Timeout when sending a channel message directly from one user to another

Hi all, hoping one of you experts might be able to point me to the error of my ways…

I’m attempting to allow a user to send a direct message to another user via a socket. I am finding every message times out, and I’m trying to find out why!

Here’s my code:

user_socket.ex assigns a UUID to the socket, and then identifies the socket using that UUID. I am expecting that it will then be possible to reference the socket topic by user:<uuid>.
(see Phoenix.Socket — Phoenix v1.8.8)

defmodule MyAppWeb.UserSocket do
  use Phoenix.Socket

  ## Channels

  # Room channel
  channel "room:*", MyAppWeb.RoomChannel

  @impl true
  def connect(_params, socket, _connect_info) do
    {:ok, assign(socket, :user_id, UUID.uuid4())}
  end

  @impl true
  def id(socket), do: "user:#{socket.assigns.user_id}"
end

In room_channel.ex I then provide a handler for the message topic which is the one I’ve chosen for users to be able to send one another direct messages:

defmodule MyAppWeb.RoomChannel do
  require Logger

  use Phoenix.Channel
  alias MyApp.RoomPresence

  def join("room:" <> _roomId, _message, socket) do
    # Send back the id that we have assigned to this particular user
    {:ok, %{ id: socket.assigns.user_id }, socket}
  end

  def handle_in("message", %{"to" => to, "body" => body}, socket) do
    socket.endpoint.broadcast("user:" <> to, "message", %{body: body, to: to})
    {:noreply, socket}
  end
end

When I attempt to send the message:

1. It appears to have been successfully posted (no errors are visible)
2. A few seconds later, a timeout is acknowledge on the client in the chan_reply_n topic, where n is a number representing the message sequence

My questions are:

1. Am I doing something wrongly?
2. How can I debug timeouts?

Thanks very much!!