Building Real-time Applications with Phoenix & Elixir/

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Keep Channels Asynchronous

Keep Channels Asynchronous

Learn how to use asynchronous channels.

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Asynchronous channels

Elixir is a parallel execution machine. Each Channel can leverage the principles of OTP design to execute work in parallel with other Channels since the BEAM executes multiple processes at once. Every message processed by a Channel, whether incoming or outgoing, must go through the Channel process to execute. This can stop working well if we’re not careful about how our Channel is designed. This is easiest to see when we have an example of the problem in front of us.

We’ll leverage our existing StatsChannel to see the effect of process slowness. Let’s add a new message handler that responds very slowly.

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# hello_sockets/lib/hello_sockets_web/channels/stats_channel.ex
  def handle_in("slow_ping", _payload, socket) do
    Process.sleep(3_000)
    {:reply, {:ok, %{ping: "pong"}}, socket}
  end

We have copied our existing ping handler but have ensured that every request takes a full three seconds to complete. We can add this into our JavaScript to see how slow it is.

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// hello_sockets/assets/js/socket.js
const slowStatsSocket = new Socket("/stats_socket", {})
slowStatsSocket.connect()
const slowStatsChannel = slowStatsSocket.channel("valid")
slowStatsChannel.join()
for (let i = 0; i < 5; i++) {
  slowStatsChannel.push("slow_ping")
    .receive("ok", () => console.log("Slow ping response received", i))
}
console.log("5 slow pings requested")

When we load the link in the widget, we will see messages each time the slow_ping message receives a ...