus:ring-blue-500"></textarea>
        <p class="text-xs text-gray-500 mt-1" data-text="(160 - ($tabState.general.data.bio || '').length) + ' characters remaining'"></p>
      </div>
      <div>
        <label class="block font-medium mb-1">Profile Picture</label>
        <div class="flex items-center gap-4">
          <div class="w-16 h-16 bg-gray-200 rounded-full flex items-center justify-center text-gray-400">
            <svg class="w-8 h-8" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M16 7a4 4 0 11-8 0 4 4 0 018 0zM12 14a7 7 0 00-7 7h14a7 7 0 00-7-7z"/></svg>
          </div>
          <button class="px-4 py-2 border rounded-lg hover:bg-gray-50 text-sm">Upload New</button>
        </div>
      </div>
    </div>

    <!-- Security Tab -->
    <div data-show="$activeTab === 'security'" class="space-y-6 p-2">
      <div class="bg-yellow-50 border border-yellow-200 p-4 rounded-lg">
        <div class="flex items-start gap-3">
          <svg class="w-5 h-5 text-yellow-600 mt-0.5 flex-shrink-0" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M12 9v2m0 4h.01m-6.938 4h13.856c1.54 0 2.502-1.667 1.732-3L13.732 4c-.77-1.333-2.694-1.333-3.464 0L3.34 16c-.77 1.333.192 3 1.732 3z"/></svg>
          <div>
            <h4 class="font-bold text-yellow-800">Security Alerts</h4>
            <ul class="list-disc list-inside text-yellow-700 text-sm mt-1 space-y-1">
              <li>Two-factor authentication is not enabled</li>
              <li>Password was last changed 90 days ago</li>
            </ul>
          </div>
        </div>
      </div>
      <div class="space-y-4">
        <div>
          <label class="block font-medium mb-1">Current Password</label>
          <input type="password" data-bind="tabState.security.data.currentPassword"
                 class="w-full border rounded-lg px-3 py-2" />
        </div>
        <div>
          <label class="block font-medium mb-1">New Password</label>
          <input type="password" data-bind="tabState.security.data.newPassword"
                 class="w-full border rounded-lg px-3 py-2" />
          <div class="mt-2 flex gap-1">
            <div class="h-1 flex-1 rounded-full transition-colors"
                 data-class="{'bg-red-500': ($tabState.security.data.newPassword || '').length < 8, 'bg-yellow-500': ($tabState.security.data.newPassword || '').length >= 8 && ($tabState.security.data.newPassword || '').length < 12, 'bg-green-500': ($tabState.security.data.newPassword || '').length >= 12}"></div>
            <div class="h-1 flex-1 rounded-full transition-colors"
                 data-class="{'bg-gray-200': ($tabState.security.data.newPassword || '').length < 8, 'bg-yellow-500': ($tabState.security.data.newPassword || '').length >= 8 && ($tabState.security.data.newPassword || '').length < 12, 'bg-green-500': ($tabState.security.data.newPassword || '').length >= 12}"></div>
            <div class="h-1 flex-1 rounded-full transition-colors"
                 data-class="{'bg-gray-200': ($tabState.security.data.newPassword || '').length < 12, 'bg-green-500': ($tabState.security.data.newPassword || '').length >= 12}"></div>
          </div>
          <p class="text-xs text-gray-500 mt-1">Password strength indicator</p>
        </div>
        <div>
          <label class="block font-medium mb-1">Confirm New Password</label>
          <input type="password" data-bind="tabState.security.data.confirmPassword"
                 class="w-full border rounded-lg px-3 py-2"
                 data-class="{'border-red-500': $tabState.security.data.confirmPassword && $tabState.security.data.confirmPassword !== $tabState.security.data.newPassword}" />
          <p data-show="$tabState.security.data.confirmPassword && $tabState.security.data.confirmPassword !== $tabState.security.data.newPassword" 
             class="text-red-500 text-sm mt-1">Passwords do not match</p>
        </div>
      </div>
      <div class="border-t pt-4">
        <h4 class="font-medium mb-3">Two-Factor Authentication</h4>
        <div class="flex items-center justify-between p-3 border rounded-lg">
          <div class="flex items-center gap-3">
            <div class="w-10 h-10 bg-gray-100 rounded-lg flex items-center justify-center">
              <svg class="w-5 h-5 text-gray-600" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M12 15v2m-6 4h12a2 2 0 002-2v-6a2 2 0 00-2-2H6a2 2 0 00-2 2v6a2 2 0 002 2zm10-10V7a4 4 0 00-8 0v4h8z"/></svg>
            </div>
            <div>
              <p class="font-medium">Authenticator App</p>
              <p class="text-sm text-gray-500">Use Google Authenticator or similar</p>
            </div>
          </div>
          <button data-on:click="@get('/api/2fa/setup')" class="px-4 py-2 bg-blue-600 text-white rounded-lg text-sm hover:bg-blue-700">Enable</button>
        </div>
      </div>
    </div>

    <!-- Notifications Tab -->
    <div data-show="$activeTab === 'notifications'" class="space-y-4 p-2">
      <div class="flex items-center justify-between p-4 border rounded-lg hover:bg-gray-50 transition-colors">
        <div class="flex items-center gap-3">
          <div class="w-10 h-10 bg-blue-100 rounded-lg flex items-center justify-center">
            <svg class="w-5 h-5 text-blue-600" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M3 8l7.89 5.26a2 2 0 002.22 0L21 8M5 19h14a2 2 0 002-2V7a2 2 0 00-2-2H5a2 2 0 00-2 2v10a2 2 0 002 2z"/></svg>
          </div>
          <div>
            <p class="font-medium">Email Notifications</p>
            <p class="text-sm text-gray-500">Receive updates via email</p>
          </div>
        </div>
        <label class="relative inline-flex items-center cursor-pointer">
          <input type="checkbox" data-bind="tabState.notifications.data.emailEnabled" class="sr-only peer" />
          <div class="w-11 h-6 bg-gray-200 peer-focus:ring-4 peer-focus:ring-blue-300 rounded-full peer peer-checked:after:translate-x-full peer-checked:after:border-white after:content-[''] after:absolute after:top-[2px] after:left-[2px] after:bg-white after:border-gray-300 after:border after:rounded-full after:h-5 after:w-5 after:transition-all peer-checked:bg-blue-600"></div>
        </label>
      </div>

      <div data-show="$tabState.notifications.data.emailEnabled" class="ml-14 space-y-3">
        <div class="flex items-center justify-between p-3 border rounded-lg">
          <span class="text-sm">New mentions</span>
          <input type="checkbox" data-bind="tabState.notifications.data.mentions" class="rounded" />
        </div>
        <div class="flex items-center justify-between p-3 border rounded-lg">
          <span class="text-sm">Project updates</span>
          <input type="checkbox" data-bind="tabState.notifications.data.projects" class="rounded" />
        </div>
        <div class="flex items-center justify-between p-3 border rounded-lg">
          <span class="text-sm">Weekly digest</span>
          <input type="checkbox" data-bind="tabState.notifications.data.digest" class="rounded" />
        </div>
      </div>

      <div class="flex items-center justify-between p-4 border rounded-lg hover:bg-gray-50 transition-colors">
        <div class="flex items-center gap-3">
          <div class="w-10 h-10 bg-purple-100 rounded-lg flex items-center justify-center">
            <svg class="w-5 h-5 text-purple-600" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M15 17h5l-1.405-1.405A2.032 2.032 0 0118 14.158V11a6.002 6.002 0 00-4-5.659V5a2 2 0 10-4 0v.341C7.67 6.165 6 8.388 6 11v3.159c0 .538-.214 1.055-.595 1.436L4 17h5m6 0v1a3 3 0 11-6 0v-1m6 0H9"/></svg>
          </div>
          <div>
            <p class="font-medium">Push Notifications</p>
            <p class="text-sm text-gray-500">Browser push notifications</p>
          </div>
        </div>
        <label class="relative inline-flex items-center cursor-pointer">
          <input type="checkbox" data-bind="tabState.notifications.data.pushEnabled" class="sr-only peer" />
          <div class="w-11 h-6 bg-gray-200 peer-focus:ring-4 peer-focus:ring-blue-300 rounded-full peer peer-checked:after:translate-x-full peer-checked:after:border-white after:content-[''] after:absolute after:top-[2px] after:left-[2px] after:bg-white after:border-gray-300 after:border after:rounded-full after:h-5 after:w-5 after:transition-all peer-checked:bg-blue-600"></div>
        </label>
      </div>
    </div>

  </div>

  <!-- Save Button -->
  <div class="mt-6 flex justify-end gap-2">
    <button data-on:click="$tabState = {general: {scroll: 0, data: {}, valid: false}, security: {scroll: 0, data: {}, valid: false}, notifications: {scroll: 0, data: {}, valid: false}}"
            class="px-4 py-2 border rounded-lg hover:bg-gray-50 transition-colors">Reset</button>
    <button data-on:click="@post('/api/settings', $tabState)"
            class="px-6 py-2 bg-blue-600 text-white rounded-lg hover:bg-blue-700 transition-colors">
      Save All Changes
    </button>
  </div>

</div>

4.11 Authentication: Login Page, OAuth Sessions & Logout

Auth is where the server-first rule (§6) is non-negotiable. The single most important design decision:

The OAuth token NEVER touches Datastar signals, localStorage, or any client-readable storage. Tokens live server-side (in PostgreSQL); the browser holds only an opaque, HttpOnly, Secure session cookie. Because cookies ride along automatically, every @get/@post/SSE stream after login is authenticated with zero extra client code — that is how you "use the token everywhere".

┌──────────┐  1. GET /login           ┌──────────┐  3. redirect to provider    ┌───────────────┐
│ Browser  │─────────────────────────▶│  Server  │────────────────────────────▶│ OAuth Provider│
│          │  2. click "Google"       │          │  4. /auth/callback?code=…   │ (Google etc.) │
│ cookie:  │◀─────────────────────────│  stores  │◀────────────────────────────│               │
│ sid=opaque  5. Set-Cookie (HttpOnly)│ tokens   │   exchange code → tokens    └───────────────┘
└──────────┘                          │ in PG    │
     │ 6. every @get / @post / SSE    └──────────┘
     │    carries sid automatically → middleware loads session → user attached

Sessions table (PostgreSQL — ties into §7.7/§7.8)

CREATE TABLE sessions (
    sid              UUID PRIMARY KEY DEFAULT uuidv7(),  -- opaque cookie value
    user_id          BIGINT NOT NULL REFERENCES users(id),
    provider         VARCHAR(32) NOT NULL,               -- 'google', 'github', 'password'
    access_token     TEXT,                               -- encrypt at rest (pgcrypto / app-level)
    refresh_token    TEXT,
    token_expires_at TIMESTAMPTZ,
    revoked_at       TIMESTAMPTZ,
    created_at       TIMESTAMPTZ NOT NULL DEFAULT NOW(),
    last_seen_at     TIMESTAMPTZ NOT NULL DEFAULT NOW()
);

-- Hot path: cookie validated on EVERY request → partial index on live sessions only (§7.8)
CREATE INDEX idx_sessions_live ON sessions (sid)
    INCLUDE (user_id, token_expires_at)
    WHERE revoked_at IS NULL;

-- "Sign out everywhere" support
CREATE INDEX idx_sessions_by_user ON sessions (user_id) WHERE revoked_at IS NULL;

The login page (Datastar)

<body data-signals="{
        form: {email: '', password: ''},
        meta: {isSubmitting: false, error: null}
      }">
  <div class="min-h-screen flex items-center justify-center bg-gray-50">
    <div class="w-full max-w-sm bg-white rounded-xl shadow p-8 space-y-5">
      <h1 class="text-xl font-semibold text-center">Sign in</h1>

      <!-- Error from server (patched into $meta.error) -->
      <div data-show="$meta.error"
           class="bg-red-50 text-red-700 text-sm p-3 rounded-lg"
           data-text="$meta.error"></div>

      <!-- Password login: capture → submit ONCE → server decides (§6.9 single-flight) -->
      <input data-bind="form.email" type="email" placeholder="Email"
             class="w-full border rounded-lg px-3 py-2"
             data-on:keydown="if(evt.key==='Enter') $loginBtn.click()" />
      <input data-bind="form.password" type="password" placeholder="Password"
             class="w-full border rounded-lg px-3 py-2"
             data-on:keydown="if(evt.key==='Enter') $loginBtn.click()" />

      <button data-ref="loginBtn"
              data-on:click="$meta.isSubmitting = true; $meta.error = null; @post('/auth/login')"
              data-attr:disabled="$meta.isSubmitting || !$form.email || !$form.password"
              class="w-full bg-blue-600 text-white rounded-lg py-2 disabled:opacity-50">
        <span data-show="!$meta.isSubmitting">Sign in</span>
        <span data-show="$meta.isSubmitting">Signing in…</span>
      </button>

      <div class="text-center text-xs text-gray-400">or</div>

      <!-- OAuth: a plain navigation, NOT an @get — the browser must follow
           cross-origin redirects to the provider's consent screen -->
      <a href="/auth/oauth/google"
         class="w-full flex items-center justify-center gap-2 border rounded-lg py-2 hover:bg-gray-50">
        Continue with Google
      </a>
    </div>
  </div>
</body>

Note the discipline: the password lives in a form.* draft signal, there's one single-flight POST, and the server owns the verdict. The OAuth button is a real <a> link — the code/PKCE dance is invisible to Datastar.

Server: password login, OAuth redirect, callback (Node.js)

import crypto from 'node:crypto';

// ---- Password login: validate, create session, redirect via SSE ----
export async function POST_authLogin(req, res) {
    const { form } = await readSignals(req);              // {email, password}
    const user = await verifyPassword(form.email, form.password);

    res.writeHead(200, { 'Content-Type': 'text/event-stream' });

    if (!user) {
        res.write(`event: datastar-patch-signals\n` +
                  `data: signals {"meta": {"isSubmitting": false, "error": "Invalid email or password"}, "form": {"password": ""}}\n\n`);
        return res.end();
    }

    const sid = await createSession({ userId: user.id, provider: 'password' });
    setSessionCookie(res, sid);                            // HttpOnly; Secure; SameSite=Lax
    // Patch a script element — it executes on insert and navigates
    res.write(`event: datastar-patch-elements\n` +
              `data: mode append\n` +
              `data: selector body\n` +
              `data: elements <script id="nav">window.location = "/dashboard"</script>\n\n`);
    res.end();
}

// ---- OAuth: redirect to provider (Authorization Code + PKCE) ----
export async function GET_oauthGoogle(req, res) {
    const state = crypto.randomBytes(16).toString('hex');
    const verifier = crypto.randomBytes(32).toString('base64url');
    await stashPkce(state, verifier);                      // short-lived row or signed cookie

    const challenge = crypto.createHash('sha256').update(verifier).digest('base64url');
    const url = new URL('https://accounts.google.com/o/oauth2/v2/auth');
    url.search = new URLSearchParams({
        client_id: process.env.GOOGLE_CLIENT_ID,
        redirect_uri: `${process.env.BASE_URL}/auth/callback`,
        response_type: 'code',
        scope: 'openid email profile',
        state,
        code_challenge: challenge,
        code_challenge_method: 'S256'
    });
    res.writeHead(302, { Location: url.toString() }).end();
}

// ---- Callback: exchange code → store tokens in PG → set cookie ----
export async function GET_authCallback(req, res) {
    const { code, state } = Object.fromEntries(new URL(req.url, process.env.BASE_URL).searchParams);
    const verifier = await consumePkce(state);             // rejects replay / CSRF

    const tokens  = await exchangeCodeForTokens(code, verifier); // POST to provider's token endpoint
    const profile = await fetchUserInfo(tokens.access_token);
    const user    = await upsertUser(profile);             // find-or-create by provider + sub

    const { rows: [s] } = await pool.query(`
        INSERT INTO sessions (user_id, provider, access_token, refresh_token, token_expires_at)
        VALUES ($1, 'google', $2, $3, NOW() + make_interval(secs => $4))
        RETURNING sid`,
        [user.id, encrypt(tokens.access_token), encrypt(tokens.refresh_token), tokens.expires_in]);

    setSessionCookie(res, s.sid);
    res.writeHead(302, { Location: '/dashboard' }).end();  // plain redirect — full page load
}

function setSessionCookie(res, sid) {
    res.setHeader('Set-Cookie',
        `sid=${sid}; Path=/; HttpOnly; Secure; SameSite=Lax; Max-Age=${60 * 60 * 24 * 30}`);
}

"Using the token everywhere": one middleware, zero client code

Because the cookie is attached by the browser to every request — including Datastar's @get/@post and the long-lived SSE stream — a single middleware authenticates everything:

export async function requireAuth(req, res, next) {
    const sid = parseCookies(req).sid;
    const { rows: [session] } = await pool.query(`
        SELECT s.*, u.name, u.email, u.role
        FROM sessions s JOIN users u ON u.id = s.user_id
        WHERE s.sid = $1 AND s.revoked_at IS NULL`, [sid]); // hits idx_sessions_live

    if (!session) {
        if (req.headers['datastar-request']) {
            // Datastar request → patch a redirect instead of a useless 401 body
            res.writeHead(200, { 'Content-Type': 'text/event-stream' });
            res.write(`event: datastar-patch-elements\ndata: mode append\ndata: selector body\n` +
                      `data: elements <script id="nav">window.location = "/login"</script>\n\n`);
            return res.end();
        }
        return res.writeHead(302, { Location: '/login' }).end();
    }

    // Refresh the OAuth access token server-side when it's about to expire —
    // the client never knows or cares
    if (session.token_expires_at && session.token_expires_at < new Date(Date.now() + 60_000)) {
        const fresh = await refreshAccessToken(decrypt(session.refresh_token));
        await pool.query(`
            UPDATE sessions
            SET access_token = $2, token_expires_at = NOW() + make_interval(secs => $3)
            WHERE sid = $1`,
            [sid, encrypt(fresh.access_token), fresh.expires_in]);
        session.access_token = encrypt(fresh.access_token);
    }

    req.user = { id: session.user_id, name: session.name, role: session.role };
    req.oauthToken = () => decrypt(session.access_token);  // for provider API calls, server-side only
    next();
}

// Now EVERY handler is authenticated, including SSE streams (§7):
app.get('/api/events/stream', requireAuth, streamEvents);  // channel = `user_${req.user.id}`
app.post('/api/orders',       requireAuth, createOrder);
app.get('/api/drive/files',   requireAuth, async (req, res) => {
    // Need the provider's API? Use the stored token — server-to-server only.
    const files = await fetch('https://www.googleapis.com/drive/v3/files', {
        headers: { Authorization: `Bearer ${req.oauthToken()}` }
    }).then(r => r.json());
    // …render rows and patch #file-list back to the client
});

After login, the server renders read-only identity into server.* signals (§6.4) for display purposes only — name, avatar, role flag; never the token:

<!-- Rendered server-side into the authenticated layout -->
<body data-signals="{server: {user: {name: 'Priya', avatar: '/a/12.png', role: 'admin'}}, ui: {loggingOut: false}}">
  <header class="flex items-center gap-3 p-4 border-b">
    <img data-attr:src="$server.user.avatar" class="w-8 h-8 rounded-full" />
    <span data-text="$server.user.name" class="font-medium"></span>
    <!-- data-show is cosmetic only; the server re-checks the role on every request -->
    <a href="/admin" data-show="$server.user.role === 'admin'" class="text-sm text-blue-600">Admin</a>

    <button data-on:click="$ui.loggingOut = true; @post('/auth/logout')"
            data-attr:disabled="$ui.loggingOut"
            class="ml-auto text-sm text-gray-500 hover:text-gray-900">
      <span data-show="!$ui.loggingOut">Log out</span>
      <span data-show="$ui.loggingOut">Logging out…</span>
    </button>
  </header>
</body>

Logout: revoke server-side, kill live streams via NOTIFY

Logout is a server mutation like any other — and it composes with §7: revoking the session NOTIFYs so any open SSE streams for that session shut down immediately (other tabs, other devices on "sign out everywhere").

export async function POST_authLogout(req, res) {
    const sid = parseCookies(req).sid;

    await pool.query(`
        WITH dead AS (
            UPDATE sessions SET revoked_at = NOW()
            WHERE sid = $1 AND revoked_at IS NULL
            RETURNING sid
        )
        SELECT pg_notify('session_revoked', sid::text) FROM dead`, [sid]);

    res.setHeader('Set-Cookie', 'sid=; Path=/; HttpOnly; Secure; SameSite=Lax; Max-Age=0');
    res.writeHead(200, { 'Content-Type': 'text/event-stream' });
    res.write(`event: datastar-patch-elements\ndata: mode append\ndata: selector body\n` +
              `data: elements <script id="nav">window.location = "/login"</script>\n\n`);
    res.end();
}

// In the SSE layer: one listener closes streams the moment a session dies
sessionListener.query(`LISTEN session_revoked`);
sessionListener.on('notification', (msg) => {
    const controllers = streamsBySession.get(msg.payload);  // Map<sid, Set<controller>>
    controllers?.forEach(c => c.close());                   // open tabs hit middleware on reconnect → /login
    streamsBySession.delete(msg.payload);
});

For "sign out of all devices", revoke by user_id instead (served by idx_sessions_by_user) and pg_notify each returned sid.

Auth anti-patterns checklist

  • ❌ data-signals="{token: '…'}" — a token in a signal is visible in the debug panel, serialized into every request payload, and readable by any XSS. Never.
  • ❌ localStorage.setItem('jwt', …) — same exposure, and it outlives the session.
  • ❌ Client-side route guards as the only protection (data-show="$user.role === 'admin'") — cosmetic only; middleware re-checks every request.
  • ❌ Refreshing OAuth tokens from the browser — refresh happens in middleware; the client never sees expiry.
  • ✅ HttpOnly; Secure; SameSite=Lax cookie + server-side session row + partial index on live sessions
  • ✅ Logout = revoke row + clear cookie + pg_notify to close live SSE streams instantly
  • ✅ CSRF: SameSite=Lax covers Datastar's same-origin requests; add a CSRF token header if the app is ever embedded cross-site

4.12 File Explorer / Tree View (lazy, server-rendered)

Design thesis. Never ship the whole tree — a real filesystem or org chart can have millions of nodes. The client holds only chrome: which nodes are expanded, which is selected, which have already loaded. The server renders each level on demand when a folder first opens. Re-opening a loaded folder is instant (the DOM is cached — just toggle visibility); the first open lazy-fetches. This is the §6 server-first rule applied to hierarchy: structure is domain data, expansion is presentation.

<!-- Tree state is pure UI chrome (§6.9): open nodes, selection, and which nodes
     have loaded their children. NO file data lives in signals — the server renders
     each level on demand, so this scales to arbitrarily deep / wide trees. -->
<div data-signals="{tree: {open: {}, loaded: {}, loading: '', selected: ''}}"
     class="w-72 bg-white border-r h-dvh overflow-y-auto text-sm select-none">

  <!-- Root level is fetched once on load; children follow the recursive node pattern -->
  <ul id="tree-root" data-init="@get('/api/tree?path=/')"></ul>
</div>

A folder node, rendered by the server (the literal node id n42 and depth padding are server-inlined):

<li>
  <!-- Row click toggles open state. The FIRST open lazy-loads children and shows a
       spinner; subsequent opens just toggle visibility — no re-fetch. -->
  <div data-on:click="
         $tree.open.n42 = !$tree.open.n42;
         if ($tree.open.n42 && !$tree.loaded.n42) {   /* fetch children once */
           $tree.loading = 'n42';
           @get('/api/tree?path=/src/components');     /* server clears loading + marks loaded */
         }"
       data-class="{'bg-blue-50 text-blue-700': $tree.selected === 'n42'}"
       class="flex items-center gap-1 py-1 px-2 rounded cursor-pointer hover:bg-gray-100"
       style="padding-left: 1.5rem">              <!-- depth × 0.75rem, server-computed -->

    <!-- Chevron rotates 90° on open — CSS transition driven by the signal -->
    <svg data-class="{'rotate-90': $tree.open.n42}"
         class="w-3 h-3 text-gray-400 transition-transform shrink-0" viewBox="0 0 12 12">
      <path d="M4 2l4 4-4 4" fill="none" stroke="currentColor" stroke-width="2"/>
    </svg>
    <svg class="w-4 h-4 text-amber-500 shrink-0"><!-- folder icon --></svg>
    <span class="truncate">components</span>
  </div>

  <!-- Children container: hidden until open. Server patches child <li>s into here on
       first load; they persist in the DOM so reopening is instant. -->
  <ul id="children-n42" data-show="$tree.open.n42" class="overflow-hidden">
    <li data-show="$tree.loading === 'n42'" class="py-1 text-gray-400"
        style="padding-left: 2.25rem">Loading…</li>
  </ul>
</li>

A leaf (file) node selects itself and drives a detail pane (master-detail, §4.1):

<li>
  <div data-on:click="$tree.selected = 'f88'; @get('/api/file?id=88')"  <!-- one action: load preview -->
       data-class="{'bg-blue-50 text-blue-700': $tree.selected === 'f88'}"
       class="flex items-center gap-1 py-1 px-2 rounded cursor-pointer hover:bg-gray-100"
       style="padding-left: 2.25rem">
    <svg class="w-4 h-4 text-gray-400 shrink-0"><!-- file icon --></svg>
    <span class="truncate">Button.tsx</span>
  </div>
</li>

Server response on first folder open — patch children, clear the spinner, mark loaded:

event: datastar-patch-elements
data: selector #children-n42
data: mode append
data: elements <li>…child nodes…</li>

event: datastar-patch-signals
data: signals {"tree": {"loading": "", "loaded": {"n42": true}}}

Scale notes. Only visible/expanded subtrees ever exist in the DOM or cross the wire. For huge expanded folders, have the server paginate children (a "load 200 more" sentinel using data-on-intersect, §4.7). Keyboard nav (↑/↓ to move, →/← to expand/collapse) is pure chrome: a data-on:keydown__window handler that moves $tree.selected between server-rendered data-key attributes.

4.13 Gantt Chart (server-computed geometry, optimistic drag)

Design thesis. Bar position is derived from domain data (each task's start/end against the visible window) — so the server computes every bar's left%/width% and renders them (§6.2: domain-derived layout is a server concern). The client owns only three things: the zoom level (a view param), hover tooltips, and drag-to-reschedule intent — an optimistic visual nudge confirmed and snapped by the server (§6.5). A small but powerful trick keeps the date math sane: the server hands the client one number, pxPerDay, so a pixel drag converts to a whole-day delta.

<!-- gantt.drag = the in-progress reschedule (id + pixel delta). gantt.pxPerDay and the
     bar left/width are SERVER-COMPUTED from dates — the client never lays out the timeline. -->
<div data-signals="{gantt: {scale: 'week', pxPerDay: 24, drag: {id: 0, startX: 0, dx: 0}}}"
     class="bg-white border rounded-xl overflow-hidden">

  <!-- ░ Zoom control: a view param. Changing it re-fetches the whole chart with new geometry ░ -->
  <div class="flex gap-1 p-2 border-b text-sm">
    <button data-on:click="$gantt.scale='day';   @get('/api/gantt')"
            data-class="{'bg-blue-600 text-white': $gantt.scale==='day'}"   class="px-2 py-1 rounded">Day</button>
    <button data-on:click="$gantt.scale='week';  @get('/api/gantt')"
            data-class="{'bg-blue-600 text-white': $gantt.scale==='week'}"  class="px-2 py-1 rounded">Week</button>
    <button data-on:click="$gantt.scale='month'; @get('/api/gantt')"
            data-class="{'bg-blue-600 text-white': $gantt.scale==='month'}" class="px-2 py-1 rounded">Month</button>
  </div>

  <!-- Server patches the timeline header (date columns, weekend shading, today line) here -->
  <div id="gantt-header" class="relative h-8 border-b bg-gray-50"></div>

  <!-- Task rows + bars are server-rendered into #gantt-body. One row template below. -->
  <div id="gantt-body" class="relative"></div>
</div>

A task bar, server-rendered with its computed position; the client adds drag:

<!-- Bar position (left/width) is server-computed from the task's dates. The pointer
     handlers implement optimistic drag; the server validates + snaps on drop. -->
<div class="relative h-9 border-b">
  <div data-on:pointerdown="$gantt.drag = {id: 7, startX: evt.clientX, dx: 0};
                            el.setPointerCapture(evt.pointerId)"
       data-on:pointermove="$gantt.drag.id === 7 && ($gantt.drag.dx = evt.clientX - $gantt.drag.startX)"
       data-on:pointerup="
         if ($gantt.drag.id === 7) {
           /* px delta → whole-day delta via the server-provided scale */
           const days = Math.round($gantt.drag.dx / $gantt.pxPerDay);
           $gantt.drag = {id: 0, startX: 0, dx: 0};
           if (days !== 0) { $gantt.moveDays = days; @patch('/api/task/7/move'); }
         }"
       data-style:transform="$gantt.drag.id === 7 ? 'translateX(' + $gantt.drag.dx + 'px)' : ''"
       data-class="{'opacity-60 ring-2 ring-blue-400 z-10': $gantt.drag.id === 7}"
       class="absolute top-1.5 h-6 rounded bg-blue-500 text-white text-xs px-2
              flex items-center cursor-grab active:cursor-grabbing transition-shadow"
       style="left: 22%; width: 14%">      <!-- ← server-computed from task dates -->
    Design phase
  </div>
</div>

Server response on drop — validate against dependencies/bounds, snap to the grid, re-render the row authoritatively (the optimistic translateX is replaced by the real left):

event: datastar-patch-elements
data: selector #task-row-7
data: mode outer
data: elements <div id="task-row-7" class="relative h-9 border-b">…bar at new left%…</div>

event: datastar-patch-signals
data: signals {"gantt": {"moveDays": 0}}

Why this split. If the client computed bar positions, two clients with different clocks/timezones would disagree, and a dependency-violating move would render before being rejected. Letting the server own geometry means every client sees the same authoritative timeline, and an illegal drag simply snaps back. Dependency arrows are server-rendered <svg> paths over the body; redraw them in the same patch as the moved row.

4.14 Multi-Level Dropdown (hover-intent, edge-flip, keyboard)

Design thesis. Opening and closing submenus is pure presentation — it must never touch the server (§6.9). A single ui.menuPath array represents the open chain (['file','export']), so exactly one branch is open at each depth and closing a parent closes its descendants for free. Only a leaf item's action hits the server. Three details separate a good menu from a frustrating one: hover-intent (open on a brief rest, not instantly, to survive diagonal mouse paths), edge-flip (submenus open leftward near the viewport edge), and Escape to collapse.

<!-- Menu open/close is pure chrome — zero server traffic. ui.menuPath is the open
     chain; reassigned wholesale (reactive-safe). Only leaf actions call the server. -->
<nav data-signals="{ui: {menuPath: []}}"
     data-on:keydown__window="if (evt.key === 'Escape') $ui.menuPath = []"
     class="relative flex gap-1 text-sm">

  <!-- ░░ Top-level: "File" ░░ -->
  <div class="relative" data-on:mouseleave="$ui.menuPath = []">  <!-- leaving the branch closes it -->
    <button data-on:click="$ui.menuPath = $ui.menuPath[0] === 'file' ? [] : ['file']"
            data-class="{'bg-gray-100': $ui.menuPath[0] === 'file'}"
            class="px-3 py-1.5 rounded hover:bg-gray-100">File</button>

    <!-- Level 1 submenu -->
    <div data-show="$ui.menuPath[0] === 'file'"
         class="absolute left-0 top-full mt-1 w-48 bg-white border rounded-lg shadow-lg py-1 z-50">

      <!-- Leaf → one server action, then close -->
      <button data-on:click="$ui.menuPath = []; @post('/api/file/new')"
              class="w-full text-left px-3 py-1.5 hover:bg-gray-100">New File</button>

      <!-- ░ Nested "Export ▸": opens level 2 on HOVER-INTENT (120ms rest via __debounce) ░ -->
      <div class="relative"
           data-on:mouseenter__debounce.120ms="$ui.menuPath = ['file', 'export']">
        <button data-class="{'bg-gray-100': $ui.menuPath[1] === 'export'}"
                class="w-full flex items-center justify-between px-3 py-1.5 hover:bg-gray-100">
          Export <span class="text-gray-400">▸</span>
        </button>

        <!-- Level 2: flies right, or flips LEFT when too close to the viewport edge -->
        <div data-show="$ui.menuPath[1] === 'export'"
             data-class="{
               'left-full ml-1':  window.innerWidth - el.parentElement.getBoundingClientRect().right > 200,
               'right-full mr-1': window.innerWidth - el.parentElement.getBoundingClientRect().right <= 200
             }"
             class="absolute top-0 w-44 bg-white border rounded-lg shadow-lg py-1 z-50">
          <button data-on:click="$ui.menuPath = []; @post('/api/export/pdf')"
                  class="w-full text-left px-3 py-1.5 hover:bg-gray-100">as PDF</button>
          <button data-on:click="$ui.menuPath = []; @post('/api/export/png')"
                  class="w-full text-left px-3 py-1.5 hover:bg-gray-100">as PNG</button>
        </div>
      </div>
    </div>
  </div>
</nav>

The two tricks worth stealing. data-on:mouseenter__debounce.120ms is hover-intent — the submenu opens only after the pointer rests, so brushing past items doesn't flash menus open. And because the whole branch lives inside one .relative wrapper, mouseleave on the wrapper fires only when the pointer leaves the entire branch (moving into a submenu stays "inside"), giving correct close behavior with a single handler. For deeper trees this generalizes to $ui.menuPath[depth] === id; render it recursively server-side.

4.15 Accordion / Collapsible Panels (height-auto animation + lazy content)

Design thesis. The hard part of an accordion is animating to height: auto — max-height hacks are janky and JS height-measuring is imperative. The clean declarative fix is the CSS grid 0fr → 1fr transition, which animates intrinsic height smoothly. Combine that with lazy content loading (a 50-item FAQ shouldn't ship every answer up front) and you get an accordion that's both buttery and cheap: the panel body is empty until first open, then the server patches it in.

<!-- Animation: grid-template-rows 0fr↔1fr smoothly animates height:auto (max-height
     can't). Content lazy-loads on first open. ui.open = single open id (swap for a
     map to allow multiple open at once — see note). -->
<div data-signals="{ui: {open: '', loaded: {}}}" class="divide-y border rounded-xl bg-white">

  <div>
    <!-- Header: single-open accordion (opening one closes the rest). First open fetches body. -->
    <button data-on:click="
              $ui.open = $ui.open === 'p1' ? '' : 'p1';
              if ($ui.open === 'p1' && !$ui.loaded.p1) { $ui.loaded.p1 = true; @get('/api/faq/p1'); }"
            data-attr:aria-expanded="$ui.open === 'p1'"
            class="w-full flex items-center justify-between px-4 py-3 text-left font-medium">
      What is your refund policy?
      <svg data-class="{'rotate-180': $ui.open === 'p1'}"
           class="w-4 h-4 text-gray-400 transition-transform shrink-0" viewBox="0 0 24 24">
        <path d="M6 9l6 6 6-6" fill="none" stroke="currentColor" stroke-width="2"/>
      </svg>
    </button>

    <!-- Animated wrapper: the grid row transitions between 0fr and 1fr -->
    <div data-class="{'grid-rows-[1fr]': $ui.open === 'p1', 'grid-rows-[0fr]': $ui.open !== 'p1'}"
         class="grid transition-[grid-template-rows] duration-300 ease-out">
      <!-- overflow-hidden is REQUIRED so content is clipped as the row collapses -->
      <div class="overflow-hidden">
        <div id="faq-p1" class="px-4 pb-4 text-sm text-gray-600">
          <!-- skeleton until the server patches the real answer on first open -->
          <div data-show="!$ui.loaded.p1" class="h-4 bg-gray-100 rounded animate-pulse"></div>
        </div>
      </div>
    </div>
  </div>
  <!-- …more panels (p2, p3 …), same template… -->
</div>

Server response on first open:

event: datastar-patch-elements
data: selector #faq-p1
data: mode inner
data: elements <p>Refunds are available within 30 days…</p>

Multi-open variant. Replace the single ui.open string with a map and test membership: header toggles $ui.openMap.p1 = !$ui.openMap.p1; the wrapper uses data-class="{'grid-rows-[1fr]': $ui.openMap.p1, …}". Everything else — animation, lazy load — is identical. The grid-fr technique needs no fixed heights, so panels with images or dynamic content animate correctly regardless of size.

4.16 Date Range Picker (with availability logic)

Design thesis. Two responsibilities, cleanly split. Availability is domain data, so the server renders the calendar grid with blackout/unavailable days already disabled (§6.2) and validates the final range (max nights, no blackout inside it). Selection interaction is chrome, so the client tracks start/end/hover and the live range highlight. The elegant glue: ISO date strings compare lexicographically ('2026-06-08' < '2026-06-14' is true), so every range comparison is a plain string compare — no Date parsing in expressions. Each server-rendered cell carries its own date literal and highlights itself.

<!-- Calendar grid is SERVER-RENDERED (availability is authoritative). Client overlays
     selection + hover using ISO-string comparison. picker.start/end/hover are 'YYYY-MM-DD'. -->
<div data-signals="{picker: {start: '', end: '', hover: '', open: false, error: ''}}"
     class="relative inline-block text-sm">

  <!-- Trigger shows the chosen range -->
  <button data-on:click="$picker.open = !$picker.open"
          class="border rounded-lg px-3 py-2 min-w-64 text-left">
    <span data-show="!$picker.start" class="text-gray-400">Select dates…</span>
    <span data-show="$picker.start" data-text="$picker.start + '  →  ' + ($picker.end || '…')"></span>
  </button>

  <div data-show="$picker.open"
       class="absolute mt-1 bg-white border rounded-xl shadow-xl p-4 z-50 flex gap-4">

    <!-- ░ Presets set both ends at once (concrete dates inlined by the server on render) ░ -->
    <div class="flex flex-col gap-1 pr-4 border-r">
      <button data-on:click="$picker.start='2026-06-14'; $picker.end='2026-06-14'; @get('/api/availability')"
              class="text-left px-2 py-1 rounded hover:bg-gray-100">Today</button>
      <button data-on:click="$picker.start='2026-06-08'; $picker.end='2026-06-14'; @get('/api/availability')"
              class="text-left px-2 py-1 rounded hover:bg-gray-100">Last 7 days</button>
      <button data-on:click="$picker.start='2026-06-01'; $picker.end='2026-06-30'; @get('/api/availability')"
              class="text-left px-2 py-1 rounded hover:bg-gray-100">This month</button>
    </div>

    <!-- ░ Two-month grid: server-rendered; month nav re-fetches (keeps availability fresh) ░ -->
    <div>
      <div class="flex items-center justify-between mb-2">
        <button data-on:click="@get('/api/calendar?dir=prev')" class="px-2">‹</button>
        <span class="font-medium">June – July 2026</span>
        <button data-on:click="@get('/api/calendar?dir=next')" class="px-2">›</button>
      </div>
      <div id="cal-grid" class="grid grid-cols-7 gap-1"><!-- server renders day cells --></div>
      <!-- Server validation feedback (e.g. "max 30 nights", "contains a blackout date") -->
      <p data-show="$picker.error" data-text="$picker.error" class="text-red-600 text-xs mt-2"></p>
    </div>
  </div>
</div>

A day cell, server-rendered with its date literal baked into every expression — so each cell highlights itself with no per-cell signals. Unavailable days get a real disabled attribute (the server owns that):

<button data-on:mouseenter="$picker.hover = '2026-06-12'"
        data-on:click="
          /* selection logic, enforced client-side as a UI affordance:
             1st click → set start, clear end; 2nd → set end (swap if reversed) then
             validate server-side; a 3rd click (range complete) starts over. */
          if (!$picker.start || ($picker.start && $picker.end)) {
            $picker.start = '2026-06-12'; $picker.end = ''; $picker.error = '';
          } else if ('2026-06-12' < $picker.start) {
            $picker.end = $picker.start; $picker.start = '2026-06-12'; @get('/api/availability');
          } else {
            $picker.end = '2026-06-12'; @get('/api/availability');
          }"
        data-class="{
          'bg-blue-600 text-white':
              $picker.start === '2026-06-12' || $picker.end === '2026-06-12',          /* endpoints */
          'bg-blue-100':
              $picker.start && $picker.end &&
              '2026-06-12' > $picker.start && '2026-06-12' < $picker.end,              /* committed range */
          'bg-blue-50':                                                                /* live hover preview */
              $picker.start && !$picker.end && (
                ('2026-06-12' > $picker.start && '2026-06-12' <= $picker.hover) ||
                ('2026-06-12' < $picker.start && '2026-06-12' >= $picker.hover))
        }"
        class="h-9 w-9 rounded-full hover:bg-gray-100 disabled:opacity-30 disabled:line-through disabled:hover:bg-transparent"
        >12</button>

Server validation response — accept and summarize, or reject with a reason and clamp:

event: datastar-patch-signals
data: signals {"picker": {"error": "Range can't exceed 30 nights", "end": ""}}

Why server-validate when the client already ordered the dates? Ordering (start ≤ end) is an affordance — cheap to do client-side for instant feedback. But "is every night in this range actually bookable?" and "does it exceed the plan's max?" are business rules that depend on live, server-owned data; a client check would be both stale and trivially bypassable (§6.2). So the client makes the interaction feel instant, and the server has the final say — re-rendering disabled cells and clamping an invalid range. ISO-string comparison means the highlight logic stays declarative and timezone-proof.

4.17 Tab System with Drag-Reorder, Overflow & URL Sync

Design thesis. Tabs look trivial until five concerns collide. Each lands on a different side of the §6 boundary, and naming that split is the design:

Concern Owner Why
Which tab is active Client chrome → URL (§6.10) Navigation pointer; bookmarkable as ?tab=…
Tab order (if persisted) Server A saved user preference is durable data — drag is optimistic, server confirms
Panel content Server, lazy Loaded on first activation, cached after — never ship all panels
Overflow scroll / drag visuals Client chrome Pure presentation; never hits the server
Disabled tabs Server-declared The server renders disabled + an enabled id list the client uses to skip them

The neat trick that makes keyboard nav clean: the server hands the client a flat tabs.enabled array (disabled ids omitted), so "next/prev tab" is just index math over that array — disabled tabs are skipped for free.

<!-- tabs.active → URL-synced (§6.10). tabs.enabled → server-rendered list of NON-disabled
     ids (powers keyboard skip). tabs.loaded → lazy-load cache. drag/over → DnD chrome.
     canLeft/canRight → overflow arrow visibility. -->
<div data-signals="{tabs: {active: '', enabled: [], loaded: {},
                           dragFrom: -1, overTo: -1, reorder: {},
                           canLeft: false, canRight: false, overflowOpen: false}}"
     data-init="
       /* hydrate active tab FROM the url on load, then lazy-load its panel (§6.10) */
       $tabs.active = new URLSearchParams(location.search).get('tab') || 'home';
       $tabs.loaded[$tabs.active] = true; @get('/api/tab/' + $tabs.active)"
     class="bg-white border rounded-xl">

  <!-- ░░ Tab strip row: ◂ scroll | scrollable tablist | ▸ scroll | ▾ overflow menu ░░ -->
  <div class="flex items-center border-b">

    <!-- Left scroll arrow: only shown when the strip is scrolled off the left edge -->
    <button data-show="$tabs.canLeft"
            data-on:click="$strip.scrollBy({left: -200, behavior: 'smooth'})"
            class="px-2 py-2 text-gray-400 hover:text-gray-700 shrink-0">‹</button>

    <!-- The scrollable tablist. role=tablist + keyboard handler = ARIA tabs pattern.
         Arrow keys walk tabs.enabled, so DISABLED tabs are skipped automatically. -->
    <div data-ref="strip" role="tablist"
         data-on:keydown="
           const e = $tabs.enabled, i = e.indexOf($tabs.active);
           if (evt.key === 'ArrowRight') { evt.preventDefault(); activateTab(e[(i + 1) % e.length]) }
           else if (evt.key === 'ArrowLeft') { evt.preventDefault(); activateTab(e[(i - 1 + e.length) % e.length]) }
           else if (evt.key === 'Home') { evt.preventDefault(); activateTab(e[0]) }
           else if (evt.key === 'End')  { evt.preventDefault(); activateTab(e[e.length - 1]) }"
         data-on:scroll__throttle.100ms="
           /* recompute arrow visibility as the user scrolls */
           $tabs.canLeft  = el.scrollLeft > 0;
           $tabs.canRight = el.scrollLeft + el.clientWidth < el.scrollWidth - 1"
         class="flex overflow-x-auto scrollbar-none scroll-smooth">

      <!-- Tab buttons are server-rendered into here; one template below -->
      <div id="tab-strip" class="flex"></div>
    </div>

    <!-- Right scroll arrow -->
    <button data-show="$tabs.canRight"
            data-on:click="$strip.scrollBy({left: 200, behavior: 'smooth'})"
            class="px-2 py-2 text-gray-400 hover:text-gray-700 shrink-0">›</button>

    <!-- Overflow dropdown: jump directly to ANY tab (esp. ones scrolled out of view) -->
    <div class="relative shrink-0 border-l">
      <button data-on:click="$tabs.overflowOpen = !$tabs.overflowOpen"
              class="px-2 py-2 text-gray-400 hover:text-gray-700">▾</button>
      <!-- Server renders the full tab list here; clicking an item activates + scrolls it in -->
      <div data-show="$tabs.overflowOpen" id="tab-overflow-menu"
           data-on:click="$tabs.overflowOpen = false"
           class="absolute right-0 top-full mt-1 w-48 bg-white border rounded-lg shadow-lg py-1 z-50"></div>
    </div>
  </div>

  <!-- ░░ Panel: lazy-loaded, cached after first activation ░░ -->
  <div id="tab-panel" class="p-5" role="tabpanel">
    <div class="h-24 animate-pulse bg-gray-100 rounded-lg"></div>  <!-- skeleton until first load -->
  </div>
</div>

A single tab button (server-rendered; idx, id, and disabled are server-inlined). It carries activation, lazy-load, URL sync, and native drag-reorder:

<!-- draggable for reorder; data-idx is the server's authoritative position.
     A disabled tab omits the click/drag handlers and isn't in $tabs.enabled. -->
<button role="tab" draggable="true" data-idx="2"
        data-on:click="
          $tabs.active = 'billing';
          history.replaceState(null, '', '?tab=billing');          /* secondary nav → replaceState (§6.10) */
          if (!$tabs.loaded.billing) { $tabs.loaded.billing = true; @get('/api/tab/billing'); }"
        data-on:dragstart="$tabs.dragFrom = 2"
        data-on:dragover__prevent="$tabs.overTo = 2"                <!-- preventDefault REQUIRED to allow drop -->
        data-on:dragend="$tabs.dragFrom = -1; $tabs.overTo = -1"
        data-on:drop__prevent="
          if ($tabs.dragFrom !== 2 && $tabs.dragFrom !== -1) {
            /* send indices; server persists the new order + re-renders the strip authoritatively */
            $tabs.reorder = {from: $tabs.dragFrom, to: 2};
            @patch('/api/tabs/reorder');
          }
          $tabs.dragFrom = -1; $tabs.overTo = -1"
        data-attr:aria-selected="$tabs.active === 'billing'"
        data-attr:tabindex="$tabs.active === 'billing' ? 0 : -1"    <!-- roving tabindex -->
        data-class="{
          'border-blue-600 text-blue-600':        $tabs.active === 'billing',
          'border-transparent text-gray-500':     $tabs.active !== 'billing',
          'border-l-2 border-l-blue-500':         $tabs.overTo === 2 && $tabs.dragFrom !== 2,  /* drop indicator */
          'opacity-50': $tabs.dragFrom === 2                          /* the tab being dragged */
        }"
        class="px-4 py-2.5 border-b-2 whitespace-nowrap shrink-0 cursor-grab active:cursor-grabbing
               text-sm font-medium transition-colors">
  Billing
</button>

activateTab(id) referenced by the keyboard handler is the same three lines as the click body (set active → replaceState → lazy-load); define it once on the page (a tiny <script> exposing window.activateTab, or inline it in each handler). On load and after any reorder, the server patches $tabs.enabled so keyboard nav always reflects the current, disabled-aware order.

Server responses. First activation lazy-loads the panel and marks it cached:

event: datastar-patch-elements
data: selector #tab-panel
data: mode inner
data: elements <div>…billing panel…</div>

Reorder persists the new order and re-renders the strip as truth (the optimistic drop indicator is replaced by the real order):

event: datastar-patch-elements
data: selector #tab-strip
data: mode inner
data: elements <button data-idx="0" …>…</button> …reordered tabs…

event: datastar-patch-signals
data: signals {"tabs": {"enabled": ["home","billing","settings"], "reorder": {}}}

Why server-authoritative reorder? Tab order here is a saved preference — durable data, so the server owns it (§6.2). The drag gives instant feedback (drop indicator + dragged-tab styling), but the server persists and re-emits the canonical order, so every device and every refresh agrees. If order were purely ephemeral per-session, you could reorder the DOM client-side and skip the round-trip — but then it wouldn't survive a reload, which defeats the point of letting users arrange tabs. For active-tab persistence across refresh, the ?tab= URL pointer (§6.10) handles it with zero server state.

5. Complex Chat UI with Rich Widgets

5.1 Full Chat Layout with Multi-Panel Design

<div class="h-screen flex bg-gray-100"
     data-signals="{
       chat: {
         rooms: [],
         activeRoomId: null,
         messages: [],
         draft: '',
         replyTo: null,
         editingMessageId: null,
         searchQuery: '',
         searchResults: [],
         isSearching: false,
         isTyping: false,
         showEmojiPicker: false,
         showAttachmentPanel: false,
         attachmentFiles: [],
         pinnedMessages: [],
         threadParentId: null
       },
       ui: {
         showRoomInfo: false,
         showMembers: false,
         showSearch: false
       }
     }">

  <!-- ═══════════════════════════════════════════════════════════════ -->
  <!--  ROOMS SIDEBAR  —  Room list + search + user profile footer     -->
  <!-- ═══════════════════════════════════════════════════════════════ -->
  <aside class="w-72 bg-white border-r flex flex-col">
    <div class="p-4 border-b">
      <div class="flex justify-between items-center mb-3">
        <h2 class="font-bold text-lg">Messages</h2>
        <!-- DS:ON — New room button triggers server GET for creation form -->
        <button data-on:click="@get('/api/rooms/new')" class="p-1.5 hover:bg-gray-100 rounded-lg">
          <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M12 4v16m8-8H4"/></svg>
        </button>
        <!-- /DS:ON -->
      </div>
      <!-- DS:BIND + DS:ON — Search input with debounced room search -->
      <input type="text" data-bind="chat.searchQuery"
             data-on:input__debounce.200ms="@get('/api/rooms/search?q=' + $chat.searchQuery)"
             placeholder="Search rooms..."
             class="w-full px-3 py-2 bg-gray-100 rounded-lg text-sm" />
      <!-- /DS:BIND + /DS:ON -->
    </div>

    <div class="flex-1 overflow-y-auto">
      <template data-for="room in $chat.rooms">
        <button class="w-full text-left p-3 hover:bg-gray-50 transition-colors flex items-start gap-3 relative"
                data-on:click="$chat.activeRoomId = room.id; $chat.messages = []; $chat.threadParentId = null; @get('/api/rooms/' + room.id + '/messages')"
                data-class="{'bg-blue-50': $chat.activeRoomId === room.id}">
          <div class="relative flex-shrink-0">
            <img data-attr:src="room.avatar" class="w-12 h-12 rounded-full bg-gray-200" />
            <span data-show="room.unread > 0" class="absolute -top-1 -right-1 w-5 h-5 bg-red-500 text-white text-xs rounded-full flex items-center justify-center font-bold" data-text="room.unread"></span>
            <span data-show="room.online" class="absolute bottom-0 right-0 w-3 h-3 bg-green-500 rounded-full border-2 border-white"></span>
          </div>
          <div class="flex-1 min-w-0">
            <div class="flex justify-between items-baseline">
              <p class="font-medium truncate" data-text="room.name"></p>
              <span class="text-xs text-gray-400" data-text="room.lastMessageTime"></span>
            </div>
            <p class="text-sm text-gray-500 truncate" data-text="room.lastMessage"></p>
          </div>
        </button>
      </template>
    </div>

    <!-- User Profile Footer -->
    <div class="p-3 border-t flex items-center gap-3">
      <img data-attr:src="$data.user.avatar" class="w-8 h-8 rounded-full bg-gray-200" />
      <div class="flex-1 min-w-0">
        <p class="text-sm font-medium truncate" data-text="$data.user.name"></p>
        <p class="text-xs text-gray-500">Online</p>
      </div>
      <button data-on:click="$ui.showRoomInfo = true" class="p-1.5 hover:bg-gray-100 rounded-lg">
        <svg class="w-5 h-5 text-gray-500" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M10.325 4.317c.426-1.756 2.924-1.756 3.35 0a1.724 1.724 0 002.573 1.066c1.543-.94 3.31.826 2.37 2.37a1.724 1.724 0 001.065 2.572c1.756.426 1.756 2.924 0 3.35a1.724 1.724 0 00-1.066 2.573c.94 1.543-.826 3.31-2.37 2.37a1.724 1.724 0 00-2.572 1.065c-.426 1.756-2.924 1.756-3.35 0a1.724 1.724 0 00-2.573-1.066c-1.543.94-3.31-.826-2.37-2.37a1.724 1.724 0 00-1.065-2.572c-1.756-.426-1.756-2.924 0-3.35a1.724 1.724 0 001.066-2.573c-.94-1.543.826-3.31 2.37-2.37.996.608 2.296.07 2.572-1.065z"/><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M15 12a3 3 0 11-6 0 3 3 0 016 0z"/></svg>
      </button>
    </div>
  </aside>

  <!-- MAIN CHAT AREA -->
  <main class="flex-1 flex flex-col min-w-0">
    <!-- Chat Header -->
    <header class="h-16 bg-white border-b flex items-center justify-between px-4 flex-shrink-0">
      <div class="flex items-center gap-3">
        <div class="relative">
          <img data-attr:src="$chat.rooms.find(r => r.id === $chat.activeRoomId)?.avatar" 
               class="w-10 h-10 rounded-full bg-gray-200" />
          <span class="absolute bottom-0 right-0 w-2.5 h-2.5 bg-green-500 rounded-full border-2 border-white"></span>
        </div>
        <div>
          <p class="font-bold" data-text="$chat.rooms.find(r => r.id === $chat.activeRoomId)?.name || 'Select a room'"></p>
          <p class="text-xs text-gray-500" data-text="$chat.rooms.find(r => r.id === $chat.activeRoomId)?.members + ' members'"></p>
        </div>
      </div>
      <div class="flex items-center gap-1">
        <button data-on:click="$ui.showSearch = true; $chat.searchQuery = ''; $chat.searchResults = []"
                class="p-2 hover:bg-gray-100 rounded-lg text-gray-500">
          <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M21 21l-6-6m2-5a7 7 0 11-14 0 7 7 0 0114 0z"/></svg>
        </button>
        <button data-on:click="$ui.showMembers = true"
                class="p-2 hover:bg-gray-100 rounded-lg text-gray-500">
          <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M17 20h5v-2a3 3 0 00-5.356-1.857M17 20H7m10 0v-2c0-.656-.126-1.283-.356-1.857M7 20H2v-2a3 3 0 015.356-1.857M7 20v-2c0-.656.126-1.283.356-1.857m0 0a5.002 5.002 0 019.288 0M15 7a3 3 0 11-6 0 3 3 0 016 0zm6 3a2 2 0 11-4 0 2 2 0 014 0zM7 10a2 2 0 11-4 0 2 2 0 014 0z"/></svg>
        </button>
        <button data-on:click="$ui.showRoomInfo = true"
                class="p-2 hover:bg-gray-100 rounded-lg text-gray-500">
          <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M13 16h-1v-4h-1m1-4h.01M21 12a9 9 0 11-18 0 9 9 0 0118 0z"/></svg>
        </button>
      </div>
    </header>

    <!-- Pinned Messages Banner -->
    <div data-show="$chat.pinnedMessages.length > 0 && !$chat.threadParentId" 
         class="bg-yellow-50 border-b border-yellow-200 px-4 py-2 flex items-center gap-2 flex-shrink-0">
      <svg class="w-4 h-4 text-yellow-600 flex-shrink-0" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M5 5a2 2 0 012-2h10a2 2 0 012 2v16l-7-3.5L5 21V5z"/></svg>
      <p class="text-sm text-yellow-800 truncate flex-1">
        <span class="font-medium">Pinned:</span> <span data-text="$chat.pinnedMessages[0].text"></span>
      </p>
      <button data-on:click="$chat.pinnedMessages = []" class="text-yellow-600 hover:text-yellow-800 text-sm">Dismiss</button>
    </div>

    <!-- Messages Area -->
    <div id="messages-container" class="flex-1 overflow-y-auto p-4 space-y-1"
         data-init="const el = document.getElementById('messages-container'); if(el) el.scrollTop = el.scrollHeight;"
         data-effect="const el = document.getElementById('messages-container'); if(el) el.scrollTop = el.scrollHeight;">

      <!-- Thread Header (when viewing thread) -->
      <div data-show="$chat.threadParentId" class="mb-4 p-3 bg-gray-50 rounded-lg border">
        <div class="flex items-center gap-2 mb-2">
          <button data-on:click="$chat.threadParentId = null" class="text-blue-600 hover:underline text-sm flex items-center gap-1">
            <svg class="w-4 h-4" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M10 19l-7-7m0 0l7-7m-7 7h18"/></svg>
            Back to room
          </button>
          <span class="text-gray-400">|</span>
          <span class="text-sm text-gray-500">Thread</span>
        </div>
        <div class="pl-4 border-l-2 border-gray-300">
          <p class="text-sm text-gray-600" data-text="$chat.messages.find(m => m.id === $chat.threadParentId)?.text"></p>
        </div>
      </div>

      <!-- Messages -->
      <template data-for="msg in $chat.messages">
        <div class="group flex items-start gap-3 py-2 px-2 rounded-lg hover:bg-gray-50 transition-colors"
             data-class="{'bg-blue-50/50': $chat.replyTo === msg.id}">

          <img data-attr:src="msg.authorAvatar" class="w-8 h-8 rounded-full bg-gray-200 flex-shrink-0 mt-1" />

          <div class="flex-1 min-w-0">
            <div class="flex items-baseline gap-2">
              <span class="font-bold text-sm" data-text="msg.authorName"></span>
              <span class="text-xs text-gray-400" data-text="msg.timestamp"></span>
              <span data-show="msg.edited" class="text-xs text-gray-400">(edited)</span>
            </div>

            <!-- Reply Reference -->
            <div data-show="msg.replyToId" class="mb-1 pl-3 border-l-2 border-gray-300 text-sm text-gray-500">
              <span class="font-medium" data-text="msg.replyToAuthor + ':'"></span>
              <span class="truncate" data-text="msg.replyToText"></span>
            </div>

            <!-- Message Content -->
            <div data-show="$chat.editingMessageId !== msg.id" class="text-sm text-gray-800 leading-relaxed">
              <span data-text="msg.text"></span>
            </div>

            <!-- Edit Mode -->
            <div data-show="$chat.editingMessageId === msg.id" class="mt-1">
              <input type="text" data-bind="chat.editText" 
                     class="w-full border rounded-lg px-3 py-2 text-sm"
                     data-ref="editInput"
                     data-effect="if($chat.editingMessageId === msg.id) { setTimeout(() => $editInput?.focus(), 50) }" />
              <div class="flex gap-2 mt-2">
                <button data-on:click="@post('/api/messages/' + msg.id + '/edit', {text: $chat.editText})"
                        class="px-3 py-1 bg-blue-600 text-white text-xs rounded hover:bg-blue-700">Save</button>
                <button data-on:click="$chat.editingMessageId = null"
                        class="px-3 py-1 text-gray-500 text-xs hover:text-gray-700">Cancel</button>
              </div>
            </div>

            <!-- File Attachments -->
            <div data-show="msg.attachments && msg.attachments.length > 0" class="flex flex-wrap gap-2 mt-2">
              <template data-for="file in msg.attachments">
                <div class="flex items-center gap-2 p-2 bg-gray-100 rounded-lg text-sm">
                  <svg class="w-4 h-4 text-gray-500" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M15.172 7l-6.586 6.586a2 2 0 102.828 2.828l6.414-6.586a4 4 0 00-5.656-5.656l-6.415 6.585a6 6 0 108.486 8.486L20.5 13"/></svg>
                  <span class="text-gray-700" data-text="file.name"></span>
                  <span class="text-gray-400 text-xs" data-text="file.size"></span>
                  <a data-attr:href="file.url" download class="text-blue-600 hover:underline text-xs">Download</a>
                </div>
              </template>
            </div>

            <!-- Reactions -->
            <div data-show="msg.reactions && Object.keys(msg.reactions).length > 0" class="flex flex-wrap gap-1 mt-2">
              <template data-for="[emoji, users] in Object.entries(msg.reactions)">
                <button data-on:click="@post('/api/messages/' + msg.id + '/react', {emoji: emoji})"
                        data-class="{'bg-blue-100 border-blue-300': users.includes($data.user.id), 'bg-gray-100 border-gray-200': !users.includes($data.user.id)}"
                        class="px-2 py-0.5 rounded-full border text-sm flex items-center gap-1 hover:bg-gray-200 transition-colors">
                  <span data-text="emoji"></span>
                  <span class="text-xs text-gray-500" data-text="users.length"></span>
                </button>
              </template>
            </div>

            <!-- Message Actions (hover) -->
            <div class="flex items-center gap-1 mt-1 opacity-0 group-hover:opacity-100 transition-opacity">
              <button data-on:click="$chat.replyTo = msg.id; $messageInput?.focus()"
                      class="text-xs text-gray-500 hover:text-blue-600 px-2 py-0.5 rounded hover:bg-gray-100">Reply</button>
              <button data-on:click="$chat.threadParentId = msg.id; @get('/api/messages/' + msg.id + '/thread')"
                      class="text-xs text-gray-500 hover:text-blue-600 px-2 py-0.5 rounded hover:bg-gray-100">
                Thread <span data-show="msg.threadCount > 0" data-text="'(' + msg.threadCount + ')'"></span>
              </button>
              <button data-on:click="$chat.editingMessageId = msg.id; $chat.editText = msg.text"
                      data-show="msg.authorId === $data.user.id"
                      class="text-xs text-gray-500 hover:text-blue-600 px-2 py-0.5 rounded hover:bg-gray-100">Edit</button>
              <button data-on:click="@post('/api/messages/' + msg.id + '/pin')"
                      class="text-xs text-gray-500 hover:text-blue-600 px-2 py-0.5 rounded hover:bg-gray-100">Pin</button>
              <button data-on:click="$chat.showEmojiPicker = msg.id"
                      class="text-xs text-gray-500 hover:text-blue-600 px-2 py-0.5 rounded hover:bg-gray-100">React</button>
            </div>
          </div>
        </div>
      </template>

      <!-- Typing Indicator -->
      <div data-show="$chat.isTyping" class="flex items-center gap-2 text-sm text-gray-500 py-2 px-2">
        <div class="flex gap-1">
          <div class="w-2 h-2 bg-gray-400 rounded-full animate-bounce"></div>
          <div class="w-2 h-2 bg-gray-400 rounded-full animate-bounce" style="animation-delay: 0.1s"></div>
          <div class="w-2 h-2 bg-gray-400 rounded-full animate-bounce" style="animation-delay: 0.2s"></div>
        </div>
        <span>Someone is typing...</span>
      </div>
    </div>

    <!-- Reply Preview -->
    <div data-show="$chat.replyTo" class="bg-gray-50 border-t px-4 py-2 flex items-center gap-2">
      <div class="flex-1 pl-3 border-l-2 border-blue-400 text-sm">
        <p class="text-gray-500 text-xs">Replying to <span class="font-medium text-gray-700" data-text="$chat.messages.find(m => m.id === $chat.replyTo)?.authorName"></span></p>
        <p class="text-gray-600 truncate" data-text="$chat.messages.find(m => m.id === $chat.replyTo)?.text"></p>
      </div>
      <button data-on:click="$chat.replyTo = null" class="text-gray-400 hover:text-gray-600 p-1">✕</button>
    </div>

    <!-- Attachment Preview -->
    <div data-show="$chat.attachmentFiles.length > 0" class="bg-gray-50 border-t px-4 py-2 flex gap-2 overflow-x-auto">
      <template data-for="(file, index) in $chat.attachmentFiles">
        <div class="flex items-center gap-2 bg-white border rounded-lg px-3 py-2 text-sm flex-shrink-0">
          <svg class="w-4 h-4 text-gray-500" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M15.172 7l-6.586 6.586a2 2 0 102.828 2.828l6.414-6.586a4 4 0 00-5.656-5.656l-6.415 6.585a6 6 0 108.486 8.486L20.5 13"/></svg>
          <span class="text-gray-700" data-text="file.name"></span>
          <button data-on:click="$chat.attachmentFiles = $chat.attachmentFiles.filter((_, i) => i !== index)"
                  class="text-gray-400 hover:text-red-500">✕</button>
        </div>
      </template>
    </div>

    <!-- Message Input -->
    <div class="p-4 bg-white border-t">
      <div class="flex items-end gap-2">
        <button data-on:click="$chat.showAttachmentPanel = !$chat.showAttachmentPanel"
                class="p-2 text-gray-500 hover:text-gray-700 hover:bg-gray-100 rounded-lg transition-colors flex-shrink-0">
          <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M15.172 7l-6.586 6.586a2 2 0 102.828 2.828l6.414-6.586a4 4 0 00-5.656-5.656l-6.415 6.585a6 6 0 108.486 8.486L20.5 13"/></svg>
        </button>

        <div class="flex-1 relative">
          <input type="text" 
                 data-ref="messageInput"
                 data-bind="chat.draft"
                 data-on:keydown__prevent.enter="if($chat.draft.trim()) { @post('/api/messages', {roomId: $chat.activeRoomId, text: $chat.draft, replyTo: $chat.replyTo, attachments: $chat.attachmentFiles}); $chat.draft = ''; $chat.replyTo = null; $chat.attachmentFiles = [] }"
                 data-on:input__throttle.300ms="@post('/api/typing', {roomId: $chat.activeRoomId})"
                 placeholder="Type a message..."
                 class="w-full px-4 py-2.5 bg-gray-100 border-transparent rounded-xl focus:bg-white focus:ring-2 focus:ring-blue-500 transition-all" />

          <!-- Emoji Picker Toggle -->
          <button data-on:click="$chat.showEmojiPicker = !$chat.showEmojiPicker"
                  class="absolute right-2 top-1/2 -translate-y-1/2 p-1.5 text-gray-500 hover:text-gray-700 hover:bg-gray-200 rounded-lg transition-colors">
            <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M14.828 14.828a4 4 0 01-5.656 0M9 10h.01M15 10h.01M21 12a9 9 0 11-18 0 9 9 0 0118 0z"/></svg>
          </button>
        </div>

        <button data-on:click="if($chat.draft.trim()) { @post('/api/messages', {roomId: $chat.activeRoomId, text: $chat.draft, replyTo: $chat.replyTo}); $chat.draft = ''; $chat.replyTo = null }"
                data-attr:disabled="!$chat.draft.trim()"
                class="p-2.5 bg-blue-600 text-white rounded-xl hover:bg-blue-700 disabled:opacity-50 disabled:cursor-not-allowed transition-colors flex-shrink-0">
          <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M12 19l9 2-9-18-9 18 9-2zm0 0v-8"/></svg>
        </button>
      </div>
    </div>
  </main>

  <!-- RIGHT INFO PANEL (collapsible) -->
  <aside data-show="$ui.showRoomInfo" 
         class="w-80 bg-white border-l flex flex-col transition-all">
    <div class="p-4 border-b flex justify-between items-center">
      <h3 class="font-bold">Room Info</h3>
      <button data-on:click="$ui.showRoomInfo = false" class="text-gray-400 hover:text-gray-600">✕</button>
    </div>
    <div class="p-4 text-center">
      <img data-attr:src="$chat.rooms.find(r => r.id === $chat.activeRoomId)?.avatar" class="w-20 h-20 rounded-full bg-gray-200 mx-auto mb-3" />
      <h4 class="font-bold" data-text="$chat.rooms.find(r => r.id === $chat.activeRoomId)?.name"></h4>
      <p class="text-sm text-gray-500 mt-1">Created Jan 2024</p>
    </div>
    <div class="border-t p-4">
      <h5 class="text-sm font-medium text-gray-500 mb-2 uppercase tracking-wide">Members</h5>
      <div id="room-members" class="space-y-2">
        <!-- Server-patched member list -->
      </div>
    </div>
  </aside>

</div>

5.2 Message Thread with Reactions

<!-- Inline thread preview within message -->
<div class="mt-2 pl-12">
  <div data-show="msg.threadCount > 0 && !$chat.threadParentId" class="flex items-center gap-2 text-sm text-blue-600 hover:underline cursor-pointer"
       data-on:click="$chat.threadParentId = msg.id; @get('/api/messages/' + msg.id + '/thread')">
    <svg class="w-4 h-4" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M8 12h.01M12 12h.01M16 12h.01M21 12c0 4.418-4.03 8-9 8a9.863 9.863 0 01-4.255-.949L3 20l1.395-3.72C3.512 15.042 3 13.574 3 12c0-4.418 4.03-8 9-8s9 3.582 9 8z"/></svg>
    <span data-text="msg.threadCount + ' replies'"></span>
    <span class="text-gray-400">·</span>
    <span class="text-gray-500" data-text="msg.lastReplyTime"></span>
  </div>
</div>

5.3 Typing Indicators & Presence

<!-- Server pushes typing state via SSE -->
<div data-show="$chat.typingUsers && $chat.typingUsers.length > 0" 
     class="flex items-center gap-2 text-sm text-gray-500 py-2 px-4">
  <div class="flex -space-x-1">
    <template data-for="user in $chat.typingUsers.slice(0, 3)">
      <img data-attr:src="user.avatar" class="w-5 h-5 rounded-full border border-white bg-gray-200" />
    </template>
  </div>
  <span data-text="$chat.typingUsers.length === 1 ? $chat.typingUsers[0].name + ' is typing...' : $chat.typingUsers.length + ' people are typing...'"></span>
  <div class="flex gap-0.5">
    <div class="w-1.5 h-1.5 bg-gray-400 rounded-full animate-bounce"></div>
    <div class="w-1.5 h-1.5 bg-gray-400 rounded-full animate-bounce" style="animation-delay: 0.1s"></div>
    <div class="w-1.5 h-1.5 bg-gray-400 rounded-full animate-bounce" style="animation-delay: 0.2s"></div>
  </div>
</div>

5.4 File Attachments & Previews

<!-- Attachment upload panel -->
<div data-show="$chat.showAttachmentPanel" class="bg-gray-50 border-t p-4">
  <div class="border-2 border-dashed border-gray-300 rounded-lg p-8 text-center hover:border-gray-400 transition-colors"
       data-on:dragover__prevent="el.classList.add('border-blue-400', 'bg-blue-50')"
       data-on:dragleave="el.classList.remove('border-blue-400', 'bg-blue-50')"
       data-on:drop__prevent="el.classList.remove('border-blue-400', 'bg-blue-50'); const files = Array.from(evt.dataTransfer.files); $chat.attachmentFiles = [...$chat.attachmentFiles, ...files.map(f => ({name: f.name, size: (f.size / 1024).toFixed(1) + 'KB', file: f}))]; $chat.showAttachmentPanel = false">
    <svg class="w-10 h-10 text-gray-400 mx-auto mb-2" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M7 16a4 4 0 01-.88-7.903A5 5 0 1115.9 6L16 6a5 5 0 011 9.9M15 13l-3-3m0 0l-3 3m3-3v12"/></svg>
    <p class="text-sm text-gray-600">Drop files here or <span class="text-blue-600">browse</span></p>
    <p class="text-xs text-gray-400 mt-1">Max 10MB per file</p>
  </div>
</div>

<!-- Image preview in message -->
<div data-show="msg.imageUrl" class="mt-2">
  <img data-attr:src="msg.imageUrl" class="max-w-sm rounded-lg border cursor-pointer hover:opacity-95 transition-opacity" 
       data-on:click="$lightboxImage = msg.imageUrl; $showLightbox = true" />
</div>

<!-- Lightbox -->
<div data-show="$showLightbox" class="fixed inset-0 z-50 bg-black/90 flex items-center justify-center"
     data-on:click="$showLightbox = false">
  <img data-attr:src="$lightboxImage" class="max-w-[90vw] max-h-[90vh] object-contain" />
</div>

5.5 Mentions & Autocomplete

<div class="relative flex-1">
  <input type="text" 
         data-ref="messageInput"
         data-bind="chat.draft"
         data-on:input="if($chat.draft.endsWith('@')) { $chat.mentionQuery = ''; $chat.showMentions = true; @get('/api/users/mentions') } else if($chat.showMentions) { const match = $chat.draft.match(/@([^\\s]*)$/); if(match) { $chat.mentionQuery = match[1]; @get('/api/users/mentions?q=' + $chat.mentionQuery) } }"
         class="w-full px-4 py-2.5 bg-gray-100 rounded-xl" />

  <!-- Mention Autocomplete Dropdown -->
  <div data-show="$chat.showMentions && $chat.mentionUsers && $chat.mentionUsers.length > 0"
       class="absolute bottom-full left-0 mb-2 w-64 bg-white border rounded-lg shadow-xl overflow-hidden">
    <div class="p-2 text-xs text-gray-500 uppercase tracking-wide font-medium">Mention someone</div>
    <template data-for="user in $chat.mentionUsers">
      <button class="w-full text-left px-3 py-2 hover:bg-blue-50 flex items-center gap-2 transition-colors"
              data-on:click="$chat.draft = $chat.draft.replace(/@[^\s]*$/, '@' + user.username + ' '); $chat.showMentions = false; $messageInput?.focus()">
        <img data-attr:src="user.avatar" class="w-6 h-6 rounded-full bg-gray-200" />
        <div>
          <p class="text-sm font-medium" data-text="user.name"></p>
          <p class="text-xs text-gray-500" data-text="'@' + user.username"></p>
        </div>
      </button>
    </template>
  </div>
</div>

5.6 Message Search & Filtering

<!-- Search Overlay -->
<div data-show="$ui.showSearch" class="fixed inset-0 z-50 bg-black/50 flex items-start justify-center pt-20">
  <div class="bg-white rounded-xl shadow-2xl w-full max-w-2xl mx-4 overflow-hidden"
       data-on:click__stop>
    <div class="p-4 border-b flex items-center gap-3">
      <svg class="w-5 h-5 text-gray-400" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M21 21l-6-6m2-5a7 7 0 11-14 0 7 7 0 0114 0z"/></svg>
      <input type="text" 
             data-ref="searchInput"
             data-bind="chat.searchQuery"
             data-on:input__debounce.300ms="$chat.isSearching = true; @get('/api/messages/search?q=' + $chat.searchQuery + '&roomId=' + $chat.activeRoomId)"
             placeholder="Search messages..."
             class="flex-1 outline-none text-lg"
             data-effect="setTimeout(() => $searchInput?.focus(), 100)" />
      <button data-on:click="$ui.showSearch = false; $chat.searchQuery = ''; $chat.searchResults = []" class="text-gray-400 hover:text-gray-600">✕</button>
    </div>

    <div class="max-h-[60vh] overflow-y-auto">
      <div data-show="$chat.isSearching" class="p-8 text-center">
        <div class="w-8 h-8 border-4 border-blue-600 border-t-transparent rounded-full animate-spin mx-auto"></div>
      </div>

      <div data-show="!$chat.isSearching && $chat.searchResults.length === 0 && $chat.searchQuery" class="p-8 text-center text-gray-500">
        No results found for "<span data-text="$chat.searchQuery"></span>"
      </div>

      <template data-for="result in $chat.searchResults">
        <button class="w-full text-left p-4 hover:bg-gray-50 border-b transition-colors"
                data-on:click="$ui.showSearch = false; @get('/api/messages/' + result.id + '/context')">
          <div class="flex items-start gap-3">
            <img data-attr:src="result.authorAvatar" class="w-8 h-8 rounded-full bg-gray-200 flex-shrink-0" />
            <div class="flex-1">
              <div class="flex items-baseline gap-2">
                <span class="font-medium text-sm" data-text="result.authorName"></span>
                <span class="text-xs text-gray-400" data-text="result.timestamp"></span>
              </div>
              <p class="text-sm text-gray-700 mt-1">
                <span data-text="result.text.substring(0, result.highlightIndex)"></span>
                <span class="bg-yellow-200" data-text="result.highlightText"></span>
                <span data-text="result.text.substring(result.highlightIndex + result.highlightText.length)"></span>
              </p>
              <p class="text-xs text-gray-400 mt-1" data-text="'in ' + result.roomName"></p>
            </div>
          </div>
        </button>
      </template>
    </div>

    <div class="p-3 bg-gray-50 border-t text-xs text-gray-500 flex justify-between">
      <span data-text="$chat.searchResults.length + ' results'"></span>
      <span>Press Enter to jump to message</span>
    </div>
  </div>
</div>

6. Server-First State Management

Datastar's philosophy is server-driven reactivity: the server owns the canonical state, and the client is a thin projection layer. This section covers how to architect your application so that state lives where it belongs — on the server — while the client handles only presentation and user input capture.

6.1 The Golden Rule: Server Owns Truth

┌─────────────────────────────────────────────────────────────────────┐
│                         STATE OWNERSHIP                              │
├─────────────────────────────────────────────────────────────────────┤
│  SERVER (Source of Truth)          │  CLIENT (Projection Layer)      │
│  ───────────────────────────────── │  ─────────────────────────────  │
│  • Domain data (users, orders)     │  • UI chrome (sidebar open?)    │
│  • Business rules & validation     │  • Active selections            │
│  • Access control & permissions    │  • Form drafts (temporary)      │
│  • Computed aggregates             │  • Scroll positions             │
│  • Audit trails & history          │  • Animation states             │
│  • Cross-user consistency          │  • Ephemeral filters            │
└─────────────────────────────────────────────────────────────────────┘

When to keep state on the server:

State Type Example Why Server?
Domain data User profiles, product catalog Single source of truth, ACID guarantees, access control
Shared state Chat messages, collaborative docs Multiple users need consistency
Computed data Dashboard KPIs, search rankings Expensive to compute, must be consistent
Validation rules Password strength, unique emails Security-critical, must not be client-bypassable
Workflow state Order status, approval chains Business logic, audit requirements

When to keep state on the client:

State Type Example Why Client?
UI chrome Sidebar collapsed, theme dark User preference, no server value
Active selection Selected row, highlighted text Ephemeral, per-session
Form drafts Unsaved input in a text field Reduces server round-trips, lost on refresh
View state Scroll position, zoom level Purely presentational
Transient filters Search query being typed Debounced before server submission

6.2 The Anti-Patterns to Avoid

❌ Pattern 1: Client-Side "Micro-Databases"

<!-- BAD: Client stores and manages full domain data -->
<div data-signals="{
  users: [],           // ← 10,000 user records in browser memory
  orders: [],          // ← Full order history
  products: []         // ← Entire product catalog
}">

Why this breaks:

  • Browser memory balloons (MBs of JSON)
  • No access control — sensitive data leaks to client
  • Stale data — client cache drifts from server truth
  • Complex client-side filtering/sorting logic duplicates server work

✅ The fix: Keep data on server; client holds only IDs and display labels.

<!-- GOOD: Client holds only what's needed for the current view -->
<div data-signals="{
  userIds: [1, 2, 3],           // ← Just IDs
  selectedUserId: null,         // ← Active selection
  filterQuery: ''               // ← Will be sent to server
}">
  <!-- Server patches the actual user cards into #user-list -->
  <div id="user-list"></div>
</div>

❌ Pattern 2: Client-Side Business Logic

<!-- BAD: Business rules in expressions -->
<div data-computed:canCheckout="$cart.total > 0 && $user.verified && $cart.items.every(i => i.inStock)">

Why this breaks:

  • Business rules are bypassable (user can edit signals in DevTools)
  • Rules drift between client and server implementations
  • Complex expressions become unmaintainable
  • Cannot access server-only data (inventory levels, fraud checks)

✅ The fix: Server computes; client displays.

<!-- GOOD: Server returns computed boolean -->
<div data-signals="{checkout: {canProceed: false, reasons: []}}"
     data-init="@get('/api/cart/validate')">
  <button data-attr:disabled="!$checkout.canProceed">
    Checkout
  </button>
  <div data-show="$checkout.reasons.length > 0">
    <p data-text="$checkout.reasons.join(', ')"></p>
  </div>
</div>

❌ Pattern 3: Optimistic Updates Without Rollback

<!-- BAD: Updates UI before server confirms -->
<button data-on:click="$likes++; @post('/api/like')">
  Like (<span data-text="$likes"></span>)
</button>

Why this breaks:

  • If the POST fails, the UI shows incorrect state
  • Race conditions: two rapid clicks = two increments, one server save
  • Network errors leave the UI permanently out of sync

✅ The fix: Server confirms, then UI updates.

<!-- GOOD: Server returns new count; UI reflects truth -->
<button data-on:click="@post('/api/like')">
  Like (<span data-text="$likes"></span>)
</button>

The server responds with:

event: datastar-patch-signals
data: signals {"likes": 42}

❌ Pattern 4: Chatty Client-Server Communication

<!-- BAD: Every keystroke hits the server -->
<input data-on:input="@post('/api/search?q=' + $query)" />

Why this breaks:

  • Server overwhelmed with requests
  • Race conditions: response order != request order
  • Poor UX: results flicker as responses arrive out of order

✅ The fix: Debounce, batch, and let the server own search.

<!-- GOOD: Debounced search with server-owned results -->
<input data-bind="query"
       data-on:input__debounce.300ms="@get('/api/search')"
       placeholder="Search..." />
<div id="search-results">
  <!-- Server patches results here -->
</div>

6.3 The Server-First Request Lifecycle

Every user interaction should follow this flow:

┌─────────────┐     ┌─────────────┐     ┌─────────────┐     ┌─────────────┐
│   User      │────▶│   Client    │────▶│   Server    │────▶│   Client    │
│  Action     │     │  Captures   │     │  Processes  │     │  Reflects   │
│             │     │   Input     │     │   & Decides │     │   Truth     │
└─────────────┘     └─────────────┘     └─────────────┘     └─────────────┘
      │                   │                     │                     │
      │                   │                     │                     │
      ▼                   ▼                     ▼                     ▼
  Click button      Update signals        Validate, compute,    Patch signals
  Type text         (optimistic only      query DB, enforce    Patch elements
  Drag item         for UI chrome)         business rules       Show result

Key principles:

  1. Client captures, does not decide. The client records user intent (button click, text input) and forwards it to the server. It does not apply business logic.

  2. Server validates, computes, persists. The server is the only place where business rules run. It checks permissions, validates data, queries the database, and computes derived values.

  3. Server returns the new truth. The response includes both signal patches (for scalar state) and element patches (for complex UI). The client applies these without interpretation.

  4. Client reflects, does not interpret. The client updates the DOM based on the server's instructions. It does not re-run business logic or re-compute derived values.

6.4 Signal Design for Server-First Architecture

Minimal Client Signals

Design your signals to be as small as possible:

<!-- BEFORE: Bloated client state -->
<div data-signals="{
  user: {id: 1, name: 'John', email: 'john@example.com', role: 'admin', department: 'Engineering', ...},
  orders: [{id: 1, items: [...], total: 100, status: 'pending', ...}],
  dashboard: {kpi1: 100, kpi2: 200, chartData: [...]}
}">

<!-- AFTER: Minimal client state -->
<div data-signals="{
  ui: {screen: 'dashboard', sidebarOpen: true},
  data: {userId: 1, activeOrderId: null},
  meta: {isLoading: false, error: null}
}">
  <!-- Server patches #user-card, #order-list, #dashboard-widgets -->

Signal Naming Conventions

Use namespaces to separate concerns:

Namespace Purpose Examples
ui.* Purely presentational $ui.sidebarOpen, $ui.activeModal, $ui.theme
data.* References to server data $data.userId, $data.selectedOrderId
form.* Unsaved user input $form.draftEmail, $form.searchQuery
meta.* Loading/error states $meta.isLoading, $meta.error, $meta.lastSync
server.* Server-computed read-only $server.checkoutValid, $server.permissions

Rule: server.* signals should never be written by the client. They are read-only projections of server state.

The _ prefix: keep client-only signals out of every request

Datastar serializes all signals into each backend request except those whose name (or any ancestor key) starts with _. So ui.* as written above is still sent on every @get/@post — the namespace is only a naming discipline. For state the server never needs (open/closed panels, drag offsets, hover, scroll), prefix it so it's automatically excluded and your payloads stay lean:

<!-- _ui and _drag are NEVER sent to the server (underscore = local-only) -->
<div data-signals="{_ui: {sidebarOpen: true}, _drag: {dx: 0}, form: {email: ''}}">
  <!-- only $form.email rides along in requests; $_ui / $_drag stay on the client -->
</div>

Equivalently, scope what's sent per request with filterSignals: @post('/save', {filterSignals: {include: /^form\./}}). Underscore-prefixing is the simpler default; reserve filterSignals for one-off overrides. (Many examples in this guide use plain ui.* for readability — in production, make those _ui.*.)

6.5 When to Use Optimistic UI (Sparingly)

Optimistic updates — showing the result before server confirmation — should be used only for:

  1. UI chrome toggles (sidebar open/close, theme switch)
  2. Local-only preferences (font size, compact mode)
  3. High-confidence, low-stakes actions (marking a notification as read)

Never use optimistic UI for:

  • Financial transactions
  • Data mutations with side effects
  • Actions requiring server validation
  • Cross-user shared state
<!-- ✅ SAFE: Optimistic toggle for UI chrome -->
<button data-on:click="$ui.sidebarOpen = !$ui.sidebarOpen">
  Toggle Sidebar
</button>

<!-- ❌ DANGEROUS: Optimistic financial mutation -->
<!-- DON'T DO THIS -->
<button data-on:click="$balance -= 100; @post('/api/transfer', {amount: 100})">
  Transfer $100
</button>

<!-- ✅ SAFE: Server computes new balance -->
<button data-on:click="@post('/api/transfer', {amount: 100})">
  Transfer $100
  <!-- Server patches $server.balance after processing -->
</button>

6.6 Handling Network Failures

Since the server owns truth, network failures should not corrupt client state:

<div data-signals="{meta: {isSaving: false, lastError: null, retryCount: 0}}">
  <button data-on:click="$meta.isSaving = true; @post('/api/save')"
          data-attr:disabled="$meta.isSaving">
    <span data-show="!$meta.isSaving">Save</span>
    <span data-show="$meta.isSaving">Saving...</span>
  </button>

  <!-- Error display -->
  <div data-show="$meta.lastError" class="bg-red-50 text-red-700 p-3 rounded-lg mt-2">
    <p data-text="$meta.lastError"></p>
    <button data-on:click="$meta.retryCount++; @post('/api/save')"
            class="text-sm underline mt-1">
      Retry (<span data-text="$meta.retryCount"></span>)
    </button>
  </div>
</div>

Server response on failure:

event: datastar-patch-signals
data: signals {"meta": {"isSaving": false, "lastError": "Network timeout. Please try again."}}

For long-lived SSE streams (not one-shot actions), failure handling is different: Datastar auto-retries on transient drops but stops on graceful disconnects and error responses. See §13.9 for the keep-alive/reconnection workaround and connection-status UI.

6.7 State Hydration on Page Load

When the page first loads, the server should send the initial state, not the client:

<!-- Server renders initial signals in the HTML -->
<body data-signals="{ui: {screen: 'dashboard'}, data: {userId: 1}, meta: {isLoading: true}}"
      data-init="@get('/api/dashboard/initial')">

Why this matters:

  • No "flash of empty content" — data is present immediately
  • SEO-friendly — crawlers see populated HTML
  • No extra round-trip on load — signals are in the initial HTML

6.8 The "Thin Client" Checklist

Before shipping a feature, verify:

  • No domain data in signals — only IDs, flags, and references
  • No business logic in expressions — all rules run on server
  • No optimistic mutations — unless purely UI chrome
  • Server returns full state after mutation — client doesn't compute deltas
  • Signals are namespaced — ui.*, data.*, form.*, meta.*, server.*
  • Network failures handled — error states, retry logic, loading indicators
  • Initial state from server — no client-side data fetching on load

6.9 Action Discipline: Don't Overdo the Client Side

Even with a server-first mindset, it's easy to drift into "chatty client" territory or, the opposite failure, smuggling logic back into expressions. Use this budget:

Rule of thumb: one user interaction → at most one server action → one SSE patch. If an interaction fires multiple requests, or a request triggers client-side recomputation of domain data, the design is wrong.

What belongs in a client expression vs. a server action

User does… ❌ Over-engineered client ✅ Right-sized
Types in search box @get on every keystroke data-on:input__debounce.300ms="@get('/search')"
Edits 5 form fields 5 separate @patch calls (one per field) One @post('/save') on submit or __debounce.1000ms on the form container
Drags a slider @post per pixel of movement data-on:input__throttle.250ms while dragging + final @post on change
Resizes / sorts a 10k-row table Client Array.sort() over a giant signal @get('/table?sort=price') — server sorts with an index, patches #table-body
Opens a dashboard 6 chained @get calls (waterfall) One @get('/dashboard/initial') returning combined element + signal patches
Watches live data setInterval polling every 2s One SSE stream (data-init="@get('/stream')"), server pushes on change
Toggles dark mode @post('/api/theme') round-trip Pure client signal: $ui.theme = 'dark' — no server value at all
Clicks "Save" twice quickly Two in-flight POSTs racing Single-flight: disable while $meta.isSaving is true

Note the last two rows: discipline cuts both ways. Pure UI chrome should not generate server traffic, and anything the server cares about should generate exactly one request.

The single-flight pattern

First, what you get for free: Datastar already cancels any in-flight request to the same URL + HTTP method when a new one fires from the same element (the default requestCancellation: 'auto'). So rapid double-clicks won't produce two concurrent @posts to /api/orders. What it does not prevent is a second request firing after the first completes, or the UI looking clickable mid-flight. The guard below is therefore about UX and intent (disable + show progress + prevent a deliberate second submit), not about racing concurrency:

<button data-on:click="$meta.isSaving = true; @post('/api/orders')"
        data-attr:disabled="$meta.isSaving"
        data-class="{'opacity-50 cursor-not-allowed': $meta.isSaving}">
  <span data-show="!$meta.isSaving">Place Order</span>
  <span data-show="$meta.isSaving">Placing…</span>
</button>

(If you genuinely want concurrent requests to the same endpoint — rare — opt out with @post('/x', {requestCancellation: 'disabled'}).)

The server must clear the flag in its response — success and failure paths:

event: datastar-patch-signals
data: signals {"meta": {"isSaving": false}}

For idempotency under retries, have the client mint a key once per logical action and let the server deduplicate:

<div data-signals="{form: {idemKey: crypto.randomUUID()}}">
  <button data-on:click="@post('/api/orders')">Place Order</button>
  <!-- Server reads $form.idemKey; on success it patches a NEW idemKey -->
</div>

Coalesce, don't spray

When several signals change together (multi-field edit, bulk select), send them in one request — Datastar already serializes the whole signal store, so the server can read everything it needs from a single POST:

<!-- BAD: per-field autosave -->
<input data-bind="form.name"  data-on:input="@patch('/api/profile')" />
<input data-bind="form.email" data-on:input="@patch('/api/profile')" />

<!-- GOOD: one debounced autosave on the container -->
<div data-on:input__debounce.1500ms="@patch('/api/profile')">
  <input data-bind="form.name" />
  <input data-bind="form.email" />
</div>

Decision checklist: should this run on the client at all?

Walk these in order; the first "yes" decides:

  1. Does another user, system, or audit trail care about it? → Server.
  2. Would it matter if the user edited it in DevTools? (price, permission, validation) → Server.
  3. Is it derived from domain data? (totals, counts, rankings) → Server computes; client displays.
  4. Is it purely presentational and per-session? (accordion open, hover state, scroll) → Client, and don't tell the server.
  5. Is it draft input not yet committed? → Client (form.*), flushed to server on debounce/submit.

Red flags that you've overdone the client

  • A data-computed expression longer than one line of business meaning
  • Any data-effect that issues a server action (effects firing requests = hidden request loops)
  • A signal holding more than ~50 array items of domain data
  • A bloated payload in the network tab (use data-json-signals to inspect; prefix client-only signals with _)
  • Filtering/sorting/aggregating arrays in expressions instead of asking the server
  • The same rule implemented twice — once in an expression, once in the backend

6.10 State Synchronization: URL, Server & Refresh-Resilient Flows

Most apps have three places state can live, and bugs come from letting them disagree. The discipline is to give each a single, distinct job:

Layer Holds Job Lifetime
Server The actual data + draft progress Canonical truth (§6.1) Durable
URL A pointer: which step/tab/filter you're on — never the data itself Shareable, bookmarkable, back-button Per navigation
Client signals Live UI: hydrated copy of the above Instant interaction Per page load

Rule: the URL encodes where you are, not what the data is. ?step=3&tab=billing is good; ?formData={...} is not — that belongs on the server. On load, signals are hydrated from the server (§6.7), and the server reconstructs "where you are" from the URL pointer.

Writing the URL: pushState vs replaceState

This choice is the whole back-button experience:

// pushState → adds a history entry. Use when "back" should undo the navigation:
//   wizard steps, opening a detail view, switching a primary tab the user may want to return from.
history.pushState(null, '', '?step=3');

// replaceState → no history entry. Use for incidental view state you don't want
//   cluttering history: filter changes, sort order, scroll position, secondary tabs.
history.replaceState(null, '', '?sort=price&dir=desc');
<!-- Wizard "Next": persist to server, push a history entry, advance -->
<button data-on:click="
          $wizard.step++;
          history.pushState(null, '', '?step=' + $wizard.step);
          @post('/api/wizard/next')">                  <!-- server saves draft + returns next step -->
  Next
</button>

<!-- Filter change: update the URL silently (no history spam), refetch -->
<select data-bind="filter.range"
        data-on:change="
          history.replaceState(null, '', '?range=' + $filter.range);
          @get('/api/report')">…</select>

Reading the URL on load & handling back/forward

<!-- On load, hydrate the pointer FROM the url, then ask the server to restore that state.
     The server is what actually rebuilds the step/tab — the client just forwards the pointer. -->
<body data-signals="{wizard: {step: 1}}"
      data-init="
        $wizard.step = Number(new URLSearchParams(location.search).get('step')) || 1;
        @get('/api/wizard/resume')"                     <!-- signals (incl. step) ride along as query -->

      data-on:popstate__window="@get('/api/wizard/resume?step=' +
                                    (new URLSearchParams(location.search).get('step') || 1))">
</body>

popstate fires on back/forward; re-fetching from the URL keeps the rendered step in lock-step with the address bar. Because the server owns the draft, going back to step 2 still shows the data the user entered there.

Server-side resume: the refresh-resilience guarantee

The payoff — refresh mid-flow and nothing is lost — comes from persisting draft progress server-side as the user advances, then reconstructing from URL pointer + draft on any (re)load. Each step's data is saved when the user moves on:

// POST /api/wizard/next — persist this step's fields into a per-user draft row, return next step
export async function wizardNext(stream, headers) {
    const { wizard, form } = await readSignals(stream, headers);
    const userId = await authenticate(headers);                       // §4.11

    // Upsert the draft keyed by (user, flow). PG: JSONB column; Mongo: one draft doc (§8).
    await db.query(`
        INSERT INTO wizard_drafts (user_id, flow, step, data)
        VALUES ($1, 'onboarding', $2, $3)
        ON CONFLICT (user_id, flow)
        DO UPDATE SET step = $2, data = wizard_drafts.data || $3, updated_at = NOW()`,
        [userId, wizard.step, JSON.stringify(form)]);

    sseHead(stream);
    patchElements(stream, renderStep(wizard.step), { selector: '#wizard-body', mode: 'inner' });
    stream.end();
}

// GET /api/wizard/resume — rebuild the flow from the URL step + the saved draft
export async function wizardResume(stream, headers) {
    const userId = await authenticate(headers);
    const url = new URL(headers[':path'], 'https://x');
    const draft = await loadDraft(userId, 'onboarding');              // {step, data} or null

    // URL pointer wins for "which step"; draft supplies the data. Clamp to a valid step.
    const step = Math.min(Number(url.searchParams.get('step')) || draft?.step || 1,
                          draft?.step || 1);

    sseHead(stream);
    // 1) Restore the step markup …
    patchElements(stream, renderStep(step), { selector: '#wizard-body', mode: 'inner' });
    // 2) … AND rehydrate the saved field values into form.* signals (§6.7)
    patchSignals(stream, { wizard: { step }, form: draft?.data ?? {} });
    stream.end();
}

On final submit, promote the draft to the real record and delete it. If the user never finishes, a TTL/cron sweep (PG DELETE WHERE updated_at < …, or a Mongo TTL index, §8.4) reclaims stale drafts.

Why not just keep wizard data in localStorage?

localStorage survives refresh but not device switches, can't be validated, drifts out of sync with server truth, and silently breaks if your schema changes between visits. Server-side drafts give you cross-device resume, server validation at each step, and one source of truth — localStorage (or data-persist) is appropriate only for pure UI preferences (theme, collapsed panels), never for flow data.

Sync checklist

  • URL carries a pointer (step/tab/filter ids), never the data payload
  • pushState for navigations "back" should undo; replaceState for incidental view state
  • data-on:popstate__window re-fetches from the URL so back/forward stays correct
  • On load, signals hydrate from the server (§6.7); the server rebuilds state from the URL pointer
  • Multi-step flows persist a server-side draft per step → refresh/revisit restores step + data
  • Drafts cleaned up on submit, and swept by TTL/cron if abandoned
  • Sensitive/validated data never round-trips through the URL or localStorage

7. PostgreSQL + LISTEN/NOTIFY + SSE for Real-Time State

For applications requiring real-time updates — live dashboards, chat, collaborative editing, notifications — PostgreSQL's built-in LISTEN/NOTIFY pub/sub system combined with Datastar's SSE integration provides a powerful, zero-infrastructure alternative to Redis, Kafka, or WebSockets.

7.1 Architecture Overview

┌─────────────────┐     ┌─────────────────┐     ┌─────────────────┐     ┌─────────────────┐
│   Browser       │◄────│   Datastar      │◄────│   Application   │◄────│   PostgreSQL    │
│   (Client)      │ SSE │   SSE Parser    │     │   Server        │     │   LISTEN/NOTIFY │
│                 │     │                 │     │                 │     │                 │
│  data-init=     │     │  event: datastar│     │  pgClient.on(   │     │  CREATE TRIGGER │
│  @get('/stream')│     │  -patch-signals │     │  'notification' │     │  → pg_notify()  │
└─────────────────┘     └─────────────────┘     └─────────────────┘     └─────────────────┘
       ▲                                               │
       │                                               │
       └───────────────────────────────────────────────┘
                    Server pushes updates
                    (no client polling)

Why this pattern works:

  1. No extra infrastructure — PostgreSQL already has LISTEN/NOTIFY (since 2000)
  2. Transactional guarantees — notifications fire inside the same transaction as the data change
  3. Low latency — typically < 10ms from DB commit to client receive
  4. Simple mental model — database change → notification → SSE → DOM patch

7.2 PostgreSQL Schema Design by Business Use Case

Use Case A: Live Activity Feed (Append-Only Log)

Best for: audit logs, event streams, notification feeds, chat messages.

-- Table: events (append-only, never updated or deleted)
CREATE TABLE events (
    id          BIGSERIAL PRIMARY KEY,
    channel     VARCHAR(64) NOT NULL,        -- 'orders', 'chat_room_42', 'user_99'
    event_type  VARCHAR(32) NOT NULL,        -- 'created', 'updated', 'deleted'
    payload     JSONB NOT NULL,              -- the actual event data
    created_at  TIMESTAMPTZ DEFAULT NOW(),

    -- Index for time-range queries (dashboard history)
    CONSTRAINT idx_events_channel_time 
        UNIQUE (channel, created_at, id)
);

-- BRIN index for time-series data (very efficient for append-only)
CREATE INDEX idx_events_created_brin ON events USING BRIN (created_at);

-- Hot-path index for "latest N per channel" queries
-- NOTE: you CANNOT write `WHERE created_at > NOW() - INTERVAL '7 days'` as a
-- partial index — predicates must be IMMUTABLE and NOW() is not. To keep the
-- hot working set small, use time-range PARTITIONING instead (see §7.7).
CREATE INDEX idx_events_recent ON events (channel, created_at DESC);

-- Trigger: notify on insert
CREATE OR REPLACE FUNCTION notify_event_insert()
RETURNS TRIGGER AS $$
BEGIN
    PERFORM pg_notify(
        NEW.channel,
        json_build_object(
            'type', 'append',
            'event_type', NEW.event_type,
            'payload', NEW.payload,
            'timestamp', NEW.created_at
        )::text
    );
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER trg_events_notify
    AFTER INSERT ON events
    FOR EACH ROW
    EXECUTE FUNCTION notify_event_insert();

Datastar client:

<div data-signals="{events: [], hasMore: true}"
     data-init="@get('/api/events/stream?channel=orders')">

  <div id="event-feed" class="space-y-2">
    <template data-for="evt in $events">
      <div class="p-3 border rounded-lg"
           data-class="{'bg-green-50': evt.event_type === 'created', 'bg-yellow-50': evt.event_type === 'updated'}">
        <p class="text-sm" data-text="evt.payload.message"></p>
        <p class="text-xs text-gray-400" data-text="new Date(evt.timestamp).toLocaleTimeString()"></p>
      </div>
    </template>
  </div>
</div>

Server SSE handler (Node.js/pg):

import { Client } from 'pg';

export async function GET(request) {
    const url = new URL(request.url);
    const channel = url.searchParams.get('channel') || 'default';

    const pgClient = new Client({ connectionString: process.env.DATABASE_URL });
    await pgClient.connect();
    await pgClient.query(`LISTEN ${pgClient.escapeIdentifier(channel)}`);

    const stream = new ReadableStream({
        start(controller) {
            // Send initial batch of recent events
            pgClient.query(
                `SELECT * FROM events WHERE channel = $1 ORDER BY created_at DESC LIMIT 50`,
                [channel]
            ).then(result => {
                controller.enqueue(new TextEncoder().encode(
                    `event: datastar-patch-signals
` +
                    `data: signals {"events": ${JSON.stringify(result.rows.reverse())}}

`
                ));
            });

            // Listen for new notifications
            pgClient.on('notification', (msg) => {
                const payload = JSON.parse(msg.payload);
                controller.enqueue(new TextEncoder().encode(
                    `event: datastar-patch-signals
` +
                    `data: signals {"events": [${msg.payload}]}

`
                ));
            });

            // Heartbeat every 30s
            const heartbeat = setInterval(() => {
                controller.enqueue(new TextEncoder().encode(': heartbeat

'));
            }, 30000);

            // Cleanup
            request.signal.addEventListener('abort', () => {
                clearInterval(heartbeat);
                pgClient.end();
            });
        }
    });

    return new Response(stream, {
        headers: {
            'Content-Type': 'text/event-stream',
            'Cache-Control': 'no-cache',
            'Connection': 'keep-alive'
        }
    });
}

Use Case B: Live Dashboard KPIs (Snapshot Table)

Best for: counters, gauges, metrics that change frequently but don't need history.

-- Table: metrics (single row per metric, updated in place)
CREATE TABLE metrics (
    metric_name VARCHAR(64) PRIMARY KEY,
    value       NUMERIC NOT NULL,
    labels      JSONB DEFAULT '{}',          -- {region: 'us-east', service: 'api'}
    updated_at  TIMESTAMPTZ DEFAULT NOW()
);

-- Trigger: notify on any metric change
CREATE OR REPLACE FUNCTION notify_metric_change()
RETURNS TRIGGER AS $$
BEGIN
    PERFORM pg_notify(
        'metrics',
        json_build_object(
            'metric', NEW.metric_name,
            'value', NEW.value,
            'labels', NEW.labels,
            'timestamp', NEW.updated_at
        )::text
    );
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER trg_metrics_notify
    AFTER UPDATE ON metrics
    FOR EACH ROW
    WHEN (OLD.value IS DISTINCT FROM NEW.value)
    EXECUTE FUNCTION notify_metric_change();

Batch update pattern (prevents notification storms):

-- Instead of updating one row at a time, batch and notify once
CREATE OR REPLACE FUNCTION batch_update_metrics(updates JSONB)
RETURNS VOID AS $$
BEGIN
    -- Disable trigger temporarily
    ALTER TABLE metrics DISABLE TRIGGER trg_metrics_notify;

    -- Apply all updates
    UPDATE metrics SET
        value = (updates->metric_name->>'value')::NUMERIC,
        updated_at = NOW()
    WHERE metric_name IN (SELECT jsonb_object_keys(updates));

    -- Re-enable trigger
    ALTER TABLE metrics ENABLE TRIGGER trg_metrics_notify;

    -- Send single batch notification
    PERFORM pg_notify('metrics', json_build_object('type', 'batch', 'updates', updates)::text);
END;
$$ LANGUAGE plpgsql;

Use Case C: Collaborative Document State (CRDT-Style)

Best for: multi-user editing, whiteboards, shared cursors.

-- Table: document_ops (operation log — append-only, ordered)
CREATE TABLE document_ops (
    id          BIGSERIAL PRIMARY KEY,
    doc_id      UUID NOT NULL REFERENCES documents(id),
    op_type     VARCHAR(16) NOT NULL,        -- 'insert', 'delete', 'retain'
    position    INT NOT NULL,
    content     TEXT,                        -- null for delete/retain
    author_id   INT NOT NULL,
    version     INT NOT NULL,                -- monotonic per document
    created_at  TIMESTAMPTZ DEFAULT NOW(),

    UNIQUE (doc_id, version)
);

-- Index for fast op retrieval
CREATE INDEX idx_doc_ops ON document_ops (doc_id, version ASC);

-- Trigger: notify on new operation
CREATE OR REPLACE FUNCTION notify_doc_op()
RETURNS TRIGGER AS $$
BEGIN
    PERFORM pg_notify(
        'doc_' || NEW.doc_id::text,
        json_build_object(
            'type', 'op',
            'version', NEW.version,
            'op_type', NEW.op_type,
            'position', NEW.position,
            'content', NEW.content,
            'author_id', NEW.author_id
        )::text
    );
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER trg_doc_ops_notify
    AFTER INSERT ON document_ops
    FOR EACH ROW
    EXECUTE FUNCTION notify_doc_op();

Datastar client for collaborative editing:

<div data-signals="{doc: {ops: [], version: 0, cursors: {}}}"
     data-init="@get('/api/docs/' + docId + '/stream')">

  <div id="editor" contenteditable="true"
       data-on:input__throttle.100ms="@post('/api/docs/' + docId + '/ops')">
    <!-- Document content rendered from ops -->
  </div>

  <!-- Remote cursors -->
  <template data-for="[userId, cursor] in Object.entries($doc.cursors)">
    <div class="absolute pointer-events-none"
         data-style:left="cursor.x + 'px'"
         data-style:top="cursor.y + 'px'">
      <div class="w-0.5 h-5" data-style:background-color="cursor.color"></div>
      <span class="text-xs text-white px-1 rounded" data-style:background-color="cursor.color"
            data-text="cursor.name"></span>
    </div>
  </template>
</div>

Use Case D: User-Specific Notifications (Per-User Queue)

Best for: in-app notifications, alerts, messages targeted to individual users.

-- Table: user_notifications (per-user queue with read tracking)
CREATE TABLE user_notifications (
    id          BIGSERIAL PRIMARY KEY,
    user_id     INT NOT NULL,
    type        VARCHAR(32) NOT NULL,        -- 'mention', 'assignment', 'alert'
    title       VARCHAR(255) NOT NULL,
    body        TEXT,
    data        JSONB DEFAULT '{}',          -- arbitrary payload
    read_at     TIMESTAMPTZ,
    created_at  TIMESTAMPTZ DEFAULT NOW()
);

-- Index for user's unread count (hot query)
CREATE INDEX idx_user_notifs_unread ON user_notifications (user_id, read_at)
    WHERE read_at IS NULL;

-- Trigger: notify on new notification for user
CREATE OR REPLACE FUNCTION notify_user_notification()
RETURNS TRIGGER AS $$
BEGIN
    PERFORM pg_notify(
        'user_' || NEW.user_id::text,
        json_build_object(
            'type', 'notification',
            'id', NEW.id,
            'notif_type', NEW.type,
            'title', NEW.title,
            'body', NEW.body,
            'created_at', NEW.created_at
        )::text
    );
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER trg_user_notif_notify
    AFTER INSERT ON user_notifications
    FOR EACH ROW
    EXECUTE FUNCTION notify_user_notification();

Server-side channel management:

// Map: userId -> Set of SSE connections
const userConnections = new Map();

export async function GET(request) {
    const url = new URL(request.url);
    const userId = authenticate(request); // Your auth logic
    const channel = `user_${userId}`;

    const pgClient = new Client({ connectionString: process.env.DATABASE_URL });
    await pgClient.connect();
    await pgClient.query(`LISTEN ${pgClient.escapeIdentifier(channel)}`);

    const stream = new ReadableStream({
        start(controller) {
            // Track this connection
            if (!userConnections.has(userId)) {
                userConnections.set(userId, new Set());
            }
            userConnections.get(userId).add(controller);

            // Send unread count on connect
            pgClient.query(
                `SELECT COUNT(*) as count FROM user_notifications WHERE user_id = $1 AND read_at IS NULL`,
                [userId]
            ).then(result => {
                controller.enqueue(new TextEncoder().encode(
                    `event: datastar-patch-signals
` +
                    `data: signals {"unreadCount": ${result.rows[0].count}}

`
                ));
            });

            pgClient.on('notification', (msg) => {
                const payload = JSON.parse(msg.payload);
                controller.enqueue(new TextEncoder().encode(
                    `event: datastar-patch-signals
` +
                    `data: signals {"unreadCount": $unreadCount + 1}

`
                ));
                controller.enqueue(new TextEncoder().encode(
                    `event: datastar-patch-elements
` +
                    `data: selector #notif-dropdown
` +
                    `data: mode prepend
` +
                    `data: elements <div class="notif-item p-3 border-b">${payload.title}</div>

`
                ));
            });

            // Cleanup
            request.signal.addEventListener('abort', () => {
                const connections = userConnections.get(userId);
                if (connections) {
                    connections.delete(controller);
                    if (connections.size === 0) {
                        userConnections.delete(userId);
                        pgClient.query(`UNLISTEN ${pgClient.escapeIdentifier(channel)}`);
                    }
                }
                pgClient.end();
            });
        }
    });

    return new Response(stream, {
        headers: {
            'Content-Type': 'text/event-stream',
            'Cache-Control': 'no-cache',
            'Connection': 'keep-alive'
        }
    });
}

7.3 LISTEN/NOTIFY Limitations & Mitigations

Limitation Impact Mitigation
8KB payload limit Cannot send large JSON blobs Send only IDs + minimal metadata; client fetches full data via separate GET
No persistence Missed notifications if listener down Combine with events table for replay; client requests "since" timestamp on reconnect
~1000 listeners/connection Scalability ceiling Use connection pooling (PgBouncer); shard channels by user/modulo; fall back to polling for high-scale
Single DB bottleneck All traffic through one Postgres Use read replicas for queries; keep LISTEN/NOTIFY on primary; consider Redis for >1000 concurrent streams
No cross-DB routing Notifications don't cross databases Use application-level fan-out if multi-DB; or centralize on one Postgres instance

7.4 The Notification Queue Pattern

For high-throughput scenarios, decouple the trigger from the notification:

-- Queue table: buffers notifications for batch processing
CREATE TABLE notification_queue (
    id          BIGSERIAL PRIMARY KEY,
    channel     VARCHAR(64) NOT NULL,
    payload     JSONB NOT NULL,
    created_at  TIMESTAMPTZ DEFAULT NOW(),
    processed   BOOLEAN DEFAULT FALSE
);

-- Trigger: queue instead of notify directly
CREATE OR REPLACE FUNCTION queue_notification()
RETURNS TRIGGER AS $$
BEGIN
    INSERT INTO notification_queue (channel, payload)
    VALUES (
        TG_TABLE_NAME || '_changes',
        json_build_object('id', NEW.id, 'action', TG_OP)
    );
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

-- Background worker (runs every 1s via cron/pg_cron)
CREATE OR REPLACE FUNCTION process_notification_queue()
RETURNS VOID AS $$
DECLARE
    batch JSONB;
BEGIN
    SELECT json_agg(json_build_object('channel', channel, 'payload', payload))
    INTO batch
    FROM notification_queue
    WHERE NOT processed
      AND created_at < NOW() - INTERVAL '100 milliseconds'  -- small delay for batching
    LIMIT 100;

    IF batch IS NOT NULL THEN
        PERFORM pg_notify('batch', batch::text);
        UPDATE notification_queue SET processed = TRUE WHERE id IN (
            SELECT id FROM notification_queue WHERE NOT processed LIMIT 100
        );
    END IF;
END;
$$ LANGUAGE plpgsql;

7.5 Datastar + LISTEN/NOTIFY Integration Checklist

  • Channel naming convention — use predictable patterns: user_{id}, doc_{uuid}, org_{id}_orders
  • Connection lifecycle — always UNLISTEN on disconnect; use request.signal for cleanup
  • Heartbeat — send `: heartbeat

` every 30s to prevent proxy timeouts

  • Payload minimization — send IDs only; let Datastar fetch full fragments via separate GET
  • Error boundaries — wrap pgClient in try/catch; send error signals to client on failure
  • Reconnection handling — client should request "events since X" on reconnect after disconnect
  • Rate limiting — throttle high-frequency channels (e.g., cursor positions) to 10Hz
  • Authentication — validate user can access channel before LISTEN; reject unauthorized

7.6 PostgreSQL 18 State Management Playbook

PostgreSQL 18 (released Sept 2025) ships several features that map directly onto the state tables in §7.2. Use them — they remove triggers, shrink indexes, and speed up exactly the query shapes that SSE backends hammer.

PG 18 feature What it gives a Datastar backend
uuidv7() Time-ordered UUIDs as primary keys for event/message tables. Inserts land at the right edge of the B-tree (no random-page churn like uuidv4), keys still globally unique across partitions and services, and sort order ≈ creation order.
B-tree skip scan A composite index like (tenant_id, status, created_at) can now serve queries that omit the leading column(s) when their cardinality is low — fewer near-duplicate indexes on hot state tables.
RETURNING OLD.* / NEW.* Build notify payload diffs in the application layer without AFTER UPDATE triggers: UPDATE orders SET status='shipped' WHERE id=$1 RETURNING OLD.status AS prev, NEW.status AS curr. One round-trip, then pg_notify() from app code with the diff.
Async I/O (io_method = worker) Faster sequential scans and vacuums — matters for the "replay events since X" reconnect query and for vacuuming high-churn queue tables.
Virtual generated columns (now the default) Derive display fields (lower(email), computed labels) with zero storage; index them if queried. Keeps the server, not the client, the place where derived values live (§6).
Temporal constraints (WITHOUT OVERLAPS) Enforce "one active booking/subscription per resource per time range" in the schema instead of application code — the database becomes the business-rule enforcer that §6.2 demands.
EXPLAIN shows buffers by default Cheaper diagnosis of why an SSE handler's catch-up query is slow.

Pattern: trigger-less notify with RETURNING OLD/NEW:

-- One statement: mutate + capture diff for the notification payload
WITH changed AS (
    UPDATE orders
    SET    status = 'shipped', updated_at = NOW()
    WHERE  id = $1
    RETURNING id, OLD.status AS prev_status, NEW.status AS new_status
)
SELECT pg_notify(
    'org_' || $2 || '_orders',
    json_build_object(
        'id', id, 'from', prev_status, 'to', new_status
    )::text
) FROM changed;

Why prefer this over AFTER UPDATE triggers for app-driven mutations: the payload logic lives in versioned application code, you can skip notifying on no-op updates, and there's no trigger overhead on bulk/maintenance writes. Keep triggers (§7.2) for tables mutated by many writers you don't control.

7.7 Table Partitioning by Business Use Case

LISTEN/NOTIFY handles the push; partitioning keeps the tables behind the push fast as they grow. Pick the strategy from the access pattern, not from row count alone.

Business use case Partition strategy Why
Event feeds, chat, audit logs (§7.2 A) RANGE by time (monthly/weekly) Reads are "recent N"; old data is dropped/archived by detaching a partition instead of a million-row DELETE
Multi-tenant SaaS state HASH by tenant_id (or LIST for few large tenants) Tenant queries prune to one partition; one noisy tenant's bloat/vacuum doesn't hurt others
Job / notification queues (§7.4) LIST by status (pending vs done) — or just a partial index, see §7.8 The hot pending partition stays tiny and cache-resident no matter how much history accumulates
Metrics snapshots (§7.2 B) Don't partition Single row per metric; partitioning adds overhead with zero pruning benefit

Range-partitioned events table (PG 18, with uuidv7)

CREATE TABLE events (
    id          UUID NOT NULL DEFAULT uuidv7(),
    channel     VARCHAR(64) NOT NULL,
    event_type  VARCHAR(32) NOT NULL,
    payload     JSONB NOT NULL,
    created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
    PRIMARY KEY (id, created_at)            -- partition key must be in the PK
) PARTITION BY RANGE (created_at);

CREATE TABLE events_2026_06 PARTITION OF events
    FOR VALUES FROM ('2026-06-01') TO ('2026-07-01');
CREATE TABLE events_2026_07 PARTITION OF events
    FOR VALUES FROM ('2026-07-01') TO ('2026-08-01');
-- Safety net for out-of-range rows
CREATE TABLE events_default PARTITION OF events DEFAULT;

-- Indexes declared on the parent cascade to every partition
CREATE INDEX idx_events_channel_time ON events (channel, created_at DESC);
CREATE INDEX idx_events_created_brin ON events USING BRIN (created_at);

-- Row-level triggers work on partitioned parents (PG 13+),
-- so the notify trigger from §7.2 attaches unchanged:
CREATE TRIGGER trg_events_notify
    AFTER INSERT ON events
    FOR EACH ROW
    EXECUTE FUNCTION notify_event_insert();

Operational notes:

  • Pruning is the payoff. WHERE channel = $1 AND created_at > NOW() - INTERVAL '1 day' touches only the current partition — this replaces the (invalid) NOW()-based partial index idea from earlier drafts.
  • Retention = detach, not delete. ALTER TABLE events DETACH PARTITION events_2026_06 CONCURRENTLY; then archive/drop. No vacuum debt, no bloat, no notify storm.
  • Automate partition creation with pg_partman or a nightly pg_cron job; always keep next month's partition pre-created plus a DEFAULT partition as a safety net.
  • SSE reconnect replay (WHERE created_at > $since) prunes to 1–2 partitions, so catch-up queries stay fast even with years of history retained.

Hash-partitioned multi-tenant state

CREATE TABLE tenant_state (
    tenant_id   BIGINT NOT NULL,
    key         VARCHAR(128) NOT NULL,
    value       JSONB NOT NULL,
    updated_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
    PRIMARY KEY (tenant_id, key)
) PARTITION BY HASH (tenant_id);

CREATE TABLE tenant_state_p0 PARTITION OF tenant_state FOR VALUES WITH (MODULUS 8, REMAINDER 0);
CREATE TABLE tenant_state_p1 PARTITION OF tenant_state FOR VALUES WITH (MODULUS 8, REMAINDER 1);
-- ... p2 through p7

Every SSE handler query includes tenant_id, so every read prunes to one partition. Pair with per-tenant notify channels (org_{id}_*, §7.5) and you get isolation at both the push layer and the storage layer.

7.8 Partial Index Cookbook for State Tables

Partial indexes are the cheapest performance win for state tables: they index only the hot subset the SSE/UI layer actually queries, so they stay small, cache-resident, and cheap to maintain.

Golden rule: the predicate must be IMMUTABLE — column comparisons against constants, IS NULL, etc. NOW(), CURRENT_DATE, and other volatile functions are rejected by Postgres. For "recent rows", partition by time (§7.7) instead.

-- 1. Unread notifications (the badge count query, fired on every SSE connect)
CREATE INDEX idx_notifs_unread ON user_notifications (user_id)
    INCLUDE (type, title, created_at)        -- covering: index-only scan
    WHERE read_at IS NULL;

-- 2. Pending jobs (the worker's polling query)
CREATE INDEX idx_queue_pending ON notification_queue (created_at)
    WHERE NOT processed;

-- 3. Soft deletes — index only live rows
CREATE INDEX idx_docs_live ON documents (owner_id, updated_at DESC)
    WHERE deleted_at IS NULL;

-- 4. Business rule as a partial UNIQUE index:
--    "a user may have only ONE active subscription"
CREATE UNIQUE INDEX uniq_active_sub ON subscriptions (user_id)
    WHERE status = 'active';

-- 5. Failed items needing retry (error dashboards / dead-letter view)
CREATE INDEX idx_queue_failed ON notification_queue (created_at)
    WHERE NOT processed AND attempts >= 3;

Why these matter for the LISTEN/NOTIFY + SSE flow specifically:

  • Index 1 makes the "send unread count on connect" query in §7.2-D an index-only scan, no matter how many millions of read notifications exist.
  • Index 2 keeps queue polling O(hot rows): a queue with 50M processed rows and 200 pending ones scans an index covering only those 200.
  • Index 4 moves a business invariant out of application code into the schema — exactly the §6 principle that the server (here, the database itself) owns the rules. Inserting a second active subscription fails atomically; no race window.
  • PG 18 skip scan synergy: with (tenant_id, status, created_at) composite indexes, admin queries that omit tenant_id can still use the index — you often need fewer total indexes. Still prefer an explicit partial index for the very hottest predicates.

Sizing check — verify the win:

SELECT relname, pg_size_pretty(pg_relation_size(indexrelid)) AS size, idx_scan
FROM pg_stat_user_indexes
WHERE relname LIKE 'idx_%';

A partial index that's a few MB with a high idx_scan next to a multi-GB full index is the pattern you want. Drop full indexes the partial ones have obsoleted.

7.9 Putting It Together: Partitioned Queue → NOTIFY → SSE

The production-grade composition of everything above — a durable, partition-pruned queue where NOTIFY is only a wake-up signal (so the 8KB limit and lost-notification problem in §7.3 disappear), and the table is the source of truth:

-- Durable outbox, range-partitioned by time, uuidv7 PKs
CREATE TABLE outbox (
    id          UUID NOT NULL DEFAULT uuidv7(),
    channel     VARCHAR(64) NOT NULL,
    payload     JSONB NOT NULL,
    processed   BOOLEAN NOT NULL DEFAULT FALSE,
    attempts    INT NOT NULL DEFAULT 0,
    created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
    PRIMARY KEY (id, created_at)
) PARTITION BY RANGE (created_at);

CREATE TABLE outbox_2026_06 PARTITION OF outbox
    FOR VALUES FROM ('2026-06-01') TO ('2026-07-01');
CREATE TABLE outbox_default PARTITION OF outbox DEFAULT;

-- Hot-path partial index: only unprocessed rows
CREATE INDEX idx_outbox_pending ON outbox (created_at) WHERE NOT processed;

-- Write path: insert + tiny wake-up ping IN THE SAME TRANSACTION
-- (payload is just the channel name — never the data itself)
BEGIN;
INSERT INTO outbox (channel, payload)
VALUES ('orders', jsonb_build_object('order_id', 42, 'event', 'created'));
SELECT pg_notify('outbox_wake', 'orders');
COMMIT;
// Dispatcher: woken by NOTIFY, reads truth from the table.
// FOR UPDATE SKIP LOCKED lets multiple dispatcher instances run safely.
pgListener.on('notification', async () => {
    const { rows } = await pool.query(`
        WITH batch AS (
            SELECT id, created_at FROM outbox
            WHERE NOT processed
            ORDER BY created_at
            LIMIT 100
            FOR UPDATE SKIP LOCKED
        )
        UPDATE outbox o
        SET processed = TRUE
        FROM batch b
        WHERE o.id = b.id AND o.created_at = b.created_at
        RETURNING o.channel, o.payload, o.created_at
    `);

    for (const row of rows) {
        fanOutToSSE(row.channel, row.payload);   // your per-channel connection map (§7.2-D)
    }
});

// Belt-and-suspenders: poll every 5s in case a NOTIFY was missed
setInterval(() => pgListener.emit('notification'), 5000);

Why this composition wins:

Concern (§7.3) How this pattern solves it
8KB NOTIFY limit Payload lives in outbox.payload (JSONB, unlimited); NOTIFY carries only a wake-up ping
Lost notifications Table is durable; the fallback poll + WHERE NOT processed guarantees delivery
Reconnect replay Client sends since; server queries outbox — partition pruning keeps it fast
Queue bloat Old partitions with fully-processed rows are detached and dropped — never DELETEd
Worker contention FOR UPDATE SKIP LOCKED allows horizontal dispatcher scaling with zero double-delivery
Hot-path speed Partial index WHERE NOT processed stays tiny regardless of total history

Checklist for this pattern:

  • NOTIFY payload ≤ channel name + maybe an ID — never domain data
  • Insert + pg_notify in the same transaction (notify fires only on commit)
  • Dispatcher reads the table, not the notification, as truth
  • Fallback poll (3–10s) covers missed wake-ups and dispatcher restarts
  • Monthly partitions pre-created via pg_cron/pg_partman; old ones detached CONCURRENTLY
  • attempts counter + partial index on failures for dead-letter visibility
  • Per-channel SSE fan-out maps remain in app memory (§7.2-D), keyed by the same channel naming convention (§7.5)

7.10 Maximum Performance: Connection Architecture, Batching & Server Tuning

The schema work in §7.2–§7.9 makes individual queries fast. This section makes the system fast under thousands of concurrent SSE streams. The wins, in rough order of impact: get the connection topology right, fan out from one listener, coalesce patches, then tune queries and the server.

1. The fan-out architecture — one listener, N streams (never one connection per browser)

The single biggest mistake is opening a Postgres LISTEN connection per SSE stream. Postgres connections are expensive (≈10MB each, hard max_connections ceiling), so this caps you at a few hundred users and melts the database. Instead: one long-lived connection LISTENs per channel, and the Node process fans each notification out to an in-memory map of SSE subscribers.

// ── ONE shared listener for the whole process ──
import pg from 'pg';
const listener = new pg.Client(process.env.DIRECT_DATABASE_URL); // direct, NOT pooled (see #2)
await listener.connect();

// channel -> Set<http2stream> : the fan-out registry lives in app memory
const subscribers = new Map();

listener.on('notification', (msg) => {
    const streams = subscribers.get(msg.channel);
    if (!streams) return;
    // msg.payload is just an id/wake-up (§7.9) — fan out to every subscriber
    for (const stream of streams) enqueuePatch(stream, msg.payload);
});

async function subscribe(channel, stream) {
    if (!subscribers.has(channel)) {
        subscribers.set(channel, new Set());
        await listener.query(`LISTEN ${pg.escapeIdentifier(channel)}`); // LISTEN once per channel
    }
    subscribers.get(channel).add(stream);
    stream.on('close', () => {
        const set = subscribers.get(channel);
        set.delete(stream);
        if (set.size === 0) {                       // last subscriber gone → stop listening
            subscribers.delete(channel);
            listener.query(`UNLISTEN ${pg.escapeIdentifier(channel)}`);
        }
    });
}

This collapses 10,000 browser streams onto one database connection for notifications. Query traffic (the catch-up reads, the writes) goes through a separate pooled path (#2).

2. Pooling: the LISTEN-vs-transaction-mode gotcha

Route normal query/write traffic through PgBouncer in transaction mode — a pool of ~25 backend connections can serve thousands of clients because each connection is held only for the microseconds of a transaction. But: LISTEN is a session-lifetime feature and is broken by transaction pooling (the backend is handed to another client mid-session). NOTIFY, being a single statement, works fine through the pool.

Rule: the listener from #1 connects directly to Postgres (bypass PgBouncer, or point it at a session-mode port). Everything else — writes that pg_notify, catch-up SELECTs, the dispatcher in §7.9 — goes through transaction-mode PgBouncer.

# pgbouncer.ini — transaction mode for the app's query pool
[pgbouncer]
pool_mode = transaction
default_pool_size = 25          # backend connections per (db,user) — start small, watch saturation
max_client_conn = 10000         # cheap: ~2KB/client in transaction mode
max_prepared_statements = 256   # PgBouncer 1.21+ : enables protocol prepared statements in tx mode
Connection Path Why
The LISTEN listener (one per process) Direct / session-mode LISTEN needs a stable backend
Writes + pg_notify Transaction-mode pool NOTIFY is statement-scoped, pools fine
Catch-up / initial-state reads Transaction-mode pool (or read replica) Short, high-volume

For very high stream counts (>~10k concurrent), consider moving the pub/sub layer to Redis and keeping Postgres as the durable store — but the one-listener pattern above takes you a long way first.

3. Coalesce patches — don't flush per-notification

A burst of writes can fire many NOTIFYs in milliseconds. Pushing one SSE frame per notification floods the client and the network. Buffer per-stream and flush on a short timer (a server-side analog of __debounce, §6.9):

const pending = new Map();  // stream -> Set<id>

function enqueuePatch(stream, id) {
    if (!pending.has(stream)) pending.set(stream, new Set());
    pending.get(stream).add(id);
}

// Flush at ~30fps: one coalesced patch instead of N
setInterval(async () => {
    for (const [stream, ids] of pending) {
        const rows = await pool.query(                 // pooled read (#2)
            `SELECT id, channel, payload FROM events WHERE id = ANY($1)`, [[...ids]]);
        patchElements(stream, renderBatch(rows.rows), { selector: '#feed', mode: 'prepend' });
    }
    pending.clear();
}, 33);

This also batches the catch-up read: one WHERE id = ANY($1) instead of a query per event.

4. Query-side tuning

  • Covering indexes for index-only scans. Add INCLUDE columns so the hot read never touches the heap (§7.8). The badge query, the catch-up query, and the dispatcher poll should all show Index Only Scan in EXPLAIN (ANALYZE, BUFFERS).
  • BRIN over B-tree for append-only time columns. On a partitioned, time-ordered events table, a BRIN index on created_at is kilobytes vs gigabytes and is ideal for the WHERE created_at > $since replay scan: 
    CREATE INDEX idx_events_brin ON events USING BRIN (created_at) WITH (pages_per_range = 32);
  • Prepared statements for the handful of hot queries — parsed/planned once, executed many times. With transaction-mode PgBouncer you need 1.21+ and max_prepared_statements > 0 (above); the pg driver's named queries then work through the pool.
  • Keep fullDocument-style lookups off the hot path: the dispatcher should select only the columns the patch renders, never SELECT *.

5. Write-path tuning (the queue/outbox is high-churn)

  • synchronous_commit = off for the notification/outbox writes. These are recoverable (the table is the source of truth and the worker re-reads), so trading a few ms of durability for a large throughput gain is usually right. Scope it per-transaction, not globally: 
    BEGIN;
SET LOCAL synchronous_commit = off;     -- this txn only
INSERT INTO outbox (channel, payload) VALUES ('orders', $1);
SELECT pg_notify('outbox_wake', 'orders');
COMMIT;
  • HOT updates + fillfactor. The processed = true flip (§7.9) churns the table. Leave free space per page and don't index the processed column with the columns you update, so Postgres can do Heap-Only-Tuple updates that skip index maintenance: 
    ALTER TABLE outbox SET (fillfactor = 80);
  • Aggressive autovacuum on queue tables. High insert+update+delete churn bloats fast; tune per-table so vacuum keeps up: 
    ALTER TABLE outbox SET (
    autovacuum_vacuum_scale_factor = 0.02,   -- vacuum at 2% dead tuples, not the 20% default
    autovacuum_vacuum_cost_delay   = 2
);
  • Prefer DELETE/partition-drop over an ever-growing processed flag. A table that only accumulates processed=true rows bloats indefinitely; either delete in batches or use the time-partition + DETACH retention from §7.7.

6. Server configuration starting points

Tune to your RAM and workload; these are sane defaults for a real-time state box (16GB example):

Setting Value Rationale
shared_buffers 25% RAM (~4GB) Hot index/page cache
effective_cache_size 50–75% RAM Planner's view of OS cache → favors index scans
work_mem 16–64MB Per-sort/hash; raise for catch-up aggregations, watch concurrency
max_connections 100–200 Low — PgBouncer multiplexes; high values waste RAM
wal_compression on Smaller WAL on high write churn
io_method (PG 18) worker Async I/O speeds catch-up scans + vacuum (§7.6)
track_io_timing on So EXPLAIN (BUFFERS) shows real I/O cost

7. Performance checklist (PG)

  • One listener per process, fanning out to in-memory subscribers — never a DB connection per stream
  • Listener connects directly; query/write traffic through transaction-mode PgBouncer
  • Patches coalesced on a ~16–50ms timer; catch-up reads batched with id = ANY($1)
  • Hot reads are Index-Only Scans (verified in EXPLAIN (ANALYZE, BUFFERS))
  • BRIN index on append-only time columns; partition pruning confirmed
  • synchronous_commit = off (scoped) on recoverable queue writes
  • fillfactor + HOT updates on the high-churn flag column; aggressive autovacuum set
  • Retention via partition DETACH or batched DELETE — no unbounded processed table
  • Read replicas absorb catch-up/initial-state reads; primary kept for writes + NOTIFY

7.11 Advanced Optimizations (when §7.10 isn't enough)

These are the heavier tools — reach for them when profiling shows the fundamentals (§7.10) are saturated.

1. Trigger-free change capture: logical decoding & pg_logical_emit_message

Triggers (§7.2) add write-path overhead and run inside every transaction. At high write volume, the production-grade alternative is logical decoding — Postgres already writes every change to the WAL, so you stream that instead of firing triggers. This is the exact PG analog of Mongo change streams (§8), and it's how CDC tools (Debezium, etc.) work.

Two tiers:

(a) pg_logical_emit_message — a wake-up ping with zero table writes. The §7.9 outbox still inserts a row just to signal "something changed." If you only need the signal (the data already lives in its real table), emit a WAL message directly — no table, no bloat, no autovacuum debt:

-- transactional=true → message is only visible if the txn commits (ordering preserved).
-- Consumers read it via logical decoding; nothing is written to any heap table.
SELECT pg_logical_emit_message(true, 'sse', json_build_object('ch','orders','id',42)::text);

(b) Stream the WAL in Node with a logical replication slot. A single replication connection receives every change in commit order, with built-in resumability via the slot's LSN (the PG equivalent of a Mongo resume token):

import { LogicalReplicationService, PgoutputPlugin } from 'pg-logical-replication';

// One slot, one connection — replaces ALL per-table NOTIFY triggers.
const repl = new LogicalReplicationService(
  { connectionString: process.env.DIRECT_DATABASE_URL },
  { acknowledge: { auto: false, timeoutSeconds: 0 } }   // we ack manually after fan-out
);

const plugin = new PgoutputPlugin({ protoVersion: 2, publication: 'sse_pub' });

repl.on('data', (lsn, log) => {
  // log.tag is 'insert' | 'update' | 'delete' | 'message' (for emit_message)
  if (log.tag === 'insert' && log.relation.name === 'events') {
    fanOutToSSE(log.new.channel, log.new);    // your in-memory subscriber map (§7.10 #1)
  }
  repl.acknowledge(lsn);                        // advance the slot only after delivery
});

// publication + slot are created once via SQL:
//   CREATE PUBLICATION sse_pub FOR TABLE events;
//   (the library creates the slot on subscribe)
await repl.subscribe(plugin, 'sse_slot');

Why graduate to this: no trigger overhead on writes, exact commit-order delivery, durable replay from the last acked LSN across restarts, and one connection regardless of table count. The cost: a replication slot that must be consumed — an abandoned slot pins WAL and can fill the disk. Monitor pg_replication_slots.active and lag, and drop slots you no longer use. Keep trigger+NOTIFY (§7.2) for low/medium volume where its simplicity wins.

2. Bulk ingest: COPY and UNNEST instead of N inserts

For high-volume event/outbox ingestion, per-row INSERT is the bottleneck. Two faster paths:

// (a) COPY — the fastest possible ingest path; streams rows in PG's binary/text protocol.
import { from as copyFrom } from 'pg-copy-streams';
const stream = client.query(copyFrom(
  `COPY events (channel, event_type, payload) FROM STDIN WITH (FORMAT csv)`));
for (const e of batch) stream.write(`${e.channel},${e.type},"${jsonCsv(e.payload)}"\n`);
stream.end();   // one round-trip for the whole batch
-- (b) UNNEST multi-row insert — parameterized, set-based, far fewer round-trips than
--     looping single INSERTs. Pass arrays; PG expands them into rows.
INSERT INTO events (channel, event_type, payload)
SELECT * FROM unnest($1::text[], $2::text[], $3::jsonb[]);

COPY wins for large batches (thousands of rows); UNNEST is cleaner for moderate batches and keeps full parameterization. Both produce far fewer WAL records and lock acquisitions than row-by-row inserts.

3. Keyset (seek) pagination — kill OFFSET

OFFSET 10000 LIMIT 50 makes Postgres scan and discard 10,000 rows every page — O(offset). For feeds, infinite scroll (§4.7), and admin tables (§9.9), paginate by the last seen sort key instead — O(1) regardless of depth, and it never skips/duplicates rows when new data arrives mid-scroll:

-- First page:
SELECT id, channel, created_at FROM events
WHERE channel = $1
ORDER BY created_at DESC, id DESC          -- id breaks ties → stable total order
LIMIT 50;

-- Next page: pass the LAST row's (created_at, id) as the cursor.
-- Row-value comparison does the right thing across both sort keys at once.
SELECT id, channel, created_at FROM events
WHERE channel = $1 AND (created_at, id) < ($2, $3)
ORDER BY created_at DESC, id DESC
LIMIT 50;

The client holds only the opaque cursor (created_at, id) of the last row — a perfect fit for the SSE "load more" sentinel (§4.7). Backed by the (channel, created_at DESC) index, each page is an index range scan that starts exactly where the last one stopped.

4. JSONB indexing for payload queries

The payload JSONB columns (§7.2) are opaque to B-tree indexes unless you index into them:

-- (a) Expression B-tree index on a single hot key — for equality/range on one field:
CREATE INDEX idx_events_status ON events (((payload->>'status')));
-- query MUST use the same expression:  WHERE payload->>'status' = 'open'

-- (b) GIN with jsonb_path_ops — for containment (@>) queries; smaller & faster than
--     the default jsonb_ops, at the cost of only supporting @>:
CREATE INDEX idx_events_payload ON events USING GIN (payload jsonb_path_ops);
-- query:  WHERE payload @> '{"priority": "high"}'

Use the expression index when you filter one known key; the jsonb_path_ops GIN when you do flexible containment matching. Avoid the default jsonb_ops GIN unless you need key-existence (?) operators — it's larger.

5. Pre-computed dashboards: materialized views

The PG analog of Mongo's $merge rollup (§8.7). Instead of aggregating on every dashboard read, materialize it and refresh on a schedule — CONCURRENTLY so readers never block (requires a unique index on the matview):

CREATE MATERIALIZED VIEW dashboard_kpis AS
SELECT tenant_id,
       count(*)        AS orders_today,
       sum(total)      AS revenue_today
FROM orders WHERE created_at >= date_trunc('day', now())
GROUP BY tenant_id;

CREATE UNIQUE INDEX ON dashboard_kpis (tenant_id);     -- required for CONCURRENTLY

-- Refresh every 10s via pg_cron; readers keep seeing the old snapshot until it swaps in:
REFRESH MATERIALIZED VIEW CONCURRENTLY dashboard_kpis;

The SSE handler then SELECTs one tiny pre-aggregated row per tenant instead of scanning the orders table per connection.

6. Safety valves (cheap, prevent outages)

-- Per-role or per-transaction: stop a runaway query from pinning a connection forever
SET statement_timeout = '5s';
-- Reclaim connections leaked by clients that BEGIN and vanish (deadly with pooling)
SET idle_in_transaction_session_timeout = '10s';

Set these on the application role (not the replication/listener role, whose long-lived connection is intentional). They turn "one bad query takes down the pool" into "one bad query errors out."

8. MongoDB 8.x + Change Streams + SSE for Real-Time State

Everything §7 does with PostgreSQL — server-owned state pushed to Datastar over SSE — has a MongoDB equivalent. The pub/sub primitive is Change Streams (built on the replica-set oplog), and the data modeling flips from normalized tables to access-pattern-driven documents. MongoDB 8.x adds meaningful performance wins for exactly these workloads: faster reads and bulk writes, time series block processing, and improved $lookup/aggregation throughput.

Prerequisite: Change Streams require a replica set (even a single-node one in dev: mongod --replSet rs0). Plain standalone mongod won't emit them.

8.1 Architecture Overview

┌──────────┐      ┌──────────┐      ┌─────────────────┐      ┌──────────────────┐
│ Browser  │◀─SSE─│ Datastar │◀─────│ Node.js (HTTP/2) │◀─────│ MongoDB 8.x      │
│          │      │ SSE      │      │ collection       │      │ Replica Set      │
│ data-init=      │ parser   │      │  .watch(pipeline)│      │ oplog → change   │
│ @get('/stream') │          │      │  for await (evt) │      │ stream events    │
└──────────┘      └──────────┘      └─────────────────┘      └──────────────────┘

Change Streams vs LISTEN/NOTIFY — what changes architecturally:

Concern PG LISTEN/NOTIFY (§7) MongoDB Change Streams
Trigger mechanism Explicit pg_notify() (trigger or app code) Automatic — every insert/update/delete/replace emits an event; nothing to wire up
Payload limit 8KB → forced "wake-up ping" pattern (§7.9) Full document available (fullDocument), no outbox needed for payload size
Missed events on disconnect Lost — needs outbox table + replay query Resume tokens — watch({resumeAfter: token}) replays from the oplog natively
Filtering One channel string per NOTIFY Aggregation pipeline on the stream: $match on any field of the change event
Fan-out granularity Channel naming conventions $match predicates (fullDocument.userId, operationType, …)
Durability window None (fire-and-forget) Bounded by oplog retention (size/hours) — configure oplogMinRetentionHours

The big win: the "Notification Queue / outbox" machinery from §7.4/§7.9 is largely built in — the oplog is the durable queue, and resume tokens are the replay cursor.

8.2 Document Modeling by Business Use Case (the NoSQL way)

The modeling rule in MongoDB is the inverse of normalization: model for the query the SSE handler will run, not for the entity diagram. Data that is read together lives together.

Use Case A: Live Activity Feed / Chat (Bucket Pattern)

Instead of one row per message (PG §7.2-A), group messages into bucket documents — one document per channel per time window. This cuts index size, makes "load recent history" a 1–2 document read, and keeps documents well under the 16MB cap.

// Collection: message_buckets — one doc per channel per ~100 messages
{
  _id: ObjectId(),                 // ObjectId is time-ordered (like uuidv7 in §7.6)
  channel: "chat_room_42",
  bucketStart: ISODate("2026-06-10T09:00:00Z"),
  count: 37,                       // current bucket fill
  messages: [                      // embedded — read together, stored together
    { id: "m1", authorId: 7, body: "hey", at: ISODate("...") },
    // ...
  ]
}
// Write path: push into the open bucket, roll over at 100 messages.
// upsert:true creates the next bucket atomically when the old one is full.
await db.collection('message_buckets').updateOne(
    { channel, count: { $lt: 100 } },                       // open bucket only
    {
        $push: { messages: msg },
        $inc:  { count: 1 },
        $setOnInsert: { channel, bucketStart: new Date() }, // fields for a NEW bucket
    },
    { upsert: true }
);

// Read path (SSE connect / history): newest 2 buckets = last ~200 messages, ONE index hit
const history = await db.collection('message_buckets')
    .find({ channel })
    .sort({ bucketStart: -1 })
    .limit(2)
    .toArray();
// Index: equality on channel, range/sort on bucketStart (ESR rule, §8.4)
db.message_buckets.createIndex({ channel: 1, bucketStart: -1 });

Use Case B: Live Dashboard KPIs (Single-Doc Snapshot + Computed Pattern)

The PG metrics table (§7.2-B) becomes one document per metric — or one document per dashboard if the widgets are always read together (the "computed pattern": store the aggregate, update it on write, never compute it on read):

// Collection: dashboards — ONE doc holds everything the dashboard SSE stream needs
{
  _id: "ops_dashboard",
  kpis: {
    ordersToday:   1284,
    revenueToday:  93211.50,
    activeUsers:   412,
    errorRate:     0.012
  },
  updatedAt: ISODate("...")
}

// Writers increment in place — atomic, no read-modify-write race
await db.collection('dashboards').updateOne(
    { _id: 'ops_dashboard' },
    { $inc: { 'kpis.ordersToday': 1, 'kpis.revenueToday': order.total },
      $currentDate: { updatedAt: true } }
);

One change-stream watcher on this single document feeds every connected dashboard — and because the change event's updateDescription.updatedFields contains only the fields that changed, it maps 1:1 onto a Datastar signal patch (see §8.3).

Use Case C: Collaborative Document Ops (Append-Only Op Log)

Same shape as PG §7.2-C — an op log is inherently relational-ish, so the document model stays flat. Use findOneAndUpdate on a version counter for the monotonic sequence:

// Collection: doc_ops
{ _id: ObjectId(), docId: UUID(), version: 412, opType: "insert",
  position: 1031, content: "x", authorId: 7, at: ISODate("...") }

// Unique index enforces one writer per version (optimistic concurrency)
db.doc_ops.createIndex({ docId: 1, version: 1 }, { unique: true });

Use Case D: Per-User Notifications (Outbox Collection + Partial Index)

// Collection: notifications
{ _id: ObjectId(), userId: 99, type: "mention", title: "…",
  body: "…", data: {}, readAt: null, createdAt: ISODate("...") }
// Partial index — Mongo's exact analog of PG partial indexes (§7.8):
// indexes ONLY unread docs, so the badge-count query stays O(unread)
db.notifications.createIndex(
    { userId: 1, createdAt: -1 },
    { partialFilterExpression: { readAt: null } }
);

// TTL index — the analog of dropping old PG partitions (§7.7):
// Mongo deletes read notifications 90 days after readAt, automatically
db.notifications.createIndex(
    { readAt: 1 },
    { expireAfterSeconds: 60 * 60 * 24 * 90,
      partialFilterExpression: { readAt: { $type: 'date' } } }
);

Embed vs reference — the 30-second decision table:

Relationship Model Example
Read together, bounded size Embed messages in a bucket, line items in an order
Read together, unbounded growth Bucket chat history, sensor readings
Shared across many parents Reference authorId → users collection
Aggregate needed at read time Computed pattern (store it) unread counts, dashboard KPIs
Many shapes, one stream Polymorphic (type field + shared base) the notification collection above

8.3 Change Streams → Datastar SSE (Node.js HTTP/2)

The full pipeline: filter server-side in the database, resume on reconnect, patch signals/elements per event. Uses the HTTP/2 helpers from §13.5.

import { MongoClient } from 'mongodb';
import { route, sseHead, patchSignals, patchElements } from './server.mjs';

const mongo = await MongoClient.connect(process.env.MONGO_URL);
const db = mongo.db('app');

route('GET', '/api/notifications/stream', async (stream, headers) => {
    const userId = await authenticate(headers);          // §4.11 session cookie
    sseHead(stream);

    // 1. Initial state: unread count via the partial index (covered query)
    const unread = await db.collection('notifications')
        .countDocuments({ userId, readAt: null });
    patchSignals(stream, { unreadCount: unread });

    // 2. Resume support: Datastar's SSE client sends Last-Event-ID on reconnect.
    //    We store the change stream's resume token there → zero missed events.
    const lastEventId = headers['last-event-id'];
    const resumeOpts = lastEventId ? { resumeAfter: JSON.parse(lastEventId) } : {};

    // 3. Watch ONLY this user's inserts — the $match runs inside MongoDB,
    //    so the app process never sees other users' traffic
    const changeStream = db.collection('notifications').watch(
        [{ $match: {
            operationType: 'insert',
            'fullDocument.userId': userId,
        }}],
        { fullDocument: 'updateLookup', ...resumeOpts }
    );

    const heartbeat = setInterval(() => stream.write(': heartbeat\n\n'), 30_000);

    try {
        for await (const change of changeStream) {
            const n = change.fullDocument;
            // Emit the resume token as the SSE event id → browser echoes it back
            stream.write(`id: ${JSON.stringify(change._id)}\n`);
            patchSignals(stream, { unreadCount: { $inc: 1 } } /* or send absolute count */);
            patchElements(stream,
                `<div id="notif-${n._id}" class="p-3 border-b">
                    <p class="text-sm font-medium">${escapeHtml(n.title)}</p>
                 </div>`,
                { selector: '#notif-dropdown', mode: 'prepend' }
            );
        }
    } finally {
        clearInterval(heartbeat);
        await changeStream.close();
    }

    stream.on('close', () => changeStream.close());      // h2 disconnect → release cursor
});

Change-stream options that matter for SSE backends:

Option Use it for
fullDocument: 'updateLookup' Get the whole post-update document on updates (default gives only the delta)
fullDocumentBeforeChange: 'whenAvailable' Pre-images — build "old → new" diff payloads (analog of PG 18 RETURNING OLD/NEW, §7.6); requires changeStreamPreAndPostImages enabled on the collection
resumeAfter / startAfter Replay missed events after disconnect (use startAfter to survive invalidate events)
Pipeline $match Per-user / per-tenant fan-out filtering inside the database
Pipeline $project Strip large fields before they cross the wire to the app server

Mapping update events to Datastar signal patches — updateDescription.updatedFields is already shaped like a JSON Merge Patch, so live KPI dashboards (§8.2-B) are nearly free:

const kpiStream = db.collection('dashboards').watch(
    [{ $match: { 'documentKey._id': 'ops_dashboard', operationType: 'update' } }]
);
for await (const change of kpiStream) {
    // e.g. {"kpis.ordersToday": 1285, "kpis.revenueToday": 93294.0}
    const flat = change.updateDescription.updatedFields;
    const nested = unflatten(flat);          // {"kpis": {"ordersToday": 1285, ...}}
    broadcastToAllDashboardStreams(s => patchSignals(s, nested));
}

8.4 Optimizing MongoDB 8.x for State Workloads

Indexes: the ESR rule

Order compound index keys Equality → Sort → Range, matching the SSE handler's hot query:

// Query: {channel: X} sorted by bucketStart desc, range on date
db.message_buckets.createIndex({ channel: 1, bucketStart: -1 });   // E, then S/R

// Multi-tenant: tenant equality first, ALWAYS
db.orders.createIndex({ tenantId: 1, status: 1, createdAt: -1 });

Partial indexes (cookbook — mirrors §7.8)

// Unread badges
db.notifications.createIndex({ userId: 1 }, { partialFilterExpression: { readAt: null } });

// Pending jobs only
db.jobs.createIndex({ createdAt: 1 }, { partialFilterExpression: { status: 'pending' } });

// Business rule as partial UNIQUE index: one active subscription per user
db.subscriptions.createIndex(
    { userId: 1 },
    { unique: true, partialFilterExpression: { status: 'active' } }
);

Same caveat as PG: the query must include the filter predicate (readAt: null, status: 'pending') for the planner to pick the partial index.

Retention: TTL indexes instead of partition drops

Where PG detaches old partitions (§7.7), Mongo expires documents declaratively:

// Raw events: keep 30 days, then auto-delete (background, batched)
db.events.createIndex({ createdAt: 1 }, { expireAfterSeconds: 60 * 60 * 24 * 30 });

For very high-volume telemetry, prefer a time series collection — MongoDB 8.0's block processing makes aggregations over them dramatically faster, and they bucket + compress automatically:

db.createCollection('device_metrics', {
    timeseries: { timeField: 'ts', metaField: 'device', granularity: 'seconds' },
    expireAfterSeconds: 60 * 60 * 24 * 7,
});

Sharding: the partitioning analog

PG strategy (§7.7) Mongo equivalent
HASH by tenant_id sh.shardCollection('app.tenant_state', { tenantId: 'hashed' })
RANGE by time Ranged shard key {channel: 1, ts: 1} (never time alone — hot shard!)
LIST by status (hot/cold) Don't shard for this — use a partial index; status is too low-cardinality for a shard key

MongoDB 8.x makes resharding and moving collections between shards substantially cheaper (moveCollection/reshard improvements), so a wrong early shard-key choice is no longer fatal — but tenantId-hashed remains the safe default for SaaS state.

Write-path discipline

  • Atomic field operators over replace: $inc, $push, $set on paths — never read-modify-write whole documents from app code (race conditions, fat oplog entries, fat change events).
  • bulkWrite for batches — 8.0's bulk insert path is significantly faster; one round-trip per batch also means fewer change-stream wakeups downstream.
  • writeConcern: 'majority' for anything the UI confirms (the SSE patch must reflect durable truth — §6.1).

8.5 Choosing: PostgreSQL (§7) vs MongoDB (§8)

Signal in your requirements Lean PG Lean Mongo
Multi-entity transactions, constraints, reporting SQL ✅
Schema known and stable; heavy joins ✅
Documents naturally nested; per-tenant flexible shapes ✅
Real-time fan-out is the core feature; replay/resume must be robust ✅ (resume tokens beat hand-rolled outbox)
Team already runs one of them ✅ use what you run ✅ use what you run
Business rules as schema (temporal constraints, FKs) ✅ (§7.6)
High-volume telemetry/time series with TTL ✅ (time series + block processing)

Both end at the same place: a durable store the server owns, a change feed, and SSE patches to Datastar. The client code in this guide is identical either way — that's the payoff of server-first state (§6).

8.6 MongoDB + Datastar Integration Checklist

  • Replica set enabled (change streams require it), even in dev
  • Every change stream has a $match pipeline — never watch a whole busy collection unfiltered per user
  • Resume tokens emitted as SSE id: lines; Last-Event-ID honored on reconnect
  • oplogMinRetentionHours sized to survive your longest plausible client disconnect
  • Partial indexes on every hot subset (unread, pending, active)
  • TTL indexes or time series collections for retention — no cron deletes
  • Writers use atomic field operators; documents never round-trip through app code
  • One change-stream cursor per channel/topic, fanned out to N SSE streams in app memory — not one cursor per browser tab
  • stream.on('close') closes the cursor — leaked change streams hold oplog cursors open

8.7 Maximum Performance: Change-Stream Tuning, Read Scaling & Server Config

§8.2–§8.4 made documents and indexes fast. This section makes the real-time pipeline fast at scale. Biggest levers first: trim the change-stream payload, fan out from one cursor, offload reads to secondaries, and pre-compute aggregates.

1. Trim the change-stream payload at the source

Every byte a change event carries crosses the wire to the app server and costs latency. Two big knobs:

  • Skip updateLookup when you only need the delta. fullDocument: 'updateLookup' does an extra read per event to fetch the whole post-update document. For dashboards and counters you usually only need updateDescription.updatedFields — which is already shaped like a JSON Merge Patch for Datastar (§8.3). Drop the lookup and save a read per event: 
    // Counter/dashboard stream: NO updateLookup → no extra read, tiny payload
const stream = db.collection('dashboards').watch(
  [{ $match: { operationType: 'update' } }]
  // (no fullDocument option)
);
for await (const c of stream) patchSignals(broadcast, unflatten(c.updateDescription.updatedFields));
  • $project inside the pipeline to strip large fields before they leave MongoDB, when you do need the document: 
    db.collection('orders').watch([
  { $match: { 'fullDocument.tenantId': tenantId } },
  { $project: { 'fullDocument.id': 1, 'fullDocument.status': 1, 'fullDocument.total': 1, ns: 1, operationType: 1 } },
], { fullDocument: 'updateLookup' });

2. Cursor batching: latency vs throughput

The change-stream cursor batches events. Tune the trade-off explicitly:

db.collection('events').watch(pipeline, {
    batchSize: 100,          // events per getMore — bigger = higher throughput, more latency
    maxAwaitTimeMS: 100,     // max wait before returning a partial batch — caps tail latency
});

Pair this with the same server-side patch coalescing as PG (§7.10 #3): buffer events per SSE stream and flush one combined patch every ~16–50ms instead of one frame per event.

3. Fan out from one cursor (never one change stream per browser)

A change stream holds an open oplog cursor; one per browser tab will exhaust cursors and oplog read capacity. Open one cursor per channel/topic and fan out to in-memory SSE subscribers — the exact mirror of §7.10 #1:

const subscribers = new Map();                  // channel -> Set<stream>

function watchChannel(channel) {
    const cs = db.collection('events').watch(
        [{ $match: { 'fullDocument.channel': channel } }],
        { batchSize: 100, maxAwaitTimeMS: 100 });
    (async () => {
        for await (const change of cs) {
            for (const s of subscribers.get(channel) ?? []) enqueuePatch(s, change);
        }
    })();
    return cs;
}

4. Read scaling: offload to secondaries

The change stream and writes belong on the primary, but history loads and initial-state reads (fired on every SSE connect) can read from secondaries, taking that traffic off the primary entirely:

// Initial unread count / history: read from a secondary if available
const unread = await db.collection('notifications')
    .countDocuments({ userId, readAt: null },
                    { readPreference: 'secondaryPreferred' });

Use secondaryPreferred (not secondary) so it still works on a single-node dev replica set. Reserve 'majority' read concern for reads that must reflect confirmed writes; default 'local'/'available' is faster for fan-out reads where slight staleness is fine.

5. Pre-compute dashboards — materialized views via $merge

Don't run an aggregation pipeline on every dashboard read or every change event. Compute it on a schedule (or incrementally) into a materialized collection, then watch that tiny collection:

// Scheduled (e.g. every 10s) rollup → one doc per dashboard
await db.collection('orders').aggregate([
    { $match: { createdAt: { $gte: startOfDay } } },
    { $group: { _id: '$tenantId',
                ordersToday: { $sum: 1 },
                revenueToday: { $sum: '$total' } } },
    { $merge: { into: 'dashboard_rollups', on: '_id',
                whenMatched: 'merge', whenNotMatched: 'insert' } },
]).toArray();

// SSE handlers watch the small rollup collection, not the huge orders collection
db.collection('dashboard_rollups').watch([{ $match: { 'documentKey._id': tenantId } }]);

Reads become a single-document fetch; the change stream watches a collection with one doc per tenant instead of millions of orders.

6. Covered queries

A query is covered (served entirely from the index, never touching documents) when every field it needs is in the index and _id is excluded from the projection. Make the hot SSE reads covered:

db.notifications.createIndex({ userId: 1, createdAt: -1, type: 1 });  // includes projected fields

db.notifications.find({ userId },
    { projection: { _id: 0, type: 1, createdAt: 1 } })   // only indexed fields, _id off
    .sort({ createdAt: -1 }).limit(20);                   // → COVERED, IXSCAN with no FETCH

Confirm with .explain('executionStats') — a covered query shows totalDocsExamined: 0.

7. Write-path & durability tuning

  • Atomic operators + bulkWrite (already in §8.4): $inc/$push/$set produce tiny oplog entries → tiny change events. Whole-document replaces produce fat events that slow every downstream stream.
  • Write concern trade-off: w: 'majority' for anything the UI confirms (the SSE patch must reflect durable truth, §6.1); w: 1 is acceptable for high-volume telemetry where occasional loss is tolerable.
  • $inc over read-modify-write avoids both races and the extra round-trip.

8. Server & driver configuration

Setting Guidance
WiredTiger cache Defaults to 50% of (RAM − 1GB). Keep the working set (hot indexes + docs) in cache; sustained cache pressure means scale RAM or sharding (§8.4).
maxPoolSize (driver) Default 100. Size to concurrent in-flight ops, not user count — the fan-out model (#3) means few DB ops per user.
Compression zstd block compression for storage-heavy collections; snappy (default) for lower CPU.
oplogMinRetentionHours Size the oplog to survive your longest plausible disconnect so resume tokens (§8.3) stay valid.
Sharded change streams 8.0 improves sharded change-stream throughput; for very high write volume, a good shard key (tenant-hashed, §8.4) also parallelizes the change feed.

9. Performance checklist (Mongo)

  • Drop updateLookup where only updatedFields is needed — no extra read per event
  • $project in the pipeline trims payloads before they leave the database
  • batchSize + maxAwaitTimeMS tuned; patches coalesced on a ~16–50ms timer
  • One change-stream cursor per channel, fanned out in app memory — never one per tab
  • Initial/history reads use secondaryPreferred to offload the primary
  • Dashboards pre-aggregated via $merge; streams watch the small rollup collection
  • Hot reads are covered (_id: 0, only indexed fields; totalDocsExamined: 0)
  • Atomic operators + bulkWrite; write concern matched to UI-confirmation needs
  • Working set fits WiredTiger cache; oplog sized for resume; shard key parallelizes writes

8.8 Advanced Optimizations (when §8.7 isn't enough)

Heavier MongoDB 8.x levers for when payload-trimming and read-offloading (§8.7) have been exhausted.

1. Clustered collections for time-ordered data

A normal collection stores documents in arbitrary heap order and keeps a separate _id index. A clustered collection stores documents physically ordered by the clustered key (the _id) and has no separate _id index — so range scans on _id read sequential storage, and you save the index's RAM/disk. For append-only, time-ordered collections (events, outbox, messages) keyed by a time-sortable _id (ObjectId or uuidv7-style), this is a large win:

db.createCollection('events', {
  clusteredIndex: { key: { _id: 1 }, unique: true, name: 'events_clustered' }
});
// Range scans like { _id: { $gt: lastId } } now walk contiguous storage — ideal for
// the SSE catch-up/replay query and keyset pagination (#2 below).

Caveat: only _id can be the clustered key, and secondary indexes carry the full _id as a pointer (slightly larger). Worth it precisely for time-series-shaped collections; not for write-anywhere document stores.

2. Keyset pagination — kill .skip()

.skip(10000).limit(50) walks and discards 10,000 documents per page — O(skip), same trap as SQL OFFSET. Page by the last seen sort key instead, O(1) per page and stable under concurrent inserts:

// First page
const page = await db.collection('events')
  .find({ channel })
  .sort({ _id: -1 }).limit(50).toArray();

// Next page: pass the LAST document's _id as the cursor
const next = await db.collection('events')
  .find({ channel, _id: { $lt: lastId } })   // seek, don't skip
  .sort({ _id: -1 }).limit(50).toArray();

On a clustered collection (#1) or any _id-sorted query this is a contiguous range read. For compound sorts, carry a compound cursor: {$or: [{createdAt: {$lt: c}}, {createdAt: c, _id: {$lt: id}}]}.

3. Handle >16MB change events: $changeStreamSplitLargeEvent

A change event must fit in 16MB BSON. With fullDocument + fullDocumentBeforeChange on large docs, an update can exceed it and throw BSONObjectTooLarge. First reduce the event (drop updateLookup, $project only needed fields — §8.7). If you genuinely need full pre/post images on large docs, split the event into fragments (MongoDB 7.0+); it must be the last pipeline stage:

const cs = db.collection('orders').watch(
  [
    { $match: { operationType: 'update' } },
    { $changeStreamSplitLargeEvent: {} },     // MUST be last; only one allowed
  ],
  { fullDocument: 'whenAvailable', fullDocumentBeforeChange: 'whenAvailable' }
);
for await (const evt of cs) {
  // evt.splitEvent = { fragment: n, of: total } when an event was split.
  // Reassemble fragments by resume-token order before patching SSE.
  if (evt.splitEvent) accumulateFragment(evt);
  else fanOut(evt);
}

Requires changeStreamPreAndPostImages: { enabled: true } on the collection for pre-images.

4. Pre-images for diff payloads (the Mongo RETURNING OLD/NEW)

To send the client a true "from → to" diff (§7.6's PG analog), capture the pre-image:

db.runCommand({ collMod: 'orders', changeStreamPreAndPostImages: { enabled: true } });

const cs = db.collection('orders').watch(
  [{ $match: { operationType: 'update' } }],
  { fullDocumentBeforeChange: 'whenAvailable', fullDocument: 'updateLookup' });

for await (const e of cs) {
  const diff = { from: e.fullDocumentBeforeChange.status, to: e.fullDocument.status };
  patchElements(streamsFor(e.fullDocument.id), renderStatusChange(diff), { mode: 'inner' });
}

Pre-images cost storage and are pruned by expireAfterSeconds on the pre-images collection — enable only where you need diffs.

5. Aggregation discipline (planner-friendly pipelines)

The aggregation planner can only optimize what you let it. Rules that matter for SSE-backing queries:

db.orders.aggregate([
  { $match: { tenantId, status: 'open' } },  // FIRST — uses the index, shrinks the set early
  { $sort:  { createdAt: -1 } },             // index-backed sort; with $limit → top-k, no full sort
  { $limit: 20 },                            // keep $limit RIGHT AFTER $sort so it's a bounded scan
  { $project: { _id: 0, id: 1, total: 1 } }, // trim fields LAST, after the set is small
]);
  • Put $match and the index-backed $sort + $limit first so MongoDB does a bounded index scan, not a blocking in-memory sort of the whole collection.
  • Avoid allowDiskUse: true as a crutch — if a pipeline needs it, an index is usually missing. Use it only for legitimately large analytical rollups.
  • Force a known-good plan with .hint('idx_name') when the planner mis-picks on a skewed collection.

6. Server-side running totals & rankings: $setWindowFields

Don't ship rows to the client to compute running totals or leaderboards (§6.2 — domain math is server-side). $setWindowFields does cumulative/ranked computations in one pass, then you patch the result:

db.trades.aggregate([
  { $match: { tenantId } },
  { $setWindowFields: {
      partitionBy: '$symbol',
      sortBy: { ts: 1 },
      output: {
        runningVolume: { $sum: '$qty',
          window: { documents: ['unbounded', 'current'] } },   // cumulative
        rank: { $denseRank: {} }
      }
  }},
]);

7. Unordered bulk writes for parallelism

bulkWrite defaults to ordered: true (stops at the first error, executes sequentially). For independent writes (event ingestion, fan-in), ordered: false lets the server apply them in parallel and continue past individual failures:

await db.collection('events').bulkWrite(ops, { ordered: false });   // parallel, fault-tolerant

Pair with driver-level compressors: ['zstd'] and a maxPoolSize sized to concurrent ops (not user count — the fan-out model means few ops per user, §8.7).

9. Mobile & Admin Dashboard Component Patterns

Ready-to-adapt Datastar + Tailwind patterns for the two UI families most apps ship: a mobile-first website/PWA and an admin dashboard. Every example follows the server-first rules (§6) and the action budget (§6.9), and is commented inline.

9.1 Mobile App Shell: Bottom Tab Bar + Screen Container

<!-- The shell owns two signals:
     ui.tab        → which tab is visually active (client-side, instant feedback)
     ui.isLoading  → cleared by the server when the screen patch arrives -->
<body data-signals="{ui: {tab: 'home', isLoading: false}}"
      class="min-h-dvh bg-gray-50 pb-16"> <!-- pb-16 = space for the fixed tab bar -->

  <!-- Screen container: the server patches whole screens into #screen
       with mode:inner (§13.7). The client never assembles screens itself. -->
  <main id="screen" class="px-4 pt-4">
    <!-- server-rendered initial screen lives here -->
  </main>

  <!-- Top progress bar: visible while any navigation is in flight -->
  <div data-show="$ui.isLoading"
       class="fixed top-0 inset-x-0 h-0.5 bg-blue-600 animate-pulse z-50"></div>

  <!-- Bottom tab bar: fixed, safe-area aware for iOS notch devices -->
  <nav class="fixed bottom-0 inset-x-0 bg-white border-t flex justify-around
              pb-[env(safe-area-inset-bottom)] z-40">
    <!-- Each tab: 1) flips the active state instantly (pure UI chrome → client-side, §6.9)
                   2) fires ONE @get; the server patches #screen + clears isLoading -->
    <button data-on:click="$ui.tab = 'home'; $ui.isLoading = true; @get('/screen/home')"
            data-class="{'text-blue-600': $ui.tab === 'home', 'text-gray-400': $ui.tab !== 'home'}"
            class="flex flex-col items-center gap-0.5 py-2 px-4 text-xs">
      <svg class="w-6 h-6"><!-- home icon --></svg>
      Home
    </button>

    <button data-on:click="$ui.tab = 'search'; $ui.isLoading = true; @get('/screen/search')"
            data-class="{'text-blue-600': $ui.tab === 'search', 'text-gray-400': $ui.tab !== 'search'}"
            class="flex flex-col items-center gap-0.5 py-2 px-4 text-xs">
      <svg class="w-6 h-6"><!-- search icon --></svg>
      Search
    </button>

    <button data-on:click="$ui.tab = 'inbox'; $ui.isLoading = true; @get('/screen/inbox')"
            data-class="{'text-blue-600': $ui.tab === 'inbox', 'text-gray-400': $ui.tab !== 'inbox'}"
            class="relative flex flex-col items-center gap-0.5 py-2 px-4 text-xs">
      <svg class="w-6 h-6"><!-- inbox icon --></svg>
      Inbox
      <!-- Live unread badge: $unreadCount is pushed by the SSE stream (§7.2-D / §8.3).
           The client only DISPLAYS it — the count is computed server-side. -->
      <span data-show="$unreadCount > 0"
            data-text="$unreadCount > 99 ? '99+' : $unreadCount"
            class="absolute -top-0.5 right-2 bg-red-500 text-white text-[10px]
                   rounded-full min-w-4 h-4 px-1 flex items-center justify-center"></span>
    </button>
  </nav>
</body>

9.2 Pull-to-Refresh

<!-- Touch-driven pull-to-refresh on a feed.
     Signals: pull.y      → current drag distance (purely visual)
              pull.active → finger is down at scrollTop 0
              pull.refreshing → request in flight (single-flight guard, §6.9) -->
<div data-signals="{pull: {y: 0, startY: 0, active: false, refreshing: false}}"

     data-on:touchstart="
       /* Only arm the gesture when the list is scrolled to the very top */
       if (el.scrollTop === 0) {
         $pull.active = true;
         $pull.startY = evt.touches[0].clientY;
       }"

     data-on:touchmove="
       if ($pull.active && !$pull.refreshing) {
         /* Dampen the drag (x0.4) so it feels rubbery, cap at 80px */
         $pull.y = Math.min(80, Math.max(0, (evt.touches[0].clientY - $pull.startY) * 0.4));
       }"

     data-on:touchend="
       if ($pull.y > 60 && !$pull.refreshing) {
         /* Past the threshold → exactly ONE refresh request */
         $pull.refreshing = true;
         @get('/api/feed/refresh');   /* server patches #feed and sets pull.refreshing=false */
       } else {
         $pull.y = 0;                 /* below threshold → snap back, no request */
       }
       $pull.active = false;"

     class="h-dvh overflow-y-auto overscroll-y-contain">

  <!-- Pull indicator: height tracks the drag, spinner appears past the threshold -->
  <div class="flex items-center justify-center overflow-hidden transition-[height]"
       data-style:height="($pull.refreshing ? 56 : $pull.y) + 'px'">
    <svg data-class="{'animate-spin': $pull.refreshing}"
         data-style:transform="'rotate(' + ($pull.y * 3) + 'deg)'"
         class="w-5 h-5 text-gray-400"><!-- refresh icon --></svg>
  </div>

  <!-- The feed itself: server prepends fresh items, then resets the pull signals -->
  <div id="feed" class="divide-y">
    <!-- server-rendered feed items -->
  </div>
</div>

Server response that completes the gesture:

event: datastar-patch-elements
data: selector #feed
data: mode prepend
data: elements <article id="post-991" class="p-4">…new item…</article>

event: datastar-patch-signals
data: signals {"pull": {"y": 0, "refreshing": false}}

9.3 Swipe-to-Action List Item (Archive / Delete)

<!-- Each row tracks its own swipe offset in a scoped signal (one per row id).
     Revealing the buttons is pure UI; the destructive action is ONE server call. -->
<li id="mail-204"
    data-signals="{swipe204: {x: 0, startX: 0}}"
    class="relative overflow-hidden bg-white">

  <!-- Action layer: sits UNDER the content, revealed as content slides left -->
  <div class="absolute inset-y-0 right-0 flex">
    <button data-on:click="@post('/api/mail/204/archive')"
            class="bg-blue-500 text-white px-5 text-sm">Archive</button>
    <!-- Server responds with mode:remove on #mail-204 — the row deletes itself
         only AFTER the server confirms (§6.2, no optimistic destruction) -->
    <button data-on:click="@delete('/api/mail/204')"
            class="bg-red-500 text-white px-5 text-sm">Delete</button>
  </div>

  <!-- Content layer: translated by the swipe -->
  <div data-on:touchstart="$swipe204.startX = evt.touches[0].clientX"
       data-on:touchmove="
         /* Allow leftward swipe only, clamp to the action-tray width (-144px) */
         $swipe204.x = Math.max(-144, Math.min(0,
             evt.touches[0].clientX - $swipe204.startX + $swipe204.x))"
       data-on:touchend="
         /* Snap: fully open past half-way, otherwise fully closed */
         $swipe204.x = $swipe204.x < -72 ? -144 : 0"
       data-style:transform="'translateX(' + $swipe204.x + 'px)'"
       class="relative bg-white p-4 transition-transform duration-150">
    <p class="font-medium text-sm">Invoice #1042 is ready</p>
    <p class="text-xs text-gray-500 truncate">Your June invoice has been generated…</p>
  </div>
</li>

9.4 Bottom Sheet (Mobile Modal)

<!-- ui.sheet holds WHICH sheet is open (null = none) — one signal drives all sheets.
     Content is patched in by the server so the sheet never ships stale data. -->
<div data-signals="{ui: {sheet: null}}">

  <!-- Trigger: open instantly (UI chrome), fetch content in the same tap -->
  <button data-on:click="$ui.sheet = 'filters'; @get('/api/sheets/filters')"
          class="px-4 py-2 border rounded-lg text-sm">Filters</button>

  <!-- Backdrop: tap anywhere outside to dismiss (no server call — closing is free) -->
  <div data-show="$ui.sheet"
       data-on:click="$ui.sheet = null"
       class="fixed inset-0 bg-black/40 z-40 transition-opacity"></div>

  <!-- The sheet: slides up from the bottom; __stop keeps backdrop clicks from
       bubbling out of the panel and closing it accidentally -->
  <div data-show="$ui.sheet"
       data-on:click__stop=""
       class="fixed bottom-0 inset-x-0 z-50 bg-white rounded-t-2xl shadow-2xl
              max-h-[85dvh] overflow-y-auto pb-[env(safe-area-inset-bottom)]">

    <!-- Grab handle (decorative) + swipe-down-to-close -->
    <div data-on:touchmove="if (evt.touches[0].clientY - evt.target.dataset.y > 80) $ui.sheet = null"
         data-on:touchstart="evt.target.dataset.y = evt.touches[0].clientY"
         class="flex justify-center py-3">
      <div class="w-10 h-1 bg-gray-300 rounded-full"></div>
    </div>

    <!-- Server patches sheet content here, keyed by sheet name -->
    <div id="sheet-content" class="px-5 pb-6">
      <div class="h-24 animate-pulse bg-gray-100 rounded-lg"></div> <!-- skeleton, §9.5 -->
    </div>
  </div>
</div>

9.5 Skeleton Loading States

<!-- Pattern: render the skeleton INSIDE the target container in the initial HTML.
     The server's element patch replaces it wholesale — no isLoading branching
     needed in the markup, and no flash of empty content (§6.7). -->
<div id="profile-card"
     data-init="@get('/api/profile/card')"
     class="bg-white rounded-xl border p-5">

  <!-- Skeleton: mirrors the real card's geometry so the patch doesn't jump -->
  <div class="animate-pulse space-y-3">
    <div class="flex items-center gap-3">
      <div class="w-12 h-12 rounded-full bg-gray-200"></div>  <!-- avatar -->
      <div class="space-y-2 flex-1">
        <div class="h-3 bg-gray-200 rounded w-1/3"></div>     <!-- name line -->
        <div class="h-2.5 bg-gray-100 rounded w-1/4"></div>   <!-- role line -->
      </div>
    </div>
    <div class="h-2.5 bg-gray-100 rounded"></div>             <!-- bio lines -->
    <div class="h-2.5 bg-gray-100 rounded w-5/6"></div>
  </div>
</div>

9.6 Segmented Control + Filter Chips

<!-- filters.* are draft client state (§6.4: form/filter namespace).
     Every change funnels into ONE debounced server call on the container —
     not one call per control (§6.9 coalescing). -->
<div data-signals="{filters: {range: '7d', tags: []}}"
     data-on-signal-patch__debounce.250ms="@get('/api/analytics')"
     data-on-signal-patch-filter="{include: /^filters\./}">  <!-- only refetch when filters.* change, not on the response's own patches -->

  <!-- Segmented control: classic iOS-style range picker -->
  <div class="inline-flex bg-gray-100 rounded-lg p-1 text-sm">
    <!-- Each segment just sets the signal; the container's listener does the I/O -->
    <button data-on:click="$filters.range = '24h'"
            data-class="{'bg-white shadow rounded-md': $filters.range === '24h'}"
            class="px-3 py-1">24h</button>
    <button data-on:click="$filters.range = '7d'"
            data-class="{'bg-white shadow rounded-md': $filters.range === '7d'}"
            class="px-3 py-1">7d</button>
    <button data-on:click="$filters.range = '30d'"
            data-class="{'bg-white shadow rounded-md': $filters.range === '30d'}"
            class="px-3 py-1">30d</button>
  </div>

  <!-- Multi-select chips: toggle membership in the tags array -->
  <div class="flex gap-2 mt-3 flex-wrap">
    <button data-on:click="$filters.tags = $filters.tags.includes('web')
              ? $filters.tags.filter(t => t !== 'web') : [...$filters.tags, 'web']"
            data-class="{'bg-blue-600 text-white border-blue-600': $filters.tags.includes('web'),
                         'bg-white text-gray-600': !$filters.tags.includes('web')}"
            class="px-3 py-1 rounded-full border text-xs">Web</button>
    <!-- repeat per tag… or render chips server-side from the taxonomy -->
  </div>

  <!-- Server patches the chart/table for the chosen filters -->
  <div id="analytics-panel" class="mt-4"></div>
</div>

9.7 Admin Shell: Collapsible Sidebar + Topbar

<!-- ui.sidebarOpen: desktop collapse state.  ui.mobileNav: drawer on small screens.
     Both are pure UI chrome → client-only, zero server calls (§6.9). -->
<body data-signals="{ui: {sidebarOpen: true, mobileNav: false, screen: 'overview'}}"
      class="min-h-dvh bg-gray-50">

  <div class="flex">
    <!-- Sidebar: width animates between expanded (w-60) and icon rail (w-16) -->
    <aside data-class="{'w-60': $ui.sidebarOpen, 'w-16': !$ui.sidebarOpen}"
           class="hidden md:flex flex-col bg-gray-900 text-gray-300 min-h-dvh
                  transition-[width] duration-200 shrink-0">

      <div class="h-14 flex items-center px-4 font-semibold text-white">
        <span data-show="$ui.sidebarOpen">Acme Admin</span>
        <span data-show="!$ui.sidebarOpen">A</span>
      </div>

      <!-- Nav item pattern: instant active-state flip + ONE screen fetch -->
      <button data-on:click="$ui.screen = 'overview'; @get('/admin/screen/overview')"
              data-class="{'bg-gray-800 text-white': $ui.screen === 'overview'}"
              class="flex items-center gap-3 px-4 py-2.5 text-sm hover:bg-gray-800">
        <svg class="w-5 h-5 shrink-0"><!-- chart icon --></svg>
        <span data-show="$ui.sidebarOpen">Overview</span>
      </button>

      <button data-on:click="$ui.screen = 'orders'; @get('/admin/screen/orders')"
              data-class="{'bg-gray-800 text-white': $ui.screen === 'orders'}"
              class="flex items-center gap-3 px-4 py-2.5 text-sm hover:bg-gray-800">
        <svg class="w-5 h-5 shrink-0"><!-- cart icon --></svg>
        <span data-show="$ui.sidebarOpen">Orders</span>
        <!-- Live badge from the SSE stream — server-owned count -->
        <span data-show="$ui.sidebarOpen && $pendingOrders > 0"
              data-text="$pendingOrders"
              class="ml-auto bg-amber-500 text-gray-900 text-xs rounded-full px-2"></span>
      </button>

      <!-- Collapse toggle pinned to the bottom -->
      <button data-on:click="$ui.sidebarOpen = !$ui.sidebarOpen"
              class="mt-auto p-4 text-gray-500 hover:text-white text-sm">
        <span data-show="$ui.sidebarOpen">« Collapse</span>
        <span data-show="!$ui.sidebarOpen">»</span>
      </button>
    </aside>

    <div class="flex-1 min-w-0">
      <!-- Topbar: burger (mobile), global search, identity from §4.11 -->
      <header class="h-14 bg-white border-b flex items-center gap-3 px-4 sticky top-0 z-30">
        <button data-on:click="$ui.mobileNav = true" class="md:hidden p-2">☰</button>

        <!-- Global search: debounced, results patched into a popover -->
        <div class="relative flex-1 max-w-md">
          <input data-bind="search.q"
                 data-on:input__debounce.300ms="@get('/admin/search')"
                 placeholder="Search orders, users…"
                 class="w-full border rounded-lg px-3 py-1.5 text-sm bg-gray-50" />
          <div id="search-results"
               class="absolute top-full mt-1 inset-x-0 bg-white border rounded-lg shadow-lg
                      empty:hidden z-40"></div> <!-- empty:hidden = invisible until patched -->
        </div>

        <img data-attr:src="$server.user.avatar" class="w-8 h-8 rounded-full ml-auto" />
      </header>

      <!-- Main content target for all sidebar navigation -->
      <main id="admin-content" class="p-6"></main>
    </div>
  </div>
</body>

9.8 Live KPI Stat Cards

<!-- One SSE stream feeds all cards via signal patches (§8.3 maps Mongo
     updateDescription — or §7.2-B PG notify — straight onto these signals). -->
<div data-signals="{kpis: {ordersToday: 0, revenueToday: 0, activeUsers: 0, errorRate: 0},
                    kpiTrend: {orders: 0, revenue: 0}}"
     data-init="@get('/admin/kpis/stream')"
     class="grid grid-cols-2 lg:grid-cols-4 gap-4">

  <div class="bg-white rounded-xl border p-5">
    <p class="text-xs text-gray-500 uppercase tracking-wide">Orders today</p>
    <p class="text-2xl font-semibold mt-1" data-text="$kpis.ordersToday.toLocaleString()"></p>
    <!-- Trend chip: color and arrow derived from the SERVER-computed delta —
         the client never recalculates trends (§6.2) -->
    <p data-class="{'text-green-600': $kpiTrend.orders >= 0, 'text-red-600': $kpiTrend.orders < 0}"
       class="text-xs mt-1">
      <span data-text="($kpiTrend.orders >= 0 ? '▲ ' : '▼ ') + Math.abs($kpiTrend.orders) + '%'"></span>
      vs yesterday
    </p>
  </div>

  <div class="bg-white rounded-xl border p-5">
    <p class="text-xs text-gray-500 uppercase tracking-wide">Revenue today</p>
    <p class="text-2xl font-semibold mt-1"
       data-text="'$' + $kpis.revenueToday.toLocaleString(undefined, {maximumFractionDigits: 0})"></p>
  </div>

  <div class="bg-white rounded-xl border p-5">
    <p class="text-xs text-gray-500 uppercase tracking-wide">Active users</p>
    <p class="text-2xl font-semibold mt-1" data-text="$kpis.activeUsers"></p>
    <span class="inline-flex items-center gap-1 text-xs text-green-600 mt-1">
      <span class="w-1.5 h-1.5 bg-green-500 rounded-full animate-pulse"></span> live
    </span>
  </div>

  <div class="bg-white rounded-xl border p-5">
    <p class="text-xs text-gray-500 uppercase tracking-wide">Error rate</p>
    <!-- Threshold styling: presentation-only, so an expression is fine here -->
    <p data-class="{'text-red-600': $kpis.errorRate > 0.01}"
       class="text-2xl font-semibold mt-1"
       data-text="($kpis.errorRate * 100).toFixed(2) + '%'"></p>
  </div>
</div>

9.9 Server-Paginated Admin Table (Search + Filter + Bulk)

<!-- The client holds ONLY: query draft, current page, selected ids, status filter.
     Sorting, filtering, pagination, and rendering all happen server-side —
     this table works identically at 100 rows or 10 million (§6.2 anti-pattern 1). -->
<div data-signals="{table: {q: '', page: 1, status: 'all', selected: []}}">

  <!-- Toolbar -->
  <div class="flex flex-wrap items-center gap-3 mb-4">
    <!-- Search: debounced; changing the query resets to page 1 BEFORE the request -->
    <input data-bind="table.q"
           data-on:input__debounce.300ms="$table.page = 1; @get('/admin/orders/table')"
           placeholder="Search orders…"
           class="border rounded-lg px-3 py-2 text-sm w-64" />

    <!-- Status filter: immediate (a select changes once, no debounce needed) -->
    <select data-bind="table.status"
            data-on:change="$table.page = 1; @get('/admin/orders/table')"
            class="border rounded-lg px-3 py-2 text-sm">
      <option value="all">All statuses</option>
      <option value="pending">Pending</option>
      <option value="shipped">Shipped</option>
      <option value="refunded">Refunded</option>
    </select>

    <!-- Bulk action bar: appears only with a selection; ONE request for N rows -->
    <div data-show="$table.selected.length > 0" class="flex items-center gap-2 ml-auto">
      <span class="text-sm text-gray-500"
            data-text="$table.selected.length + ' selected'"></span>
      <button data-on:click="@post('/admin/orders/bulk-ship')"
              class="bg-blue-600 text-white text-sm px-3 py-1.5 rounded-lg">
        Mark shipped
      </button>
      <!-- Server clears table.selected and re-patches #orders-tbody on success -->
    </div>
  </div>

  <table class="w-full bg-white border rounded-xl overflow-hidden text-sm">
    <thead class="bg-gray-50 text-left text-gray-500">
      <tr>
        <th class="p-3 w-10">
          <!-- Select-all toggles against the ids the SERVER rendered into this page -->
          <input type="checkbox"
                 data-on:change="$table.selected = evt.target.checked ? $pageIds : []" />
        </th>
        <!-- Sortable header: sorting is a server concern → it's just a GET param -->
        <th class="p-3 cursor-pointer hover:text-gray-900"
            data-on:click="@get('/admin/orders/table?sort=total')">Total ↕</th>
        <th class="p-3">Customer</th>
        <th class="p-3">Status</th>
      </tr>
    </thead>
    <!-- Server patches rows here; each row's checkbox edits table.selected.
         $pageIds is patched alongside the rows so select-all always matches. -->
    <tbody id="orders-tbody"></tbody>
  </table>

  <!-- Pagination: server patches #table-pager with the real page count;
       buttons mutate table.page then fetch -->
  <div id="table-pager" class="flex items-center justify-between mt-3 text-sm">
    <button data-on:click="$table.page--; @get('/admin/orders/table')"
            data-attr:disabled="$table.page <= 1"
            class="px-3 py-1.5 border rounded-lg disabled:opacity-40">‹ Prev</button>
    <span data-text="'Page ' + $table.page"></span>
    <button data-on:click="$table.page++; @get('/admin/orders/table')"
            class="px-3 py-1.5 border rounded-lg">Next ›</button>
  </div>
</div>

9.10 Component Discipline Checklist

  • Visual state flips (tabs, sidebars, sheets, swipes) are client-only — no server traffic
  • Every data-bearing interaction is one debounced/coalesced request (§6.9)
  • Destructive actions (delete, bulk ops) wait for the server's element patch — never optimistic
  • Live badges/KPIs are displayed, never computed, on the client
  • Skeletons live inside the patch target and mirror final geometry (no layout shift)
  • Touch handlers clamp and snap locally; only the committed gesture talks to the server
  • Tables/lists hold IDs and drafts in signals; rows are server-rendered HTML
  • Safe-area insets (env(safe-area-inset-bottom)) on fixed mobile chrome

10. Plugin Development & Extensions

Datastar's architecture is built around a plugin system where a compact reactive engine serves as the foundation, and all behavior — from event handling to server communication — is provided by pluggable modules. This design keeps the core minimal while allowing the ecosystem to grow organically.

10.1 Plugin Types

Datastar supports three plugin categories, each serving a distinct purpose:

Category Purpose Examples
Attribute Plugins Declarative DOM bindings triggered by data-* attributes data-text, data-bind, data-on:click, data-class, data-show, data-effect
Action Plugins Imperative functions callable from expressions or event handlers @get(), @post(), @setAll(), @toggleAll(), @peek()
Watcher Plugins Global event listeners for server-pushed SSE events datastar-patch-signals, datastar-patch-elements

10.2 Public API Surface

Datastar exposes a Public API for plugin registration and reactive primitives. These are available from the datastar module:

// Plugin registration functions
import { action, actions, attribute, watcher, load, apply } from "datastar";

// Reactive primitives
import { signal, computed, effect, root, mergePatch, mergePaths } from "datastar";

Key functions:

Function Purpose
load(plugin) Registers a plugin with Datastar's engine
apply() Scans the DOM and applies all registered plugins to existing elements
action(config) Registers a single action plugin
actions(configs[]) Registers multiple action plugins at once
attribute(config) Registers a single attribute plugin
watcher(config) Registers a single watcher plugin

10.3 Creating an Action Plugin

Action plugins are functions prefixed with @ that can be called from Datastar expressions.

import { action, load, apply } from "datastar";

// Define the action plugin
const AlertAction = action({
    name: 'alert',
    apply(ctx, value) {
        alert(value)
    }
});

// Register it
load(AlertAction);
apply();

Usage in HTML:

<button data-on:click="@alert('Hello from a custom action')">
    Alert using an action
</button>

Action Plugin Configuration:

Property Type Description
name string The action name (becomes @name in expressions)
apply function The function executed when the action is called. Receives (ctx, value)

The ctx object provides access to:

  • ctx.signals — the reactive signal store
  • ctx.el — the element that triggered the action (if applicable)
  • ctx.event — the DOM event (if triggered by an event handler)

10.4 Creating an Attribute Plugin

Attribute plugins are the heart of Datastar's declarative API. They add new data-* attributes that bind behavior to DOM elements.

import { attribute, load, apply } from "datastar";

// Define the attribute plugin
const AlertAttribute = attribute({
    name: 'alert',
    requirement: {
        key: 'denied',      // 'denied' | 'must' | 'allowed'
        value: 'must',      // 'denied' | 'must' | 'allowed'
    },
    returnsValue: true,
    apply({ el, rx }) {
        // rx() evaluates the attribute's expression and returns the value
        const callback = () => alert(rx());
        el.addEventListener('click', callback);

        // Return cleanup function
        return () => el.removeEventListener('click', callback);
    }
});

load(AlertAttribute);
apply();

Usage in HTML:

<button data-alert="'Hello from an attribute'">
    Alert using an attribute
</button>

Attribute Plugin Configuration:

Property Type Description
name string The attribute name (becomes data-name in HTML)
requirement.key 'denied' | 'must' | 'allowed' Whether the attribute requires a key (e.g., data-on:click has key click)
requirement.value 'denied' | 'must' | 'allowed' Whether the attribute requires a value expression
returnsValue boolean Whether the expression should return a value (enables rx())
apply function Setup function. Receives {el, rx, effect, runtimeErr, value} and returns cleanup function

The apply context object:

Property Description
el The DOM element the attribute is on
rx Function that evaluates the attribute expression and returns the current value
effect Function to create a reactive effect that re-runs when signals change
runtimeErr Function to throw structured runtime errors
value The raw expression string from the attribute

10.5 Creating a Watcher Plugin

Watcher plugins listen for global events (typically SSE events from the server) and react to them.

import { watcher, load, apply } from "datastar";

const CustomWatcher = watcher({
    name: 'custom-event',
    onLoad: ({ el, effect, rx }) => {
        const handler = (event) => {
            // Process the custom SSE event
            const data = event.detail;
            // Update signals or DOM based on event data
        };

        window.addEventListener('datastar-custom-event', handler);

        return () => {
            window.removeEventListener('datastar-custom-event', handler);
        };
    }
});

load(CustomWatcher);
apply();

10.6 Advanced Attribute Plugin: Reactive Text Label

This example demonstrates a more complex attribute plugin that maps signal values to human-readable labels using a global lookup object.

import { attribute, load, apply } from "datastar";

// Global label lookup (could be loaded from server)
const labels = {
    search: {
        mode: {
            music: "Music",
            sfx: "SFX",
            all: "All",
        }
    },
    status: {
        active: "Active",
        inactive: "Inactive",
        pending: "Pending Review"
    }
};

const TextlabelPlugin = attribute({
    name: 'textlabel',
    requirement: {
        key: 'denied',
        value: 'must',
    },
    returnsValue: true,
    apply: ({ el, effect, rx, runtimeErr, value }) => {
        const update = () => {
            const signalValue = rx();

            // Parse the signal path from the expression
            const pathMatch = String(value).match(/^\$([a-zA-Z_][a-zA-Z0-9_.]*)$/)?.[1];
            if (!pathMatch) {
                throw runtimeErr('InvalidLabelPath', {
                    message: 'Could not extract label path from expression'
                });
            }

            const pathSegments = pathMatch.split('.');
            let labelObj = labels;
            for (const segment of pathSegments) {
                if (labelObj && typeof labelObj === 'object') {
                    labelObj = labelObj[segment];
                } else {
                    labelObj = undefined;
                    break;
                }
            }

            const label = labelObj?.[signalValue];
            el.textContent = label !== undefined ? label : signalValue;
        };

        // Create reactive effect that re-runs when signals change
        const cleanup = effect(update);

        return cleanup;
    },
});

load(TextlabelPlugin);
apply();

Usage:

<div data-signals="{search: {mode: 'music'}}">
  <button data-on:click="$search.mode = 'music'">Music</button>
  <button data-on:click="$search.mode = 'sfx'">SFX</button>
  <span>Searching for <strong data-textlabel="$search.mode"></strong></span>
  <!-- Renders: "Searching for Music" -->
</div>

10.7 Advanced Action Plugin: Beacon API for Analytics

This action plugin integrates the browser's Beacon API for sending analytics data before page unload.

import { action, load, apply } from "datastar";

const BeaconAction = action({
    name: 'beacon',
    apply: (ctx, url) => {
        // Collect signals to send (excluding local signals prefixed with _)
        const signals = {};
        for (const [key, value] of Object.entries(ctx.signals)) {
            if (!key.startsWith('_')) {
                signals[key] = value;
            }
        }

        // Send via Beacon API (fires even if page unloads)
        navigator.sendBeacon(url, JSON.stringify(signals));
    }
});

load(BeaconAction);
apply();

Usage:

<button data-on:click="@beacon('/api/analytics/track')">
    Track Event
</button>

<!-- Automatically send on page hide -->
<div data-on:visibilitychange="@beacon('/api/analytics/session-end')"></div>

10.8 Plugin with Scoped Signals

This plugin provides isolated signal namespaces so different components don't interfere with each other.

import { attribute, action, load, apply } from "datastar";

// Scoped signal store
const scopes = new Map();

const ScopeAttribute = attribute({
    name: 'scope',
    requirement: { key: 'must', value: 'denied' },
    returnsValue: false,
    apply: ({ el, effect, rx, value }) => {
        const scopeName = value; // e.g., "user-form"

        if (!scopes.has(scopeName)) {
            scopes.set(scopeName, new Map());
        }

        // Mark element as part of this scope
        el.dataset.scopeName = scopeName;

        return () => {
            // Cleanup scope if no more elements use it
            const scopedElements = document.querySelectorAll(`[data-scope-name="${scopeName}"]`);
            if (scopedElements.length === 0) {
                scopes.delete(scopeName);
            }
        };
    }
});

const ScopedAction = action({
    name: 'scopedSet',
    apply: (ctx, args) => {
        const { scope, key, value } = args;
        if (scopes.has(scope)) {
            scopes.get(scope).set(key, value);
        }
    }
});

load(ScopeAttribute);
load(ScopedAction);
apply();

Usage:

<div data-scope="user-form">
  <input data-bind="name" />
  <input data-bind="email" />
</div>

<div data-scope="settings-form">
  <input data-bind="name" />  <!-- Isolated from user-form.name -->
  <input data-bind="theme" />
</div>

10.9 Plugin with Keyboard Shortcuts

This plugin binds keyboard events to reactive actions, supporting multi-key combinations.

import { attribute, load, apply } from "datastar";

const OnKeysPlugin = attribute({
    name: 'onKeys',
    requirement: { key: 'denied', value: 'must' },
    returnsValue: true,
    apply: ({ el, rx, effect }) => {
        const handler = (event) => {
            const binding = rx(); // e.g., "Ctrl-Shift-S" or "Escape.Enter"
            const keys = binding.split('.'); // OR logic: Escape OR Enter

            for (const keyCombo of keys) {
                const parts = keyCombo.split('-');
                const key = parts.pop().toLowerCase();
                const modifiers = parts.map(m => m.toLowerCase());

                const ctrl = modifiers.includes('ctrl');
                const shift = modifiers.includes('shift');
                const alt = modifiers.includes('alt');
                const meta = modifiers.includes('meta');

                if (evt.key.toLowerCase() === key &&
                    event.ctrlKey === ctrl &&
                    event.shiftKey === shift &&
                    event.altKey === alt &&
                    event.metaKey === meta) {
                    evt.preventDefault();
                    // Execute the action expression stored in a data attribute
                    const actionExpr = el.dataset.onKeysAction;
                    if (actionExpr) {
                        eval(actionExpr); // In production, use proper expression evaluation
                    }
                    break;
                }
            }
        };

        // Can be global or element-scoped
        const scope = el.dataset.onKeysScope || 'global';
        if (scope === 'global') {
            document.addEventListener('keydown', handler);
        } else {
            el.addEventListener('keydown', handler);
        }

        return () => {
            if (scope === 'global') {
                document.removeEventListener('keydown', handler);
            } else {
                el.removeEventListener('keydown', handler);
            }
        };
    }
});

load(OnKeysPlugin);
apply();

Usage:

<!-- Global shortcut -->
<div data-on-keys="'Ctrl-Shift-S'" data-on-keys-action="@post('/api/save')">
  Press Ctrl+Shift+S to save
</div>

<!-- Multiple keys with OR logic -->
<div data-on-keys="'Escape.Enter'" data-on-keys-action="$modalOpen = false">
  Press Escape or Enter to close
</div>

<!-- Element-scoped -->
<input data-on-keys="'Ctrl-A'" data-on-keys-scope="element" data-on-keys-action="$selectAll = true" />

10.10 Plugin with DOM Lifecycle Hooks

This plugin executes custom expressions when DOM elements are removed, useful for cleanup.

import { attribute, load, apply } from "datastar";

const OnRemovePlugin = attribute({
    name: 'onRemove',
    requirement: { key: 'denied', value: 'must' },
    returnsValue: true,
    apply: ({ el, rx, effect }) => {
        // Use MutationObserver to detect when element is removed
        const parent = el.parentNode;
        const observer = new MutationObserver((mutations) => {
            for (const mutation of mutations) {
                for (const removedNode of mutation.removedNodes) {
                    if (removedNode === el || removedNode.contains(el)) {
                        // Element is being removed — execute the expression
                        rx();
                        observer.disconnect();
                        return;
                    }
                }
            }
        });

        if (parent) {
            observer.observe(parent, { childList: true, subtree: true });
        }

        return () => observer.disconnect();
    }
});

load(OnRemovePlugin);
apply();

Usage:

<div data-on-remove="console.log('Element removed:', el.id); cleanupResources()">
  <!-- When this element is removed from DOM, the expression fires -->
</div>

10.11 Plugin with Web Speech API

This plugin wraps the Web Speech API for text-to-speech functionality.

import { action, load, apply } from "datastar";

const speechQueue = [];
let currentUtterance = null;
const synth = window.speechSynthesis;

const SpeechAction = action({
    name: 'speech',
    apply: (ctx, options) => {
        const { text, lang = 'en-US', rate = 1, pitch = 1, volume = 1, voice } = options;

        const utterance = new SpeechSynthesisUtterance(text);
        utterance.lang = lang;
        utterance.rate = rate;
        utterance.pitch = pitch;
        utterance.volume = volume;

        if (voice) {
            const voices = synth.getVoices();
            const selected = voices.find(v => v.name === voice);
            if (selected) utterance.voice = selected;
        }

        speechQueue.push(utterance);

        if (!synth.speaking) {
            playNext();
        }
    }
});

const SpeechCtrlAction = action({
    name: 'speechCtrl',
    apply: (ctx, command) => {
        switch (command) {
            case 'play':
                if (synth.paused) synth.resume();
                else if (!synth.speaking) playNext();
                break;
            case 'pause':
                synth.pause();
                break;
            case 'next':
                synth.cancel();
                playNext();
                break;
            case 'previous':
                // Would need queue index tracking
                break;
            case 'reset':
                synth.cancel();
                speechQueue.length = 0;
                break;
        }
    }
});

function playNext() {
    if (speechQueue.length > 0) {
        currentUtterance = speechQueue.shift();
        currentUtterance.onend = playNext;
        synth.speak(currentUtterance);
    }
}

load(SpeechAction);
load(SpeechCtrlAction);
apply();

Usage:

<button data-on:click="@speech({text: 'Hello world', lang: 'en-US', rate: 1.2})">
    Speak
</button>

<button data-on:click="@speechCtrl('pause')">Pause</button>
<button data-on:click="@speechCtrl('next')">Skip</button>

10.12 Plugin with CSS Custom Highlight API

This plugin integrates the CSS Custom Highlight API for syntax highlighting without wrapping elements in spans.

import { attribute, load, apply } from "datastar";

const HighlightPlugin = attribute({
    name: 'highlight',
    requirement: { key: 'denied', value: 'must' },
    returnsValue: false,
    apply: ({ el, effect, rx, value }) => {
        const highlightText = () => {
            const text = el.textContent;
            const lang = value; // e.g., "javascript", "python"

            // Create or get highlight range
            const highlightName = `datastar-highlight-${el.id || Math.random().toString(36).slice(2)}`;

            // Simple tokenization (in production, use a proper tokenizer)
            const tokens = tokenize(text, lang);

            // Create ranges for each token type
            const ranges = [];
            for (const token of tokens) {
                const range = new Range();
                // Set range boundaries based on token positions
                // ...
                ranges.push(range);
            }

            // Register with CSS Custom Highlight API
            const highlight = new Highlight(...ranges);
            CSS.highlights.set(highlightName, highlight);
        };

        const cleanup = effect(highlightText);

        return () => {
            cleanup();
            const highlightName = `datastar-highlight-${el.id}`;
            CSS.highlights.delete(highlightName);
        };
    }
});

load(HighlightPlugin);
apply();

Usage:

<style>
  ::highlight(datastar-highlight-code) .keyword { color: blue; }
  ::highlight(datastar-highlight-code) .string { color: green; }
  ::highlight(datastar-highlight-code) .comment { color: gray; }
</style>

<pre data-highlight="javascript">
function hello() {
  // This is a comment
  return "Hello, World!";
}
</pre>

10.13 Building & Distributing Plugins

Using the official starter template:

The datastar-plugin-starter template provides pre-configured build tooling with esbuild, development server, and GitHub Pages integration.

# Use the GitHub template, then:
pnpm install
pnpm dev      # Development server with hot reload
pnpm build    # Compile to dist/index.js

Manual setup (vanilla JS):

<script type="importmap">
{
  "imports": {
    "datastar": "https://cdn.jsdelivr.net/gh/starfederation/datastar@v1.0.2/bundles/datastar.js"
  }
}
</script>
<script type="module">
  import { action, attribute, load, apply } from "datastar";

  // Define your plugin
  const MyPlugin = action({
      name: 'myAction',
      apply(ctx, value) {
          console.log('My action called with:', value);
      }
  });

  // Register and apply
  load(MyPlugin);
  apply();
</script>

Distribution via CDN:

Commit your compiled dist/ files to your repo so others can use jsDelivr:

<script type="importmap">
{
  "imports": {
    "datastar": "https://cdn.jsdelivr.net/gh/starfederation/datastar@v1.0.2/bundles/datastar.js",
    "my-plugin": "https://cdn.jsdelivr.net/gh/YOUR_USERNAME/my-plugin@v1.0.0/dist/index.js"
  }
}
</script>
<script type="module">
  import "my-plugin";
</script>

10.14 Plugin Best Practices

  1. Use the Public API where possible. Internal APIs (@types, @utils/*) are available but not officially supported and may break in future versions.

  2. Always return cleanup functions from apply to prevent memory leaks when elements are removed from the DOM.

  3. Use effect() for reactivity rather than manual polling or setInterval. This ensures your plugin updates only when relevant signals change.

  4. Handle errors gracefully using runtimeErr() for structured error reporting that integrates with Datastar's error handling.

  5. Namespace your plugin names to avoid collisions with built-in plugins or other community plugins. Use a prefix like mylib-.

  6. Test with the core bundle (datastar-core.js) to ensure your plugin works when users build custom bundles without built-in plugins.

  7. Document modifier support if your plugin supports modifiers (e.g., __debounce, __throttle). Follow Datastar's double-underscore convention.

10.15 Community Plugin Ecosystem

The Datastar community has created numerous plugins extending the framework:

Plugin Type Description
datastar-attribute-on-keys Attribute Keyboard shortcuts with multi-key combinations
datastar-attribute-prop Attribute Property binding for DOM elements and web components
data-on-remove Attribute Lifecycle hook for element removal
data-persist Attribute localStorage-based signal persistence
datastar-beacon Action Beacon API for analytics and telemetry
data-highlight Attribute CSS Custom Highlight API for syntax highlighting
datastar-speech Action Web Speech API for text-to-speech
datastar-scoped-signals Attribute Isolated signal namespaces per component
datastar-inspector DevTool Browser debugger for signals and SSE events
data-lint-example Tool Tree-sitter based linter for Datastar projects

11. Tailwind CSS Integration

11.1 Utility-First with Datastar

Tailwind's utility classes work seamlessly with Datastar's reactive attributes:

<!-- Conditional styling -->
<div data-class="{
  'bg-green-100 text-green-800': $status === 'success',
  'bg-red-100 text-red-800': $status === 'error',
  'bg-yellow-100 text-yellow-800': $status === 'warning',
  'bg-gray-100 text-gray-800': $status === 'neutral'
}">
  Status Message
</div>

<!-- Dynamic spacing -->
<div data-class="{
  'p-2 gap-2': $isCompact,
  'p-6 gap-4': !$isCompact
}">
  <!-- Content -->
</div>

<!-- Responsive + reactive -->
<div class="grid"
     data-class="{
       'grid-cols-1': $columns === 1,
       'grid-cols-2': $columns === 2,
       'grid-cols-3': $columns === 3,
       'grid-cols-4': $columns >= 4
     }">
</div>

<!-- Dynamic colors -->
<div class="px-3 py-1 rounded-full text-sm font-medium"
     data-class="{
       'bg-blue-100 text-blue-800': $priority === 'low',
       'bg-yellow-100 text-yellow-800': $priority === 'medium',
       'bg-red-100 text-red-800': $priority === 'high'
     }"
     data-text="$priority.toUpperCase()">
</div>

11.2 Animation Patterns

<!-- Fade in on show -->
<div data-show="$visible"
     class="transition-all duration-300"
     data-class="{'opacity-0 translate-y-2': !$visible, 'opacity-100 translate-y-0': $visible}">
</div>

<!-- Slide in drawer -->
<div class="fixed right-0 top-0 h-full w-96 bg-white shadow-2xl transform transition-transform duration-300"
     data-class="{'translate-x-0': $drawerOpen, 'translate-x-full': !$drawerOpen}">
</div>

<!-- Scale modal with backdrop blur -->
<div data-show="$modalOpen"
     class="fixed inset-0 z-50 flex items-center justify-center bg-black/50 backdrop-blur-sm transition-all duration-200"
     data-class="{'opacity-0': !$modalOpen, 'opacity-100': $modalOpen}">
  <div class="bg-white rounded-xl shadow-xl p-6 max-w-lg w-full mx-4 transform transition-transform duration-200"
       data-class="{'scale-95': !$modalOpen, 'scale-100': $modalOpen}">
    Modal Content
  </div>
</div>

<!-- Staggered list animation -->
<div class="space-y-2">
  <template data-for="(item, index) in $items">
    <div class="p-3 border rounded-lg transition-all duration-300"
         data-style:animation-delay="(index * 50) + 'ms'"
         data-class="{'animate-slide-in': $animateItems}">
      <span data-text="item.name"></span>
    </div>
  </template>
</div>

<!-- Skeleton loading -->
<div data-show="$isLoading" class="space-y-3">
  <div class="h-4 bg-gray-200 rounded animate-pulse w-3/4"></div>
  <div class="h-4 bg-gray-200 rounded animate-pulse w-1/2"></div>
  <div class="h-20 bg-gray-200 rounded animate-pulse"></div>
</div>

11.3 Dark Mode Support

<html data-signals="{theme: 'light'}">
  <body class="transition-colors duration-300"
        data-class="{'dark': $theme === 'dark'}">

    <div class="bg-white dark:bg-gray-900 text-gray-900 dark:text-white min-h-screen">
      <button data-on:click="$theme = $theme === 'light' ? 'dark' : 'light'"
              class="p-2 rounded-lg bg-gray-200 dark:bg-gray-700 transition-colors">
        <span data-show="$theme === 'light'">🌙</span>
        <span data-show="$theme === 'dark'">☀️</span>
      </button>

      <div class="p-6 bg-gray-50 dark:bg-gray-800 rounded-lg border border-gray-200 dark:border-gray-700">
        <h2 class="text-xl font-bold text-gray-900 dark:text-white">Dark Mode Content</h2>
        <p class="text-gray-600 dark:text-gray-300">Automatically adapts to theme signal.</p>
      </div>
    </div>

  </body>
</html>

Tailwind Config:

// tailwind.config.js
module.exports = {
  darkMode: 'class', // Required for manual dark mode toggle
  // ...
}

12. Performance & Anti-Patterns

12.1 Do's

✅ Use data-show over conditional rendering for state preservation:

<!-- GOOD: Form state preserved when hidden -->
<div data-show="$step === 2">
  <input data-bind="form.email" />
</div>

<!-- BAD: State lost on re-render -->
<div id="step-2"></div> <!-- Server removes/re-adds -->

✅ Patch elements, not whole pages:

<!-- GOOD: Only update the list -->
<div id="item-list">
  <!-- Server patches this div only -->
</div>

✅ Use computed signals for expensive operations:

<div data-computed:filtered="$items.filter(i => complexPredicate(i))">
  <!-- Cached until dependencies change -->
</div>

✅ Debounce/throttle rapid events:

<input data-on:input__debounce.300ms="@get('/api/search')" />
<div data-on:scroll__throttle.100ms="$scrollPos = el.scrollTop"></div>

✅ Use __openWhenHidden for SSE streams:

<div data-init="@get('/api/stream')__openWhenHidden.true"></div>

✅ Use data-effect with explicit deps for targeted updates:

<!-- data-effect re-runs when any signal IN the expression changes; reference the
     dep ($messages.length) and the effect fires only when it changes -->
<div data-effect="$messages.length && scrollToBottom()">
  <!-- Only runs when messages.length changes, not on any signal change -->
</div>

✅ Batch signal updates:

<!-- GOOD: Single update -->
<button data-on:click="$ui.screen = 'chat'; $ui.isNavigating = true; @get('/api/screens/chat')">

<!-- BAD: Multiple separate updates causing multiple re-renders -->

12.2 Don'ts

❌ Don't store derived state in signals:

<!-- BAD: Manual sync, easy to get out of date -->
<div data-signals="{firstName: 'John', lastName: 'Doe', fullName: 'John Doe'}">

<!-- GOOD: Use computed -->
<div data-computed:fullName="$firstName + ' ' + $lastName">

❌ Don't create signals for static data:

<!-- BAD: Unnecessary reactivity -->
<div data-signals="{appName: 'My App'}">

<!-- GOOD: Static content -->
<div>My App</div>

❌ Don't over-fetch with actions:

<!-- BAD: Fetch on every keystroke -->
<input data-on:input="@get('/api/search?q=' + $query)" />

<!-- GOOD: Debounced fetch -->
<input data-on:input__debounce.300ms="@get('/api/search')" />

❌ Don't ignore cleanup in data-init:

<!-- BAD: Interval keeps running if element removed -->
<div data-on:interval__duration.1000ms="$counter++"></div>

<!-- GOOD: Use data-effect with cleanup -->
<div data-effect="const id = setInterval(() => $counter++, 1000); return () => clearInterval(id)">
</div>

❌ Don't use data-effect without deps for heavy operations:

<!-- BAD: Runs on EVERY signal change -->
<div data-effect="expensiveCalculation($data)"></div>

<!-- GOOD: Only runs when specific signals change -->
<!-- Subscribe ONLY to $data.filter; read the rest with @peek so changes to other
     $data fields don't retrigger the expensive call -->
<div data-effect="$data.filter && expensiveCalculation(@peek(() => $data))"></div>

12.3 Performance Checklist

  • Use data-show for tabs/wizards instead of server round-trips
  • Patch only changed elements, not entire containers
  • Use data-computed for filtered/sorted lists
  • Debounce search inputs (300ms typical)
  • Throttle scroll/resize events (100ms typical)
  • Use __openWhenHidden for SSE connections
  • Minimize signal payload size (exclude large objects from auto-send)
  • Use @peek(() => $x) inside data-effect/expressions to read a signal without subscribing — limits re-execution scope
  • Leverage browser caching for static assets
  • Use CDN for Datastar bundle (jsDelivr)
  • Use useViewTransition for smooth screen transitions
  • Avoid deeply nested computed signals (max 3 levels)

13. Server-Side Patterns

13.1 SSE Event Types

Datastar supports these SSE event types from the server:

Event Purpose Example
datastar-patch-signals Update signal values (JSON Merge Patch) data: signals {"count": 5}
datastar-patch-elements Insert/update DOM elements data: elements <div id="foo">...</div>
datastar-execute-script Run JS on client data: script alert('hello')

13.2 Element Patch Modes

event: datastar-patch-elements
data: selector #content
data: mode inner        <!-- Replace inner HTML (preserves element state) -->
data: mode outer        <!-- Replace entire element (default, morphs by ID) -->
data: mode replace      <!-- Replace entire element (resets state) -->
data: mode prepend      <!-- Insert at beginning of children -->
data: mode append       <!-- Insert at end of children -->
data: mode before       <!-- Insert before target as sibling -->
data: mode after        <!-- Insert after target as sibling -->
data: mode remove       <!-- Remove target element -->
data: useViewTransition true  <!-- Use View Transitions API -->

Important: Datastar requires complete HTML elements, not fragments. Always wrap content in a tag with an ID.

<!-- GOOD: Complete element with ID -->
data: elements <div id="content"><p>Hello</p></div>

<!-- BAD: Fragment without wrapper -->
data: elements <p>Hello</p><p>World</p>

13.3 JSON Merge Patch Semantics (RFC 7386)

When patching signals, Datastar uses JSON Merge Patch:

Operation Behavior Example
Add/Update Set property value {"key": "value"}
Remove Set to null {"key": null}
Nested Recursive patch {"user": {"name": "Johnny"}} 
event: datastar-patch-signals
data: signals {"user": {"name": "John", "email": null}, "count": 5}

13.4 Request Payload Structure

When Datastar sends requests, signals are serialized as JSON under the datastar namespace:

{
  "datastar": {
    "signals": {
      "user": {"name": "John", "email": "john@example.com"},
      "cart": {"items": [], "total": 0}
    }
  }
}

Note: GET and DELETE requests send signals in query params; POST/PUT/PATCH send in body.

13.5 Node.js HTTP/2 Server Foundation

All server-side examples in this guide run on a plain Node.js HTTP/2 server. HTTP/2 matters specifically for Datastar apps: browsers cap HTTP/1.1 at ~6 connections per origin, so a long-lived SSE stream plus a few in-flight actions can starve the page. HTTP/2 multiplexes everything (SSE streams included) over one TCP connection, so you can hold many SSE streams open per tab without exhausting the pool.

// server.mjs — HTTP/2 base with SSE + Datastar helpers
import http2 from 'node:http2';
import fs from 'node:fs';

const { HTTP2_HEADER_PATH, HTTP2_HEADER_METHOD } = http2.constants;

// h2 requires TLS in browsers — use mkcert for local certs
const server = http2.createSecureServer({
    key:  fs.readFileSync('./certs/localhost-key.pem'),
    cert: fs.readFileSync('./certs/localhost.pem'),
    allowHTTP1: true,                       // graceful fallback for old clients/proxies
});

// ---- Datastar helpers over an http2 stream ----
function sseHead(stream) {
    stream.respond({
        ':status': 200,
        'content-type': 'text/event-stream',
        'cache-control': 'no-cache',
        // NOTE: no 'Connection: keep-alive' — that header is illegal in HTTP/2
    });
}

export function patchSignals(stream, signals) {
    stream.write(`event: datastar-patch-signals\ndata: signals ${JSON.stringify(signals)}\n\n`);
}

export function patchElements(stream, html, { selector, mode } = {}) {
    let frame = `event: datastar-patch-elements\n`;
    if (selector) frame += `data: selector ${selector}\n`;
    if (mode)     frame += `data: mode ${mode}\n`;
    // Multi-line HTML → one `data: elements` line per source line
    frame += html.split('\n').map(l => `data: elements ${l}`).join('\n') + '\n\n';
    stream.write(frame);
}

// Signals arrive as body JSON (POST/PUT/PATCH) or `datastar` query param (GET/DELETE)
export async function readSignals(stream, headers) {
    if (['GET', 'DELETE'].includes(headers[HTTP2_HEADER_METHOD])) {
        const url = new URL(headers[HTTP2_HEADER_PATH], 'https://x');
        return JSON.parse(url.searchParams.get('datastar') ?? '{}');
    }
    let body = '';
    for await (const chunk of stream) body += chunk;
    return JSON.parse(body || '{}');
}

// ---- Minimal router ----
const routes = new Map();          // 'POST /api/counter' -> handler(stream, headers)
export const route = (m, p, h) => routes.set(`${m} ${p}`, h);

server.on('stream', async (stream, headers) => {
    const method = headers[HTTP2_HEADER_METHOD];
    const path   = new URL(headers[HTTP2_HEADER_PATH], 'https://x').pathname;
    const handler = routes.get(`${method} ${path}`);
    if (!handler) return stream.respond({ ':status': 404 }), stream.end();
    try {
        await handler(stream, headers);
    } catch (err) {
        if (!stream.headersSent) sseHead(stream);
        patchSignals(stream, { meta: { isLoading: false, error: 'Server error' } });
        stream.end();
    }
});

server.listen(8443);
export { sseHead };

HTTP/2 + SSE gotchas:

Gotcha Fix
Connection, Keep-Alive headers are forbidden in h2 Just omit them — the connection is persistent by design
Proxies (nginx) may buffer SSE proxy_buffering off; and X-Accel-Buffering: no
Client disconnect detection Listen on stream.on('close', …) (h2), not req.on('aborted')
Per-connection concurrent stream cap (default ~100) Plenty for SSE; raise peerMaxConcurrentStreams only if you truly need more

13.6 Backend Integration Example (Node.js HTTP/2)

The counter example — read incoming signals, mutate, patch both signal and element back:

import { route, sseHead, readSignals, patchSignals, patchElements } from './server.mjs';

route('POST', '/api/counter', async (stream, headers) => {
    const signals = await readSignals(stream, headers);
    const count = (signals.count ?? 0) + 1;

    sseHead(stream);
    patchSignals(stream, { count });                                  // scalar state
    patchElements(stream, `<div id="counter">${count}</div>`, {      // rendered state
        selector: '#counter',
    });
    stream.end();                                                     // one-shot action: close
});

13.7 Multi-Screen Navigation Server Pattern (Node.js HTTP/2)

route('GET', '/api/screen', async (stream, headers) => {
    const url = new URL(headers[':path'], 'https://x');
    const screen = url.searchParams.get('screen') ?? 'home';

    sseHead(stream);
    // Render the screen template (any template engine — here a function map)
    patchElements(stream, renderScreen(screen), {
        selector: '#screen-content',
        mode: 'inner',
    });
    // Update navigation state in the same response
    patchSignals(stream, { ui: { screen, isNavigating: false } });
    stream.end();
});

13.8 Long-Lived SSE Stream Pattern (Node.js HTTP/2)

For §7 (LISTEN/NOTIFY) and §8 (Change Streams) the stream stays open — same helpers, no stream.end() until disconnect:

route('GET', '/api/events/stream', async (stream, headers) => {
    sseHead(stream);

    const unsubscribe = subscribeToChannel('orders', (evt) => {
        patchElements(stream, renderEventCard(evt), {
            selector: '#event-feed', mode: 'prepend',
        });
    });

    const heartbeat = setInterval(() => stream.write(': heartbeat\n\n'), 30_000);

    stream.on('close', () => {        // h2 disconnect signal
        clearInterval(heartbeat);
        unsubscribe();
    });
});

13.9 SSE Reconnection Workaround (surviving restarts & error responses)

Datastar's SSE client auto-retries on transient drops, but by default (retry: 'auto') it retries on network errors only — not on HTTP error responses or graceful disconnects.

Try the built-in option first. The fetch actions take retry, retryInterval, retryScaler, retryMaxWait, and retryMaxCount options. For a stream that must reconnect through error responses, this is the first-class fix — no polling needed:

<!-- 'error' retries on 4xx/5xx too; 'always' retries on all non-204 responses.
     openWhenHidden keeps a dashboard stream alive in a background tab. -->
<div data-init="@get('/updates', {retry: 'always', retryMaxCount: 1000, openWhenHidden: true})"></div>

Reach for the manual poll below only when you also want an explicit connection-status indicator, or behavior the retry options don't cover (e.g. a hard cap with a visible "reconnecting" banner and custom backoff UX).

Manual approach adapted from a community workaround by @gazpachoking (via alvarolm/datastar-resources).

The pattern combines four pieces:

Attribute Role
data-indicator="_connecting" Sets $_connecting true while a fetch is in flight — guards against overlapping connection attempts
data-on-interval__duration.30s.leading Fires immediately (.leading), then every 30s — the re-connection heartbeat
data-signals:_disconnected="false" Tracks whether to show the "offline" banner
data-on:datastar-fetch Listens to Datastar's fetch lifecycle events (evt.detail.type) to flip $_disconnected 
<!-- Re-establish the SSE stream every 30s IF not already connecting.
     .leading fires once on load, then on each interval. The $_connecting
     guard prevents stacking duplicate streams (§6.9 single-flight, applied to SSE). -->
<div data-signals-_disconnected="false"
     data-indicator="_connecting"
     data-on-interval__duration.30s.leading="!$_connecting && @get('/updates')"

     data-on:datastar-fetch="
       /* Only react to THIS element's stream events */
       el === evt.detail.el && (
         /* Any datastar-* SSE event means we're connected → clear the banner */
         (evt.detail.type.startsWith('datastar') && ($_disconnected = false)) ||
         /* These lifecycle events mean the stream ended → show the banner,
            the next interval tick will attempt to reconnect */
         (['retrying', 'error', 'finished'].includes(evt.detail.type) && ($_disconnected = true))
       )">
</div>

<!-- Connection-status banner: only visible while disconnected -->
<div data-show="$_disconnected"
     class="fixed top-0 inset-x-0 bg-amber-500 text-white text-sm text-center py-1.5 z-50">
  Reconnecting to server…
</div>

How it behaves across a server restart:

  1. Server goes down → Datastar emits error/finished → $_disconnected = true → banner shows.
  2. Datastar stops its own retries (graceful/error close), but the 30s interval keeps firing.
  3. Each tick checks !$_connecting and re-issues @get('/updates').
  4. Server comes back → the new stream's first datastar-* event flips $_disconnected = false → banner hides.

Tuning notes:

  • The 30s interval is a ceiling on reconnect latency. Lower it (e.g. __duration.5s) for snappier recovery at the cost of more idle requests; pair with server-side rate limiting.
  • This composes with the resume mechanisms elsewhere in this guide: on reconnect, send Last-Event-ID so the server replays only what was missed — PG via a since offset (§7.9) or Mongo via a change-stream resume token (§8.3). The workaround handles reconnecting; resume tokens handle not losing events in the gap.
  • For one-shot actions (not streams), prefer the simpler retry button in §6.6 — this section is specifically for long-lived streams.

13.10 Intercepting & Transforming SSE Streams (advanced)

Datastar exposes no official hook to sit between the network and its SSE parser, but because it uses the standard fetch API, you can monkey-patch window.fetch to observe or rewrite SSE bytes before Datastar sees them — useful for debugging, analytics, client-side caching/replay, sanitization, or recovering from malformed payloads.

Adapted from alvarolm/datastar-resources. This is unofficial — it depends on Datastar's use of fetch internally and may need updating if internals change. Apply the patch before Datastar initializes (load this script first), and prefer the alternatives at the end when they suffice.

// Patch fetch BEFORE the Datastar bundle loads so it intercepts every request.
const originalFetch = window.fetch;

window.fetch = async function (url, options = {}) {
  try {
    const response = await originalFetch(url, options);
    if (!response.body) return response;            // nothing to transform

    // Ask the transformer for a TransformStream (or null = pass through untouched)
    const transformStream = fetchStreamTransformer({ url, options, response });
    if (!transformStream) return response;

    // Pipe the body through the transform. This stays compatible with Datastar
    // because the new Response exposes the transformed body via BOTH
    // response.body (used for SSE via getBytes) and response.text() (used for HTML/JSON).
    const transformedBody = response.body.pipeThrough(transformStream);
    return new Response(transformedBody, {
      status: response.status,
      statusText: response.statusText,
      headers: response.headers,
    });
  } catch (error) {
    console.error('Fetch interception error:', error);
    throw error;
  }
};

The transformer decides per-response whether to act, and returns a TransformStream that processes each chunk:

function fetchStreamTransformer({ url, response }) {
  // Only touch SSE streams — leave HTML/JSON responses alone
  const contentType = response.headers.get('content-type') || '';
  if (!contentType.includes('text/event-stream')) return null;

  const decoder = new TextDecoder();
  const encoder = new TextEncoder();

  return new TransformStream({
    transform(chunk, controller) {
      try {
        const text = decoder.decode(chunk, { stream: true });
        console.log(`[SSE ${url}]`, text);          // observe

        // To MODIFY the stream, re-encode edited text instead of passing the chunk:
        //   const edited = text.replace(/secret/g, '***');
        //   controller.enqueue(encoder.encode(edited));

        controller.enqueue(chunk);                  // pass through unchanged
      } catch (err) {
        console.error('Transform error:', err);
        controller.enqueue(chunk);                  // never break the stream on error
      }
    },
    flush() {
      // Handle any buffered tail when the stream ends
      const tail = decoder.decode();
      if (tail) console.log(`[SSE ${url}] final:`, tail);
    },
  });
}

Why this is safe with Datastar: the spec'd Response object surfaces the transformed body through both response.body (a ReadableStream, which Datastar consumes via getBytes() for SSE) and response.text() (which Datastar uses for HTML/JSON) — so a single transform works regardless of how Datastar reads the response.

Use cases: debugging/event logging, connection analytics, offline caching and replay of SSE streams, payload transformation, malformed-data recovery, and content sanitization.

Cautions:

  • It patches the global fetch — every request in the page goes through it. Keep the content-type guard so you only transform SSE.
  • Always try/catch inside transform and pass the original chunk through on error — a throw here kills the connection.
  • Watch performance on high-volume streams; decoding/re-encoding every chunk isn't free.
  • Prefer simpler options first: transform server-side before sending (§13.5 helpers), handle events client-side after Datastar processes them, or use a plugin (§10) if official support lands.

13.11 Advanced Node.js SSE Optimizations

The §13.5 helpers are correct but naïve about a few things that bite in production: slow clients, syscall overhead, multiple cores, and deploys.

1. Backpressure — the bug that silently eats memory

stream.write() returns false when the kernel/socket buffer is full (a slow client, a stalled mobile connection). If you keep writing anyway, Node buffers the unsent data in process memory, unbounded — one slow client can OOM the server. You must respect the signal and wait for 'drain':

// A per-stream writer that honors backpressure and drops hopeless clients.
function makeWriter(stream) {
  let backed = false;
  stream.on('drain', () => { backed = false; });
  return (frame) => {
    if (backed) return false;                  // already saturated → skip (or queue, bounded)
    const ok = stream.write(frame);
    if (!ok) {
      backed = true;                           // buffer full → stop until 'drain'
      // Safety valve: if it never drains, the client is dead — cut it loose.
      const kill = setTimeout(() => stream.destroy(), 30_000);
      stream.once('drain', () => clearTimeout(kill));
    }
    return ok;
  };
}

For a live feed, dropping intermediate frames for a backed-up client is usually correct — they'll get the next full state on the following patch. For an event log where every frame matters, buffer into a bounded queue and disconnect (forcing a resume via Last-Event-ID, §7.9/§8.3) if it overflows. The one thing you must never do is write unconditionally.

2. cork()/uncork() — fewer syscalls per patch

A patch is several write()s (event line, selector, mode, elements). Each can be its own TCP segment. cork() buffers them and uncork() flushes as one writev — fewer syscalls, fewer packets:

function patchElementsCorked(stream, html, { selector, mode } = {}) {
  stream.cork();                               // batch the following writes
  stream.write('event: datastar-patch-elements\n');
  if (selector) stream.write(`data: selector ${selector}\n`);
  if (mode)     stream.write(`data: mode ${mode}\n`);
  for (const line of html.split('\n')) stream.write(`data: elements ${line}\n`);
  stream.write('\n');
  process.nextTick(() => stream.uncork());     // flush on next tick as a single write
}

3. Precompute the constant parts of hot frames

On a high-fanout feed you serialize the same event to thousands of streams. Do the work once: build the frame Buffer a single time, then write that Buffer to every subscriber — not JSON.stringify + string-build per client.

listener.on('notification', async (msg) => {
  const rows = await pool.query('SELECT … WHERE id = $1', [msg.payload]);
  // Build the SSE frame ONCE as a Buffer (UTF-8 encoded), reuse for all subscribers.
  const frame = Buffer.from(renderFrame(rows.rows[0]), 'utf8');
  for (const stream of subscribers.get(msg.channel) ?? []) writeTo(stream, frame);
});

Writing a pre-encoded Buffer skips per-write UTF-8 encoding and string allocation — measurable at thousands of fan-outs per second.

4. Multi-core: the in-memory fan-out map doesn't cross workers

§7.10/§8.7 keep subscribers in process memory. Under cluster or multiple containers, a DB notification arriving on worker A can't reach an SSE client pinned to worker B. Two correct topologies:

(a) Dedicated dispatcher process
    PG/Mongo ──change──▶ [1 dispatcher] ──Redis pub/sub──▶ [worker 1..N] ──SSE──▶ clients
    One DB listener total; workers subscribe to Redis channels and own only their sockets.

(b) Per-worker listener (simpler, heavier on the DB)
    Each worker opens its own LISTEN / change-stream cursor. Fine for PG NOTIFY (cheap),
    but N change-stream cursors multiply oplog read load — prefer (a) for Mongo at scale.

// Sketch of (a): worker side — no DB listener, just Redis → local sockets
import { createClient } from 'redis';
const sub = createClient({ url: process.env.REDIS_URL });
await sub.connect();
await sub.pSubscribe('sse:*', (payload, channel) => {
  const ch = channel.slice(4);                 // strip 'sse:'
  for (const stream of subscribers.get(ch) ?? []) writeTo(stream, Buffer.from(payload));
});

SSE also needs sticky sessions at the load balancer (the long-lived stream must stay on one worker/box) — or topology (a), where any worker can serve any client because Redis carries the events.

5. Stream large query results — don't buffer rows

A catch-up query returning 50k rows shouldn't be .toArray()'d into memory then written. Stream rows straight to the socket (respecting backpressure from #1):

import QueryStream from 'pg-query-stream';
const q = new QueryStream('SELECT id, payload FROM events WHERE created_at > $1', [since]);
const rows = pgClient.query(q);                 // a readable stream of rows
rows.on('data', (row) => writeTo(stream, Buffer.from(renderRow(row))));
rows.on('end', () => stream.write(': catchup-complete\n\n'));
// Mongo: cursor.stream() gives the same row-by-row readable.

Peak memory stays flat regardless of result size, and the client starts receiving rows immediately instead of after the whole set loads.

6. Graceful shutdown — don't black-hole clients on deploy

On SIGTERM (deploy/scale-down), open SSE streams are killed abruptly and clients may not reconnect cleanly. Drain them: stop accepting new streams, tell existing ones to reconnect, then exit.

process.on('SIGTERM', async () => {
  server.close();                              // stop accepting new connections
  for (const set of subscribers.values())
    for (const stream of set) {
      // Nudge the client to re-establish against a healthy instance, then close.
      stream.write('event: datastar-patch-elements\ndata: mode append\ndata: selector body\n' +
                   'data: elements <script id="reconnect">setTimeout(()=>location.reload(),1500)</script>\n\n');
      stream.end();
    }
  await listener.end();                         // release the DB listener / replication slot
  process.exit(0);
});

Combined with the reconnection handling (§13.9) and resume tokens (§7.9/§8.3), a deploy becomes a sub-second blip the user never notices instead of a wall of dead dashboards.

14. Testing & Debugging

14.1 Debug Signals Panel

Add this to your page for a live signal inspector:

<div class="fixed bottom-4 left-4 z-50 bg-gray-900 text-green-400 p-4 rounded-lg font-mono text-xs max-w-md max-h-64 overflow-auto shadow-2xl border border-gray-700"
     data-show="$debugMode"
     data-init="$debugMode = false">
  <div class="flex justify-between items-center mb-2 pb-2 border-b border-gray-700">
    <span class="font-bold text-white">Signal Inspector</span>
    <button data-on:click="$debugMode = false" class="text-gray-400 hover:text-white">✕</button>
  </div>
  <!-- Built-in: data-json-signals renders ALL signals reactively, with optional filters -->
  <pre data-json-signals class="whitespace-pre-wrap"></pre>
  <!-- e.g. <pre data-json-signals="{include: /^user/, exclude: /password/}"></pre> -->
</div>
<button data-on:click="$debugMode = !$debugMode" 
        data-class="{'bg-blue-600': $debugMode, 'bg-gray-800': !$debugMode}"
        class="fixed bottom-4 left-4 z-50 text-white px-3 py-1.5 rounded-lg text-xs font-mono shadow-lg transition-colors">
  <span data-show="!$debugMode">🔍 Debug</span>
  <span data-show="$debugMode">Hide</span>
</button>

14.2 Event Logging

<div data-on:click="console.log('Click:', {evt, element: el})">
  Debug Click
</div>

<!-- Log all signal changes -->
<div data-on-signal-patch="console.log('Signal patched:', patch)">
</div>

14.3 Network Inspection

  • Open DevTools → Network → filter by datastar
  • Inspect SSE streams in the EventStream tab
  • Check request payloads for signal values
  • Look for datastar query param in GET requests

14.4 Common Issues

Issue Cause Solution
Signals not updating Missing $ prefix Use $signalName in expressions
Actions not firing Wrong attribute syntax Use data-on:click not data-on-click
FOUC (flash of content) Datastar not loaded yet Add [data-cloak] { display: none !important; } to CSS
SSE not reconnecting Tab hidden Use __openWhenHidden.true modifier
Form values not syncing Missing data-bind Add data-bind="fieldName" to inputs
Infinite loops Signal updates trigger themselves Use data-computed instead of manual updates
Elements not patching Missing ID on top-level element Ensure patched elements have id attribute
View transitions not working Missing useViewTransition Add data: useViewTransition true to SSE
Large signal payloads Too much data in signals Keep signals minimal; use server for large data

Resources

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment