Why handle_info is called at GenServer.start_link

Questions / Help
1

Hello,

I am completely new to elixir and found this code in a blog post:

defmodule OurNewApp.Counter do
  use GenServer
  require Logger

  @interval 100



  def start_link(start_from, opts \\ []) do
    GenServer.start_link(__MODULE__, start_from, opts)
  end



  def get(pid) do
    GenServer.call(pid, :get)
  end



  def stop_gracefully(pid) do
    GenServer.call(pid, :stop_gracefully)
  end



  def init(start_from) do
    Process.flag(:trap_exit, true)

    st = %{
      current: start_from,
      timer: :erlang.start_timer(@interval, self(), :tick),
      terminator: nil
    }

    {:ok, st}
  end



  def handle_call(:get, _from, st) do
    {:reply, st.current, st}
  end



  def handle_call(:stop_gracefully, from, st) do
    if st.terminator do
      {:reply, :already_stopping, st}
    else
      {:noreply, %{st | terminator: from}}
    end
  end



  def handle_info({:timeout, _timer_ref, :tick}, st) do
    :erlang.cancel_timer(st.timer)

    new_current = st.current + 1

    Logger.info("Call handle_info")

    if st.terminator && rem(new_current, 10) == 0 do
      GenServer.reply(st.terminator, :ok)
      {:stop, :normal, %{st | current: new_current, timer: nil}}
    else
      new_timer = :erlang.start_timer(@interval, self(), :tick)
      {:noreply, %{st | current: new_current, timer: new_timer}}
    end
  end



  def terminate(reason, st) do
    Logger.info("terminating with #{inspect(reason)}, counter is #{st.current}")
  end



end

when I start the GenServer by caling start_link, the handle_info will be also invoked and I dont understand why because I thought the handle_info will be only executes when I send directly messages to the Genserver by using send method. Could please someone explain why it comes to this behaviour.

Thank you

2

init is called after the GenServer starts.

This starts a timer that will send a message :tick after @interval milliseconds. These messages are handled in handle_info(:tick, ...).

2 Likes
3

Thank you for the fast answer. Now it is clear.