GenServer.handle_call/3 Where is the origin of the from arg?

dband · November 1, 2022, 11:00am

Hello, I am currently working trough the Coding Gnome for Elixir 2. There is an excercise where I am expected to save active clients in a list and remove them when they get disconnected.

There is 1 GenServer that is created by a DynamicSupervisor. The Genserver contains a register_client function, which stores the PID of the from argument into an Agent. When the client disconnects the PID is removed from the Agent in handle_info.

So far that seems to work, but I dont really understand why. I can´t trace where the contents of the fromarg originate. Also I can´t find the PID in the erlang observer. My understanding is that the field is supposed to identify the calling client, but I am stuck here.

Can anyone shed some light?

cmo · November 1, 2022, 11:20am

When you call GenServer.call you’re sending a message of a specific format and the GenServer behaviour handles receiving these messages for you. One part of that format includes the caller’s pid, retrieved with self().

The pid is created around here:

github.com

erlang/otp/blob/462bce4222def663359f2704b3f12fc655fbefb2/lib/stdlib/src/gen.erl#L256


      
          Mref = erlang:monitor(process, Process, [{alias,demonitor}]),
          
          
Tag = [alias | Mref],
          
          
%% OTP-24:
          %% Using alias to prevent responses after 'noconnection' and timeouts.
          %% We however still may call nodes responding via process identifier, so
          %% we still use 'noconnect' on send in order to try to send on the
          %% monitored connection, and not trigger a new auto-connect.
          %%
          erlang:send(Process, {Label, {self(), Tag}, Request}, [noconnect]),
          
          
receive
              {[alias | Mref], Reply} ->
                  erlang:demonitor(Mref, [flush]),
                  {ok, Reply};
              {'DOWN', Mref, _, _, noconnection} ->
                  Node = get_node(Process),
                  exit({nodedown, Node});
              {'DOWN', Mref, _, _, Reason} ->
                  exit(Reason)

And is handled around here:

github.com

erlang/otp/blob/462bce4222def663359f2704b3f12fc655fbefb2/lib/stdlib/src/gen_server.erl#L1177


      
          	   end;
          	false ->
          	    {ok, ok}
              end.
          
          

          
%%% ---------------------------------------------------
          %%% Message handling functions
          %%% ---------------------------------------------------
          
          
handle_msg({'$gen_call', From, Msg}, Parent, Name, State, Mod, HibernateAfterTimeout) ->
              Result = try_handle_call(Mod, Msg, From, State),
              case Result of
          	{ok, {reply, Reply, NState}} ->
          	    reply(From, Reply),
          	    loop(Parent, Name, NState, Mod, infinity, HibernateAfterTimeout, []);
          	{ok, {reply, Reply, NState, TimeoutOrHibernate}}
                    when ?is_timeout(TimeoutOrHibernate);
                         TimeoutOrHibernate =:= hibernate ->
          	    reply(From, Reply),
          	    loop(Parent, Name, NState, Mod, TimeoutOrHibernate, HibernateAfterTimeout, []);

An easy way to dig into the source is to click the </> on hexdocs. If you see an erlang function, e.g. :gen.call, then you’ll need to switch to the erlang source on GitHub.

dband · November 2, 2022, 4:52am

Got it, thanks. Also additionally, this answer on PID format might help others as well.