Socket topic subscription overhead


I have a messaging app where users can have many different conversations.

Ideally I’d from the client just connect/join the Phoenix socket once, instead of having to
manually join each conversation channel (there can potentially be 10s or 100s).

For that, I was thinking of doing something like this inside the Socket connect/2 callback

ids =
  |> Conversaiton.get_user_conversations
  |> conversation -> end)

for id <- ids, do: MyApp.Endpoint.subscribe("conversations" <> id)

And then dynamically subscribe the socket to any new conversations that would be created.

My question is how much overhead there is for these subscriptions and if I will run into performance problems


You can’t subscribe in UserSocket.connect since it’s in the transport process. To get what you want, have a UserChannel with topic user:[user_id]. You could then multiplex conversations by subscribing the channel join to conversation:[convo_id]

Subscriptions are ets entries, so they are very cheap. Additionally, we pool the pubsub servers, so the underlying genserver call for subscribe is pooled.


I do what chrismccord stated in my work project to have a restricted set of user channels over a single socket internally get messages and handle upwards of a hundred ‘topics’ in just those couple of channels, it works really well. We do have a restricted set of people that will use the system though, a few thousand at most.

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Awesome, thank you

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