Broadcasting to Phoenix Channel without Phoenix?

I’ve got an umbrella app with a Phoenix app, and some vanilla elixir apps. Is there a way to broadcast to a Phoenix channel from a vanilla elixir app? I’d like to avoid adding a dependency on the Phoenix app to reference its Endpoint.

I’m working on using Phoenix.PubSub to do this, but I’m not having any luck. This code:

    Phoenix.PubSub.broadcast(Www.PubSub, topic, %{
      __struct__: Phoenix.Socket.Broadcast,
      topic: topic,
      event: event,
      payload: payload
    })

Is giving me this error:

[error] GenServer #PID<0.952.0> terminating
** (UndefinedFunctionError) function Www.TimeChannel.handle_out/3 is undefined or private. Did you mean one of:

      * handle_in/3
      * handle_info/2

    (www) Www.TimeChannel.handle_out("now", %{now: "yolo"}, %Phoenix.Socket{assigns: %{user_id: 14904}, channel: Www.TimeChannel, channel_pid: #PID<0.952.0>, endpoint: Www.Endpoint, handler: Www.UserSocket, id: "users_socket:14904", joined: true, pubsub_server: Www.PubSub, ref: nil, serializer: Phoenix.Transports.WebSocketSerializer, topic: "time:now", transport: Phoenix.Transports.WebSocket, transport_name: :websocket, transport_pid: #PID<0.909.0>})
    (phoenix) lib/phoenix/channel/server.ex:234: Phoenix.Channel.Server.handle_info/2
    (stdlib) gen_server.erl:601: :gen_server.try_dispatch/4
    (stdlib) gen_server.erl:667: :gen_server.handle_msg/5
    (stdlib) proc_lib.erl:247: :proc_lib.init_p_do_apply/3
Last message: %Phoenix.Socket.Broadcast{event: "now", payload: %{now: "yolo"}, topic: "time:now"}
State: %Phoenix.Socket{assigns: %{user_id: 14904}, channel: Www.TimeChannel, channel_pid: #PID<0.952.0>, endpoint: Www.Endpoint, handler: Www.UserSocket, id: "users_socket:14904", joined: true, pubsub_server: Www.PubSub, ref: nil, serializer: Phoenix.Transports.WebSocketSerializer, topic: "time:now", transport: Phoenix.Transports.WebSocket, transport_name: :websocket, transport_pid: #PID<0.909.0>}

I’m confused about it wanting to use handle_out because I’m not using intercept anywhere, but maybe I just have my message format wrong. I was hoping to get the vanilla elixir app to broadcast directly to the web browser clients.

3 Likes

I’m curious to her the answer to this as well…

1 Like

Honestly I’d probably be lazy and make a configuration for the module to call on, then in the app I’d get that config entry and just call whateverConfig.broadcast(...) on it, and just configure my endpoint in the config. ^.^

handle_out is being called because It’s not respecting the channel subscriber fastlanining, but without seeing your code it’s hard to say why that is. In any case, instead of trying to build cheat by building the Phoenix.Socket.Broadcast, I would have the external app broadcast “raw” messages over pubsub that your phoenix app subscribes to. You could have the channel subscribe to the topic and forward the message in handle_info, or you can have server(s) in your app subscribing to external topics/events, and then relaying them to channels via MyApp.Endpoint.broadcast. This way you aren’t coupling the external app to your channel layer directly. This also gives you a place to do any necessary data munging from the external data to the client representation.

2 Likes

I did get it to work by making these changes:

  1. I removed the struct definition so my broadcast code looks like:
  defp broadcast(topic, event, payload) do
    Phoenix.PubSub.broadcast(Www.PubSub, topic, %{
      topic: topic,
      event: event,
      payload: payload
    })
  end

and I call it like:

broadcast("session:12345", "yolo", %{ yoloed: 1 })
  1. I added a handle_info into my SessionChannel:
  def handle_info(%{topic: _, event: ev, payload: payload}, socket) do
    push socket, ev, payload
    {:noreply, socket}
  end

I found that code at the bottom of the Channel docs under “Subscribing to external topics”.

It works, but seems a little fragile. I like @chrismccord’s idea of making a server (GenServer?) that uses Phoenix.PubSub to listen to my own format message, and then MyApp.Endpoint.broadcast them. How do I prevent double-broadcasts when I have 2 webservers running each with their own GenServer doing the message forwarding?

2 Likes

I’m facing a similar problem, that is, in an umbrella app there is one app that supervises a PubSub server, another one that broadcasts a message, and a third one, which is the Phoenix app that contains the relevant channel. There is also Presence in use, and apparently “fastlaning” has been disabled for that as well:

18:12:44.561 [error] GenServer #PID<0.940.0> terminating
** (UndefinedFunctionError) function Web.Presence.Channel.handle_out/3 is undefined or private. Did you mean one of:

      * handle_in/3
      * handle_info/2

    (web) Web.Presence.Channel.handle_out("presence_diff", %{joins: %{}, leaves:%{...
    (phoenix) lib/phoenix/channel/server.ex:252: Phoenix.Channel.Server.handle_info/2
    (stdlib) gen_server.erl:616: :gen_server.try_dispatch/4
    (stdlib) gen_server.erl:686: :gen_server.handle_msg/6
    (stdlib) proc_lib.erl:247: :proc_lib.init_p_do_apply/3
Last message: %Phoenix.Socket.Broadcast{event: "presence_diff", payload: %{joins: %{}, leaves: %{...

My solution was to simply implement the missing handle_out callback:

  def handle_out(event, payload, socket) do
    push(socket, event, payload)
    {:noreply, socket}
  end

As long as the Phoenix app creates the PubSub server this isn’t needed, but it’s suddenly required when you configure your endpoint to use an external PubSub server instead. Why?

@chrismccord is there documentation on what fastlaning actually means, what it does and why it’s disabled in some cases? I couldn’t find anything about it on hexdocs, and I’m not sure where to look in the code.

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Same question, I have a channel that broadcast events (from handle_in), with:

broadcast!(socket, "update", state)

This works well until I subscribe to a PubSub in the join callback:

:ok = Phoenix.PubSub.subscribe(MyAppWeb.PubSub, "topic")

Since I have added this subscription, the events broadcast trigger errors:

** (UndefinedFunctionError) function MyAppWeb.GridChannel.handle_out/3 is undefined or private

I can add the missing handle_out/3 callback but this doesn’t feel right and I don’t understand why I should add it.

What am I missing?

EDIT: I found out why: I was using the same topic name for my channel and my PubSub subscription. I didn’t realize channels and “global” PubSub shared the same namespace for topics.