Phoenix Socket draining

We’ve come up with a not-so-great workaround for now.

SocketDrano had a couple issues for us:

• it relied on channel join telemetry events, so it would miss sockets that do not join a channel
• it was oriented around draining upon sigterm, but we’re relying on CodeDeploy lifecycle events for triggering the draining

So, we ended up “extending” Phoenix.Socket by creating our own Socket module that has overrideable callbacks:

Then we create a custom init callback:

@impl Phoenix.Socket.Transport
   def init(state) do
     super(state)
     |> tap(fn
       {:ok, {_state, %Phoenix.Socket{transport_pid: pid}}} when is_pid(pid) ->
         track_socket(pid)

       _ ->
         :ok
     end)
   end

track_socket then does essentially what SocketDrano does (monitor the socket pids, disconnects them when told to).

I’ve opened a thread on the Phoenix Core mailing list asking about better solutions being added Phoenix itself: https://groups.google.com/g/phoenix-core/c/1umhh2X1oAM/m/_sfxU8xIBgAJ

For folks coming across this recently: Phoenix sockets now have built-in connection draining (since 1.7.2). You can configure it with the drainer option to socket/3: Phoenix.Endpoint — Phoenix v1.8.8

Presence.track can store key/values so you could store hostname and userid? So on a quit signal do a lookup on hostname and you have a list of connected sockets for that node? And you could use Phoenix.PubSub — Phoenix.PubSub v2.2.0 to keep the broadcast local to the node.