presence_drainer.ex

defmodule MyApp.PresenceDrainer do
  use GenServer
  require Logger

  def start_link(opts \\\\ []) do
    GenServer.start_link(__MODULE__, opts, name: __MODULE__)
  end

  @impl GenServer
  def init(_init_arg) do
    # We need to trap exits so that we receive the `terminate/2` callback during
    # a graceful shutdown
    Process.flag(:trap_exit, true)
    {:ok, []}
  end

  # NOTE: We cannot guarantee that this will run on every shutdown, it will only
  # get run on graceful shutdowns such as via a SIGTERM which is what is often
  # used for zero-downtime deploys
  @impl GenServer
  def terminate(_reason, _state) do
    Logger.info("Shutting down FeltServerWeb.Presence")
    Phoenix.Tracker.graceful_permdown(MyAppWeb.Presence)
    :ok
  end
end