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Building Real-time Applications with Phoenix & Elixir

Dynamic Topics

Explore how to implement dynamic topics in Phoenix Channels to create flexible, real-time web applications. Understand the use of wildcard patterns for routing and validating topics to enable precise and efficient communication between clients and servers in Elixir-based systems.

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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.

Elixir
# 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:

Elixir
# 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.

Shell
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 * ...