[Elixir Tip] Endpoint.broadcast versus PubSub.broadcast

This is a snippet post since I couldn’t find this documented clearly.

MyAppWeb.Endpoint.broadcast/3 is a wrapper around PubSub.broadcast.

Why would you need to use the Phoenix.PubSub.broadcast?

Well, because you want to isolate your context layer from your Web side of the code - I’m talking about in Phoenix. This allows for good divide between what is web related and what is business related. And last weekend was when I realized that I was using MyAppWeb.Endpoint.broadcast() everywhere in my context layer! Aghast!

For more information on how to divide / organize Elixir projects, please take a look at https://medium.com/very-big-things/towards-maintainable-elixir-the-core-and-the-interface-c267f0da43, post by Saša Jurić.

In order to make them call the same signature so it gets captured by following subscription function signature:

  @impl true
  def handle_info(
        %Phoenix.Socket.Broadcast{
          event: "testing",
          payload: payload,
          topic: _topic
        } = msg,
        socket
      ) do
    IO.inspect(msg, label: "TESTING BROADCST ON EVEN TTESTING")
    {:noreply, socket}
  end

You have to do the following.

1. First, subscribe. For the sake of discussion let’s say you have this setup for your LiveView page, listening in on this topic. Hence using MyAppWeb.Endpoint here:

         MyAppWeb.Endpoint.subscribe("delivery_slots:1")
   

Note for doing the same thing in Phoenix.PubSub, this is the way:

          Phoenix.PubSub.subscribe(MyApp.PubSub, "delivery_slots:1")

2. If this is how it’s broadcasted in MyAppWeb.Endpoint.Broadcast:

   MyAppWeb.Endpoint.broadcast("delivery_slots:1", "testing", %{test: "data"})

1. You have to call it this way when using PubSub.broadcast:

         Phoenix.PubSub.broadcast(MyApp.PubSub, "delivery_slots:1", %Phoenix.Socket.Broadcast{
           topic: "delivery_slots:1",
           event: "testing",
           payload: %{test: "data"}
         })
   

Of course before all of this - you have to setup your PubSub in the applications.ex:

  def start(_type, _args) do
    children = [
      # Start the Ecto repository
      MyApp.Repo,
      # Start the Telemetry supervisor
      MyAppWeb.Telemetry,
      ...
      # Start the PubSub system
      {Phoenix.PubSub, [name: MyApp.PubSub, adapter: Phoenix.PubSub.PG2]},
      ...