Genserver timeout error on :no_reply response

shijith.k

I have GenServer setup in a module. One of the handle_call/3 returns {:no_reply, state}. Every time after executing the call a timeout error raises. Can someone tell me what I am doing wrong here? This is the first time I am trying GenServer.

My code looks like this:

defmodule TestApp.Message do
  alias TestApp.Message

  use GenServer
  import Logger

  def init(args) do
    {:ok, args}
  end

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

  def handle_call({%{"message" => message}, state}, _sender, _current_state) do
    info("message")
    {:noreply, state}
  end
end

This is the error message:

{:timeout, {GenServer, :call, [MyApp.Message, {%{"message" => message}, %{id: 1, sender: "user1}}, 5000]}}

5 comments

#learning-elixir #timeout #genserver

0 1997 5

2020-03-22 21:10:05 UTC

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rvirding

Creator of Erlang

The {:noreply, state} return value tells the behaviour not to send a reply to the GenServer.call. As it has built-in timeout of 5 seconds (which can be changed) then it will timeout when a reply does not arrive within that time. You need to explicitly send back a reply by returning {:reply, reply, state}.

Post #1

shijith.k

I am planning to set up a connection with an iOT device, which does not expect a reply there. In such cases what can I do?

Post #2

kokolegorille

You need to use handle_cast in this case…

Post #3

rvirding

Creator of Erlang

Yes, one way is to use GenServer.cast/handle_cast. One problem with that is that is gives no guarantees of the destination being there or the message arriving as it literally uses :erlang.send internally. For more safety you could use GenServer.call/handle_call and return the reply :ok. NB that this is doing a synchronous call with all that it costs but you will at least be certain the message has arrived and has been processed.

It’s all a matter of deciding what you need/want and are willing to pay for it. TANSTAAFL

Post #4

lucaong

In other words, when you use handle_call, your caller (the process that calls GenServer.call) expects a reply. One can return {:noreply, state} from handle_call, but only if the reply is computed asynchronously and then sent later with GenServer.reply.

As @rvirding and @kokolegorille wrote, you have two options:

Return a simple reply like {:reply, :ok, state}. This ensures that GenServer.call will return :ok only after the operation that you perform in the GenServer completes. This is, generally, the best way, as it ensures that whatever you wanted to execute actually does execute.
Alternatively, if you really don’t care about the answer, and also you don’t need to guarantee that the GenServer actually performed the operation, you can use GenServer.cast and handle_cast.