Calling handle_info from handle_cast/handle_call order of execution

Question

I have a gen_server and i do not understand the following:

-module(myserver).
-behaviour(gen_server).
-record(state,{ count=0}).

handle_cast(Message,From,State=#state{count=C})->
     self() ! something,
     {noreply,State}.
handle_info(Message,State=#state{count=C})
      NewCount=case Message of 
                   0 -> C+1;
                   true ->C,
      {noreply,State#state{count=NewCount}}.

Given the above code i want to understand the following thing:
-assuming i enter handle_cast and i send something to the mailbox of the gen_server which fits the handle_info case, will the handle_info get called on a different thread right then and there ( self() ! Message while i am still evaluating the handle_cast?

What is the order in which things unfold ?

• A

  • handle_cast enter
  • handle_cast send to own mailbox
  • handle_cast returns
  • handle_info gets triggered

• B

  1. handle_cast enter
  2. handle_cast send to own mailbox
     3.handle_info gets triggered (concurrently) and may alter state
  3. handle_cast may or may not end before handle_info finishes , and can get state altered by handle_info

Answer 1

The answer is A. All handlers of gen_server - like handle_cast or handle_info - are always executed in the same single process. The message send to self within handle_cast will be received by the gen_server implementation after the handle_cast callback returns. Only then handle_info will be called, and handle_info will receive the state returned from handle_cast.

Generally, all gen_server handlers are always called sequentially, and the state returned by a handler is passed to the handler which is called next.