Phoenix Channels: Prevent auto-subscription to raw client topic

Phoenix Forum Chat / Discussions
, ,
1

TL;DR

Clients join a simple topic like "daily_agent_stats:total_inbound_calls".
On the server I expand it using workspace_id + agent_id from params.
But Phoenix still subscribes them to the raw topic too → I want to stop that and only keep the expanded subscription.

Hey folks :waving_hand:

I’m building a Phoenix channel for agent analytics stats.
Here’s a simplified version of my use case:

What I want

  • Clients connect with workspace_id and agent_id as query params.

  • Then they only subscribe with a simplified topic (no need to repeat IDs).

  • On the server, I expand that simplified topic into the full PubSub key and subscribe internally.

Example

Channel:

defmodule MyAppWeb.AgentChannel do
  use Phoenix.Channel

  # Client joins something like: "daily_agent_stats:total_inbound_calls"
  def join("daily_agent_stats:" <> stat_key, _payload, socket) do
    workspace_id = socket.assigns[:workspace_id]
    agent_id = socket.assigns[:agent_id]

    # Expand topic internally
    full_topic = "agent/analytics/daily_agent_stats:#{workspace_id}:#{agent_id}:#{stat_key}"

    IO.puts("Subscribing to #{full_topic}")
    Phoenix.PubSub.subscribe(MyApp.PubSub, full_topic)

    {:ok, socket}
  end

  def join(topic, _payload, _socket) do
    {:error, %{reason: "Invalid topic #{topic}"}}
  end
end

UserSocket:

defmodule MyAppWeb.UserSocket do
  use Phoenix.Socket

  channel "daily_agent_stats:*", MyAppWeb.AgentChannel

  def connect(%{"workspace_id" => ws_id, "agent_id" => agent_id}, socket, _connect_info) do
    {:ok, assign(socket, :workspace_id, ws_id) |> assign(:agent_id, agent_id)}
  end

  def id(_socket), do: nil
end

Client connection (via wscat):

wscat -c "ws://localhost:4000/v1/agent/websocket?workspace_id=c8f4a3a0-835d-4ee2-9c71-0c18550680e5&agent_id=47dbbab6-1abf-4392-a42f-dbde107c2793"

Join a topic without IDs:

{"topic":"daily_agent_stats:total_inbound_calls","event":"phx_join","payload":{},"ref":1}

The Issue:

Even though I subscribe manually inside join/3, Phoenix still also assigns the socket to the original "daily_agent_stats:total_inbound_calls" topic.

So the socket ends up listening on two topics:

  • "daily_agent_stats:total_inbound_calls"

  • "agent/analytics/daily_agent_stats:<workspace_id>:<agent_id>:total_inbound_calls"

I want to disable the first (auto) subscription and only rely on the expanded one.

My Question:
Is there a way to tell Phoenix not to subscribe the socket to the raw topic passed in by the client, so I can fully control subscriptions in join/3?

Any guidance on best practices here would be super helpful.

2

No. Channels are not an abstraction for communication between only the client and the server (1 to 1). They’re an abstraction to extend phoenix pubsub from just reaching processes on servers to their connected clients – it’s still pubsub on the channels topic.

If you want plain websockets without the semantics of channels take a look at: Bare Websockets | Benjamin Milde

Alternatively you could also make the client join the correct topic directly.

2 Likes
3

Thanks, that makes sense. If I stick with channels, is there a recommended way to avoid exposing sensitive IDs (like workspace_id, agent_id) in the topic string? Or is the expectation that topics are not security boundaries and I should enforce authorization only in join/3?

4

Yeah consider topics almost like URLs. In general a secret URL isn’t great security, you instead should perform a real authZ check on join.

1 Like