OTP gen_server Implementation

test_server.erl

-module(test_server).
-behaviour(gen_server).

% Callback exports

-export([init/1, 
         handle_call/3, 
         handle_cast/2,
         handle_info/2, 
         terminate/2, 
         code_change/3]).

% API exports

-export([start_link/0]).

% Server state

-record(state, {count = 0}).

% API functions

start_link() ->
    gen_server:start_link({local, ?MODULE}, ?MODULE, [], []).

update_request_count(#state{count = Count} = State) ->
    State#state{count = Count + 1}.

% Callback functions

init([]) ->
    {ok, #state{count=0}}.

handle_call(greeting, _From, State) ->
    Reply = "Hello World!",
    {reply, Reply, update_request_count(State)};

handle_call(get_count, _From, #state{count = Count} = State) ->
    Reply = Count,
    {reply, Reply, update_request_count(State)};

handle_call(_Request, _From, State) ->
    {reply, ok, update_request_count(State)}.

handle_cast(_Request, State) ->
    {noreply, State}.

handle_info(_Info, State) ->
    {noreply, State}.

terminate(_Reason, _State) ->
    ok.

code_change(_OldVsn, State, _Extra) ->
    {ok, State}.