Phoenix.PubSub in an umbrella app -- Phoenix in Action 1.4

al2o3cr · December 26, 2023, 3:06pm

What was involved in that? It shouldn’t have needed any “squelching”…

In particular, did you change anything in mix.exs like applications? That list is maintained automatically nowadays, but you’ll still see a lot of old advice to fiddle with it and it can flat-out break your app if misconfigured.

7stud:

...phoenix_apps/second_auction/auction_umbrella $ iex -S mix
...
...
...
> 14:25:30.946 [notice] Application eex exited: :stopped
> ** (Mix) Could not start application auction: Auction.Application.start(:normal, []) returned an error: shutdown: failed to start child: Phoenix.PubSub.Supervisor
>     ** (EXIT) shutdown: failed to start child: Phoenix.PubSub.PG2
>         ** (EXIT) shutdown: failed to start child: Auction.PubSub.Adapter
>             ** (EXIT) exited in: :gen_server.call(Phoenix.PubSub, {:join_local, Auction.PubSub.Adapter, #PID<0.4916.0>}, :infinity)
>                 ** (EXIT) no process: the process is not alive or there's no process currently associated with the given name, possibly because its application isn't started

This error is coming from PG2Worker, which is trying to connect to a pg process in pg_join:

github.com

phoenixframework/phoenix_pubsub/blob/507cea298bbfce46eba0d410fe4037dfe61923d8/lib/phoenix/pubsub/pg2.ex#L118


      
              {:noreply, pubsub}
            end
          
            @impl true
            def handle_info(_, pubsub) do
              {:noreply, pubsub}
            end
          
            if Code.ensure_loaded?(:pg) do
              defp pg_join(group) do
                :ok = :pg.join(Phoenix.PubSub, group, self())
              end
            else
              defp pg_join(group) do
                namespace = {:phx, group}
                :ok = :pg2.create(namespace)
                :ok = :pg2.join(namespace, self())
                :ok
              end
            end
          end

github.com

erlang/otp/blob/2bcabe9e94d6328767729d8ff4def9bc229955ab/lib/kernel/src/pg.erl#L133-L136


      
          -spec join(Scope :: atom(), Group :: group(), PidOrPids :: pid() | [pid()]) -> ok.
          join(Scope, Group, PidOrPids) when is_pid(PidOrPids); is_list(PidOrPids) ->
              ok = ensure_local(PidOrPids),
              gen_server:call(Scope, {join_local, Group, PidOrPids}, infinity).

The message suggests that the application supervisor for phoenix_pubsub didn’t start correctly, since that’s what makes the pg process:

github.com

phoenixframework/phoenix_pubsub/blob/507cea298bbfce46eba0d410fe4037dfe61923d8/lib/phoenix/pubsub/application.ex#L11-L13


      
          defp pg_children() do
            [%{id: :pg, start: {:pg, :start_link, [Phoenix.PubSub]}}]
          end

7stud:

** (Mix) Could not start application auction: exited in: Auction.Application.start(:normal, [])
    ** (EXIT) an exception was raised:
        ** (ArgumentError) The module Phoenix.PubSub was given as a child to a supervisor but it does not exist

On the other hand, this error suggests that running the app in apps/auction isn’t loading Phoenix at all.

A general tip: having separate PubSub instances is explicitly against the design goal of the Phoenix pubsub system. The idea is that code in the “not-web” application can’t explicitly call code in the “web” application (because the dependencies don’t point that way), but it can push messages into pubsub that are handled by the “web” application.