Ok, this sounds very good, maybe you have some more info on this topic? 
I would like to check out some example, to get my head around it. I mean how to join inside the a join, and what do I have to handle inside and so, or is it simple like that?
def join("users", %{"id" => id}, socket) do
join("users:" <> id, _, socket)
end
It’s at: Phoenix.Endpoint — Phoenix v1.8.8
To join you’d actually do:
def join("users", %{"id" => id}, socket) do
MyEndpoint.subscribe("users:" <> id)
end
At which point this same "users" process will get messages for "users:"<>id, but it will not automatically forward them to the client, to do that you just listen for them (just like any other PubSub message, Phoenix Channels are just Phoenix.PubSub underneath) and then push them to the client however you want. 
Hmm, I think I understood how to subscribe to a virtual topic. And to listen on the incoming events / firing events, seams to be clear. But the last problem I have is to understand how to
… push them to the client however you want.
I would like to push the message to the client over the “users” channel, where the client joined initially to.
…I wish there would be a simple API for like Endpoint.send(socket, payload, topic) function, where the topic process could hold many sockets, but allow direct messaging to each separately. I mean it must work somehow, like handle_in → reply to a single client, right?
Think of it this way:
broadcast messages between them via topics they are subscribed toThus you just listen for the messages via normal handle_info or whatever it was for sockets, then just push the message back out to itself just like you do any other message.
An Endpoint.send(...) makes no sense as you can already push(..., socket, ...) anyway, which is the same thing.
Oh Yeah, now I got it.. push is what I was looking for all the time… many thanks. Really strange that this feature is only mentioned in in the interceptions and outgoing events section. This makes it much much clearer..
So, I could basically manage the sockets by myself somewhere in a custom GenServer or so, and broadcast some messages to all sockets, and some for specific users, just with push.. you made my day (night actually) Thanks!
Easiest way for specific users is just to have them join a topic all on their own with their user ID or so, like you user:uuid above did, then just broadcast to it.
Don’t use any custom genserver’s, not at all useful since you can do it all (and safer and faster) via Phoenix.PubSub.
I’ll do it like you said for my MVP, but its good to know that those options exists and I’ll definitely play around with it when the time comes 