command vs event, sync vs async

gistfile1.txt

Hey,

So I've been thinking about the source of data for the delivery system which is
the creation of the delivery. I've read a bit and talk to some people and
wanted to share some info.

First, the difference between a command and an event is their intent. Both are
DTOs (just transfer data) but:

- an event is something that happened in the past in the producer of the event,
  e.g rails-backend produces Delivery Created
- an event can be handled by different event handlers (e.g increment the
  counter of creations and send an email)
- a command is like an action and is in the present tense, e.g Create Delivery
- and a command is handled by one and just one command handler (but it can do
  multiple things).

Notice that I've not talked about message brokers yet. A message broker enables
asynchronicity. You send a message (fire&forget) and optionally you consume
other messages that will arrive in the future.

The typical messages to send are events (message broker support multiple
consumers), but you could also send commands (even we just need one consumer).
Also, that would make the commands async, another key difference. The nature of
an event is fire&forget, which means you don't care about how it's handled. On
the contrary, for a command, you want a response back for consistency reasons
because you need it to know it has been executed properly before executing
other commands.

By using a message broker to send commands in an async way, you also need a
message with its response somehow (that's how WebSocket RPC systems work for
e.g). Using an HTTP API you also have a response, but it's sync: you get the
HTTP response right away.

So in the end what I'm saying is that we don't need async for creating
deliveries and it should be a plain HTTP endpoint instead of a message via
message broker like we proposed to Suraj some days ago. That makes everything
simpler.

Sorry for the long text, and happy to discuss any doubts.