Trying to understand Phoenix.Channel.broadcast/3

So this is what I’m trying to achieve:
One part of my javascript client side code joins a topic "data:data" with a payload of data to sync to the backend.

def join("data:data", %{"query" => q} = params, socket) when is_binary(q) do
    send(self(), {:sync_data, params})
    {:ok, socket}

def handle_info({:sync_data, params}, socket) do
    response = response(params)
    broadcast(socket, "data-synced", response)
    {:noreply, socket}

This part of the client side code subscribes to the "data-synced" event and does something with the response. So far this part is working:

this.dataChannel ="data:data", params);
this.dataChannel.on("data-synced", this.dataSyncedCb);

Now other parts of the code using the same socket (I’m using Phoenix.js) will join another topic "data:client:" <> id and also subscribe to the "data-synced" event and do different things with the response.

def join("data:client:" <> _id, _params, socket), do: {:ok, socket}
this.dataSyncChannel ="data:client:289", {});
this.dataSyncChannel.on("data-synced", this.dataClientSyncedCb);

But these other parts of the code are not receiving the data-synced event.
Please note that these are all the same clients but they all have different responsibilities for the event response.

So I’m thinking may be Phoenix.Channel.broadcast/3 is not the way to go.

The socket holding the channel state works on a per topic level. So broadcast(socket, "data-synced", response) sends the event data-synced to subscribers of the data:data topic, but not so subscribers of the data:client:289 topic. Imagine a chatservice with many rooms. You wouldn’t want to send all chatmessages to all rooms there as well.

Socket here is not equivalent to “the single connection between server and client”, which is certainly a place where phoenix doesn’t do a good job at wording things. Socket in the context of a channel(-state) is rather “subscriber to a certain topic of the channel”.

1 Like

Thanks @LostKobrakai. I have also tried:

def handle_info({:sync_data, params}, socket) do
    response = response(params)
    push(socket, "data-synced", response)

    #I know all the client IDs upfront.
    MyApp.Endpoint.broadcastfrom!(self(), "data:client:" <> id, "data-client-synced", response)
    {:noreply, socket}

def handle_in("data-client-synced", params, socket) do
    {:noreply, socket}

Yet handle_in("data-client-synced", params, socket) was never called.