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Dynamic Topics

Dynamic Topics

Let's dive into the details of dynamic topics and their role in Channels.

Topics

Topics are string identifiers used to connect to the correct Channel when the phx_join message is received by the Socket. They are defined in the Socket module, as we previously saw with our UserSocket example.

# hello_sockets/lib/hello_sockets_web/channels/user_socket.ex
  
  channel "ping", HelloSocketsWeb.PingChannel

A topic can be any string, but it is best to use a topic:subtopic format for the topic name. This convention allows us to have a single Socket module with different Channels associated with it. This is because channel/3 can accept a wildcard splat operator as the final part of the string.

Let’s change our topic definitions to use a wildcard operator and then observe the effects:

# hello_sockets/lib/hello_sockets_web/channels/user_socket.ex
  channel "ping", HelloSocketsWeb.PingChannel
  channel "ping:*", HelloSocketsWeb.PingChannel

We can then connect to a ping:wild Channel and send messages to it.

wscat -c 'ws://localhost:4000/socket/websocket?vsn=2.0.0' 
connected (press CTRL+C to quit)
> ["1","1","ping:wild","phx_join",{}]
< ["1","1","ping:wild","phx_reply",{"response":{},"status":"ok"}]
> ["1","1","ping:wild","ping",{}]
< ["1","1","ping:wild","phx_reply",{"response":{"ping":"pong"},"status":"ok"}]

It’s possible to use the topic of * to allow any topic to route to the Channel. Any routing is permitted as long as the * ...