Sending messages between clients NOT subscribed to channel:topic?

Phoenix Forum Questions / Help
1

User A makes a socket connection and “user:A” channel/topic is created. This channel will be used for all application specific data (will mimic classic HTTP req/res pattern) for this user. The same thing will happen when user B connects.

User A views user’s B profile, perhaps sends this user an email basically does something that user B might be interested in knowing.

These users are not subscribed to the same channel:topic. Therefore how can I propagate this specific event (for example “user A viewed your profile”) from user’s A socket over to user’s B socket?

One possible solution would be to have two channels (one public to which all users subscribe and one private) for all users but I don’t want to notify user C that user A visited user’s B profile. This event would have been propagated to all connected clients over the public channel, which is not really what I’d want (seems like a waste of resources to send a message to N users when only one is interested).

Is there a way to filter this somehow? Would I have to filter/ignore on the client side?

2

I’m on mobile so I’ll keep it short. But as I understand you, you can do a global channel that everyone joins and use handle_out to do the filtering before actually sending the message over the network.

3

Just send an Endpoint.broadcast call to whatever topic you want to receive the message.

4

You broadcast directly to their topic.

From User A’s channel:

MyApp.Endpoint.broadcast!("user:#{user_b_id}", "profile_view", %{
  by_user_id: socket.assings.user_id
})

And that’s all there is to it. The same rules apply as broadcasting to external topics from anywhere in the cluster. See this section in the docs for more details:
https://github.com/phoenixframework/phoenix/blob/f2e02d97ef79f825f6bbbdc2d7b69a1d005bff90/lib/phoenix/channel.ex#L130

3 Likes
5

Thanks guys, I appreciate your help!

6

just a questions: if I have hundred of thousands of users, Is creating a topic for every connected user a good solution ?

7

I certainly hope so because each of my sockets have about 10 topics each for various purposes… >.>

8

Yes :rocket:

3 Likes
9

Glad to be vindicated! Always worried I’d eventually hit some unknown limit. :slight_smile:

I use topics excessively for connecting to various services and for each of broadcasting. :slight_smile: