How can I receive messages sent to a PID which running inside a gen_server

Question

• I have a ejabberd server

• I have a custom module, my_apns_module.erl which is run by ejabberd server like this:

  start_link(Host, Opts) ->
    Proc = gen_mod:get_module_proc(Host, ?PROCNAME),
    ?GEN_SERVER:start_link({local, Proc}, ?MODULE,
                           [Host, Opts], []).
  start(Host, Opts) ->
    Proc = gen_mod:get_module_proc(Host, ?PROCNAME),
    ChildSpec = {Proc, {?MODULE, start_link, [Host, Opts]},
                 temporary, 1000, worker, [?MODULE]},
    supervisor:start_child(ejabberd_sup, ChildSpec).
  
  stop(Host) ->
    Proc = gen_mod:get_module_proc(Host, ?PROCNAME),
    ?GEN_SERVER:call(Proc, stop),
    supervisor:delete_child(ejabberd_sup, Proc),
    ok.
  
  init([Host, _Opts]) ->
     ...
  
  handle_call(stop, _From, State) ->
    {stop, normal, ok, State}.
  
  handle_cast(_Msg, State) -> {noreply, State}.
  
  handle_info(#offline_msg{us = _UserServer,
            from = From, to = To, packet = Packet} = _Msg, State) ->
    ...
  
  handle_info(_Info, State) ->
    {noreply, State}.
  
  terminate(_Reason, State) ->
    Host = State#state.host,
    ok.
  
  code_change(_OldVsn, State, _Extra) -> {ok, State}.
  

• Inside init I run another APNs application to send push notifictions. https://github.com/inaka/apns4erl

  init([Host, _Opts]) ->
        apns:start(),
        case apns:connect(cert, ?APNS_CONNECTION) of
             {ok, PID} -> ?INFO_MSG("apns connection successful with PID ~p~n", [PID]);
             {error, timeout} -> ?ERROR_MSG("apns connection unsuccessful reason timed out", [])
        end,
        {ok, #state{host = Host}}.
  

• This is working as I can send notifications.

• Now the documentation (https://github.com/inaka/apns4erl) says:

If network goes down or something unexpected happens the gun connection with APNs will go down. In that case apns4erl will send a message {reconnecting, ServerPid} to the client process, that means apns4erl lost the connection and it is trying to reconnect. Once the connection has been recover a {connection_up, ServerPid} message will be send.

My question is:

• what code should I write inside my_apns_module.erl to receive {reconnecting, ServerPid} or {connection_up, ServerPid}?

Answer 1

Messages sent to a gen_server process, e.g. GenServerPid ! {ok, 10}}, are handled by:

handle_info(Msg, State)

So, you could do something like this:

handle_info({reconnecting, ServerPid}=Msg, State) ->
     %%do something here, like log Msg or change State;
handl_info({connection_up, ServerPid}=Msg, State) ->
     %%do something here, like log Msg or change State;   
handle_info(#offline_msg{us = _UserServer,
          from = From, to = To, packet = Packet} = _Msg, State) ->
     %%do something.

Response to comment:

This is your current code:

init([Host, _Opts]) ->
      apns:start(),
      case apns:connect(cert, ?APNS_CONNECTION) of
           {ok, PID} -> ?INFO_MSG("apns connection successful with PID ~p~n", [PID]);
           {error, timeout} -> ?ERROR_MSG("apns connection unsuccessful reason timed out", [])
      end,
      {ok, #state{host = Host}}.

You could change that to something like this:

init([Host, _Opts]) ->
    spawn(?MODULE, start_apns, []),
    ...


start_apns() ->

          apns:start(),
          case apns:connect(cert, ?APNS_CONNECTION) of
               {ok, PID} -> ?INFO_MSG("apns connection successful with PID ~p~n", [PID]);
               {error, timeout} -> ?ERROR_MSG("apns connection unsuccessful reason timed out", [])
          end,
          apns_loop().

apns_loop() ->
    receive
        {reconnecting, ServerPid} -> %%do something;
        {connection_up, ServerPid} -> %% do something;
        Other -> %% do something
    end,
    apns_loop().
  

After you start the apns process, the apns process will enter a loop and wait for messages.