Correct GenServer implementation with persistent state

Question

I am new to some OTP concepts. I have GenServer, that will Publish events to RabbitMQ. This GenServer has the state: amqp Chanel which is initiates once during init() and is persistent between cast invokes.

defmodule Myapp.Events.AmqpTransport do

  require Logger
  use GenServer
  use AMQP

  def start_link(_) do
    GenServer.start_link(__MODULE__, [], name: __MODULE__)
  end

  def init(_opts) do
    username = get_conf(:username)
    password = get_conf(:password)
    host = get_conf(:host)
    port = get_conf(:port)
    vhost = String.replace(get_conf(:vhost), "/", "%2f")
    exchange = get_conf(:exchange)
    {:ok, conn} = Connection.open("amqp://#{username}:#{password}@#{host}:#{port}/#{vhost}")
    {:ok, chan} = Channel.open(conn)

    {:ok, chan}
  end

  def handle_cast({:emit, event}, chan) do
    payload = :jiffy.encode(%{event: event})
    Basic.publish(
      chan,
      get_conf(:exchange),
      get_conf(:routing_key),
      payload
    )
    {:noreply, :ok, chan}
  end

  def emit(event) do
    GenServer.cast(__MODULE__, {:emit, event})
  end

  defp get_conf(key) do
    conf = Application.get_env(:myapp_events, :rabbit)
    conf[key]
  end
end

When I call it using Myapp.Events.AmqpTransport.emit(%{"hop": "hej"}) I get error:

[error] Supervisor 'Elixir.Myapp.Events.Supervisor' had child
        'Elixir.Myapp.Events.AmqpTransport' started with
        'Elixir.Myapp.Events.AmqpTransport':start_link([])
        at <0.7024.0> exit with reason timeout_value
        in gen_server:loop/7 line 437
        in context child_terminated

What I missed here?

Answer 1

You should return two-element tuple {:noreply, chan}, not three-element tuple {:noreply, :ok, chan} from GenDerver.handle_cast/2.

Casts are async and hence they don’t return anything. Your three-element tuple {:noreply, _, _} is treated as a response in a form

{:noreply, new_state,
    timeout() | :hibernate | {:continue, term()}}

and the state, passed as a third element, is expected to be a timeout (it does not match :hibernate, nor the tuple), but its value is not a timeout by any means.