Presence not picking up user leave events?

I need to perform some actions when the user leaves a channel (in most cases where they close the tab voluntarily, but there may also be a connection loss/timeout etc.)

According to posts like Phoenix.Presence run some code when user leaves the channel and, intercepting the "presence_diff" event from Presence seems to be a foolproof way to go, as it should also cover the cases where the connection terminates abnormally.

Strangely, the presence_diff event seems to only be triggered when I track the user via Presence.track (i.e. when the user joins), but not when the user leaves (e.g. when I close my browser tab under dev environment)

Meanwhile, adding a terminate(reason, socket) callback in my channel correctly catches the leave event.

I wonder what could be wrong in my configuration. Or did I not understand the use of Presence correctly?

Example code:

  def join("participant:" <> participant_id, _payload, socket) do
    if socket.assigns.participant_id == participant_id do
      send(self(), :after_participant_join)
      {:ok, socket}
      {:error, %{reason: "unauthorized"}}
  def handle_info(:after_participant_join, socket) do
    experiment_id = socket.assigns.experiment_id

    Presence.track(socket, experiment_id, %{
      # keys to track

    # Broadcast something
    # broadcast(socket, ...)
    {:noreply, socket}


  def handle_out("presence_diff", payload, socket) do
    # Only gets triggered at Presence.track, but not when the connection is closed.
    IO.puts("presence_diff triggered, payload is #{inspect(payload)}")

    leaves = payload.leaves

    for {experiment_id, meta} <- leaves do
      IO.puts("Leave information: #{meta}")
      # Do stuffs

    {:noreply, socket}
  # This works, however.
  def terminate(reason, socket) do
    IO.puts("terminated. #{inspect(reason)}")

    # Do stuffs.

OK I think I know what happened: Each "participant:" <> participant_id topic is, as its name suggests, only subscribed to by one participant. Therefore, when that participant quits, nobody is subscribed to the topic whatsoever. Apparently in this case the whole Presence also ceases to function.

This is not really what I expected, as I need to run some cleanup after a participant exits, but under this mechanism, whenever the last participant that’s out there exits, my callback wouldn’t be triggered at all. It appears that Presence is not suitable for this purpose.

1 Like

The issue is that the presence leave event is triggered when the process dies. But the process itself is subscribed to this information, so you can’t really do anything in that process, because it is already dead. If there was a separate process listening for those events, it should work just fine.


I see. Thanks for the reply. I also found a related old thread at How to create a process subscribed to a channel topic to watch the handle_diff events?. Seems that if one wants really foolproof disconnection detection, one’d still need to have an external monitoring process after all then.