Erlang OTP Behaviors

What this skill does

# Erlang OTP Behaviors

## Introduction

OTP (Open Telecom Platform) behaviors provide reusable patterns for common process
types in Erlang systems. These abstractions handle complex details like message
passing, error handling, and state management, allowing developers to focus on
business logic while maintaining system reliability.

Behaviors define interfaces that processes must implement, with OTP handling the
infrastructure. Gen_server provides client-server processes, gen_statem implements
state machines, supervisors manage process lifecycles, and gen_event coordinates
event distribution. Understanding these patterns is essential for production Erlang.

This skill covers gen_server for stateful processes, gen_statem for complex state
machines, supervisor trees for fault tolerance, gen_event for event handling,
application behavior for packaging, and patterns for building robust OTP systems.

## Gen_Server Basics

Gen_server implements client-server processes with synchronous and asynchronous
communication.

```erlang
-module(counter_server).
-behaviour(gen_server).

%% API
-export([start_link/0, increment/0, decrement/0, get_value/0, reset/0]).

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

-define(SERVER, ?MODULE).

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

%%%===================================================================
%%% API
%%%===================================================================

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

increment() ->
    gen_server:cast(?SERVER, increment).

decrement() ->
    gen_server:cast(?SERVER, decrement).

get_value() ->
    gen_server:call(?SERVER, get_value).

reset() ->
    gen_server:call(?SERVER, reset).

%%%===================================================================
%%% gen_server callbacks
%%%===================================================================

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

%% Synchronous calls (with response)
handle_call(get_value, _From, State) ->
    {reply, State#state.count, State};

handle_call(reset, _From, State) ->
    {reply, ok, State#state{count = 0}};

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

%% Asynchronous casts (no response)
handle_cast(increment, State) ->
    NewCount = State#state.count + 1,
    {noreply, State#state{count = NewCount}};

handle_cast(decrement, State) ->
    NewCount = State#state.count - 1,
    {noreply, State#state{count = NewCount}};

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

%% Handle other messages
handle_info(_Info, State) ->
    {noreply, State}.

terminate(_Reason, _State) ->
    ok.

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

%%%===================================================================
%%% Complex gen_server example: Cache
%%%===================================================================

-module(cache_server).
-behaviour(gen_server).

-export([start_link/1, put/2, get/1, delete/1, clear/0, size/0]).
-export([init/1, handle_call/3, handle_cast/2, handle_info/2, terminate/2, code_change/3]).

-record(state, {
    cache = #{},
    max_size = 1000,
    hits = 0,
    misses = 0
}).

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

put(Key, Value) ->
    gen_server:call(?MODULE, {put, Key, Value}).

get(Key) ->
    gen_server:call(?MODULE, {get, Key}).

delete(Key) ->
    gen_server:cast(?MODULE, {delete, Key}).

clear() ->
    gen_server:cast(?MODULE, clear).

size() ->
    gen_server:call(?MODULE, size).

init([MaxSize]) ->
    process_flag(trap_exit, true),
    {ok, #state{max_size = MaxSize}}.

handle_call({put, Key, Value}, _From, State) ->
    Cache = State#state.cache,
    case maps:size(Cache) >= State#state.max_size of
        true ->
            {reply, {error, cache_full}, State};
        false ->
            NewCache = maps:put(Key, Value, Cache),
            {reply, ok, State#state{cache = NewCache}}
    end;

handle_call({get, Key}, _From, State) ->
    Cache = State#state.cache,
    case maps:find(Key, Cache) of
        {ok, Value} ->
            NewState = State#state{hits = State#state.hits + 1},
            {reply, {ok, Value}, NewState};
        error ->
            NewState = State#state{misses = State#state.misses + 1},
            {reply, not_found, NewState}
    end;

handle_call(size, _From, State) ->
    Size = maps:size(State#state.cache),
    {reply, Size, State};

handle_call(_Request, _From, State) ->
    {reply, {error, unknown_request}, State}.

handle_cast({delete, Key}, State) ->
    NewCache = maps:remove(Key, State#state.cache),
    {noreply, State#state{cache = NewCache}};

handle_cast(clear, State) ->
    {noreply, State#state{cache = #{}}};

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

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

terminate(Reason, State) ->
    io:format("Cache terminating: ~p~n", [Reason]),
    io:format("Stats - Hits: ~p, Misses: ~p~n", [State#state.hits, State#state.misses]),
    ok.

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

%%%===================================================================
%%% gen_server with timeouts
%%%===================================================================

-module(session_server).
-behaviour(gen_server).

-export([start_link/0, touch/0]).
-export([init/1, handle_call/3, handle_cast/2, handle_info/2, terminate/2, code_change/3]).

-define(TIMEOUT, 30000). % 30 seconds

-record(state, {
    last_activity,
    data = #{}
}).

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

touch() ->
    gen_server:cast(?MODULE, touch).

init([]) ->
    {ok, #state{last_activity = erlang:system_time(millisecond)}, ?TIMEOUT}.

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

handle_cast(touch, State) ->
    NewState = State#state{last_activity = erlang:system_time(millisecond)},
    {noreply, NewState, ?TIMEOUT};

handle_cast(_Msg, State) ->
    {noreply, State, ?TIMEOUT}.

handle_info(timeout, State) ->
    io:format("Session timed out~n"),
    {stop, normal, State};

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

terminate(_Reason, _State) ->
    ok.

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

Gen_server provides structure for stateful processes with client-server patterns.

## Gen_Statem for State Machines

Gen_statem implements finite state machines with explicit state transitions.

```erlang
-module(door_fsm).
-behaviour(gen_statem).

-export([start_link/0, open/0, close/0, lock/0, unlock/1]).
-export([init/1, callback_mode/0, terminate/3, code_change/4]).
-export([locked/3, unlocked/3, open/3]).

-define(CODE, "1234").

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

open() ->
    gen_statem:call(?MODULE, open).

close() ->
    gen_statem:call(?MODULE, close).

lock() ->
    gen_statem:call(?MODULE, lock).

unlock(Code) ->
    gen_statem:call(?MODULE, {unlock, Code}).

init([]) ->
    {ok, locked, #{}}.

callback_mode() ->
    state_functions.

%% Locked state
locked(call, {unlock, Code}, Data) when Code =:= ?CODE ->
    {next_state, unlocked, Data, [{reply, ok}]};

locked(call, {unlock, _WrongCode}, Data) ->
    {keep_state, Data, [{reply, {error, wrong_code}}]};

locked(call, _Event, Data) ->
    {keep_state, Data, [{reply, {error, door_locked}}]}.

%% Unlocked state
unlocked(call, lock, Data) ->
    {next_state, locked, Data, [{reply, ok}]};

unlocked(call, open, Data) ->
    {next_state, open, Data, [{reply, ok}]};

unlocked(call, _Event, Data) ->
    {keep_state, Data, [{reply, ok}]}.

%% Open state
open(call, close, Data) ->
    {next_state, unlocked, Data, [{reply, ok}]};

open(call, _Event, Data) ->
    {keep_state, Data, [{reply, {error, door_open}}]}.

terminate(_Reason, _State, _Data) ->
    ok.

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