gen_server:call is failing with exception exit - erlang

Question

I am running Erlang R16B03-1 (erts-5.10.4) at OS X 10.9.2. Erlang was installed by using brew.
And I am trying to run a gen_server module.
-module(logger).
-author("evangelosp").
-behaviour(gen_server).
%% API
-export([start/0, stop/0, log/2]).
%% gen_server callbacks
-export([init/1,
handle_call/3,
handle_cast/2,
handle_info/2,
terminate/2,
code_change/3]).
-define(SERVER, ?MODULE).
%%%===================================================================
%%% API
%%%===================================================================
start() -> gen_server:start_link({global, ?SERVER}, ?MODULE, [], []).
stop() -> gen_server:call(?MODULE, stop).
log(_Level, _MSG) -> gen_server:call(?MODULE, {add, {_Level, _MSG}}).
%%%===================================================================
%%% gen_server callbacks
%%%===================================================================
init([]) -> {ok, ets:new(?MODULE, [])}.
handle_call(_Request, _From, Table) -> {reply, {ok, ["Mplah!", _Request, _From, Table]}, Table}.
handle_cast(_Request, State) -> {noreply, State}.
handle_info(_Info, State) -> {noreply, State}.
terminate(_Reason, _State) -> ok.
code_change(_OldVsn, State, _Extra) -> {ok, State}.
In the erlang shell I am running:
Eshell V5.10.4 (abort with ^G)
1> c(logger).
{ok,logger}
2> logger:start().
{ok,<0.40.0>}
3> logger:log(info, "Hello World").
** exception exit: {noproc,{gen_server,call,
[logger,{add,{info,"Hello World"}}]}}
in function gen_server:call/2 (gen_server.erl, line 180)
And I can't get rid of that exception. I haven't actually found any useful resource by looking up the exception message, but this which didn't help much.
Cheers.

Answer 1

In you code start() -> gen_server:start_link({global, ?SERVER}, ?MODULE, [], [])., you use {global, ?SERVER} which means that:
If ServerName={global,GlobalName} the gen_server is registered
globally as GlobalName using global:register_name/2.
So when you send message to the server, you should write log(_Level, _MSG) -> gen_server:call({global, ?MODULE}, {add, {_Level, _MSG}}).. Please see the erlang doc:
call(ServerRef, Request, Timeout) -> Reply
Types:
ServerRef = Name | {Name,Node} | {global,GlobalName} |
ServerRef can be:
Name, if the gen_server is locally registered,
{global,GlobalName}, if the gen_server is globally registered.

Answer 2

Your server is not registered, so it is accesible only by its pid. But the interface functions use the implicit macro ?MODULE (which is replaced by logger at compilation) to access it.
You need either to change your interface functions, or, to register the server in the start function:
start() ->
{ok,Pid} = gen_server:start_link({global, ?SERVER}, ?MODULE, [], []),
register(?MODULE,Pid).
[edit] Thanks Evalon, I made the correction in the answer :o)