Hi all,

I’m creating an application where at any given point there are a number of active “rooms”. These rooms must have state, so when someone creates a room the application spins up a GenServer to associate with it via a DynamicSupervisor, and it is tracked in a Registry. Rooms can be disbanded a few ways, notably manually by a host, by inactivity, or by error (this last one is where my confusions come in).

When a room terminates, any client still in that room must be notified, so I have to broadcast to the channel topic. This is easy to do with Endpoint.broadcast!, but I’m wondering where this call should be made. I’ve come up with a few options:

1. The terminate/2 callback of the GenServer which is exiting. This seems to work well, but my understanding is terminate/2 is not always called (for example, running 1 / 0 in a callback did not call terminate/2). It feels weird not wanting to do something because a runtime exception might cause strange behavior (lol), but also this is Elixir, if there’s a runtime exception I want people to be gracefully kicked out of the room and send back to the lobby so they can create a new room if they like. It also feels a little weird having channel event logic in the GenServer, but not sure if that’s actually something I should be worried about.
2. The supervisor monitoring/trapping exits. I’ve actually had trouble getting this to work, but if this would be better in theory I would figure it out.
3. A different process being in charge of monitoring room GenServers and broadcasting messages on their exit. I kinda like this because it separates concerns of the room servers and room supervisor a bit, but not sure if there’s actually a reason to do this. Plus it would rely on the :trap_exit flag and/or Process.monitor, and I’m not sure if using those from a separate process is the best way to do this when terminate/2 and all children are supervised by an existing process already.

Any thoughts and/or resources I should read? I feel like the problem of associating a GenServer with an active topic can’t be a new one but I haven’t come across any resources that describe good patterns for this in-depth. Thank you!