Erlang Otp Behaviors

-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}.

-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}.

%%%===================================================================
%%% Connection state machine
%%%===================================================================

-module(connection_fsm).
-behaviour(gen_statem).

-export([start_link/0, connect/0, disconnect/0, send/1]).
-export([init/1, callback_mode/0, terminate/3, code_change/4]).
-export([disconnected/3, connecting/3, connected/3]).

-record(data, {
    socket = undefined,
    buffer = <<>>,
    retry_count = 0
}).

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

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

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

send(Data) ->
    gen_statem:call(?MODULE, {send, Data}).

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

callback_mode() ->
    [state_functions, state_enter].

%% Disconnected state
disconnected(enter, _OldState, _Data) ->
    io:format("Entered disconnected state~n"),
    keep_state_and_data;

disconnected(call, connect, Data) ->
    case connect_to_server() of
        {ok, Socket} ->
            {next_state, connected, Data#data{socket = Socket, retry_count = 0},
             [{reply, ok}]};
        error ->
            NewData = Data#data{retry_count = Data#data.retry_count + 1},
            case NewData#data.retry_count < 3 of
                true ->
                    {next_state, connecting, NewData, [{reply, {error, retrying}}]};
                false ->
                    {keep_state, NewData, [{reply, {error, max_retries}}]}
            end
    end.

%% Connecting state
connecting(enter, _OldState, _Data) ->
    erlang:send_after(1000, self(), retry_connect),
    keep_state_and_data;

connecting(info, retry_connect, Data) ->
    case connect_to_server() of
        {ok, Socket} ->
            {next_state, connected, Data#data{socket = Socket, retry_count = 0}};
        error ->
            NewData = Data#data{retry_count = Data#data.retry_count + 1},
            case NewData#data.retry_count < 3 of
                true ->
                    {keep_state, NewData};
                false ->
                    {next_state, disconnected, NewData}
            end
    end.

%% Connected state
connected(enter, _OldState, _Data) ->
    io:format("Connection established~n"),
    keep_state_and_data;

connected(call, {send, Data}, StateData) ->
    case send_data(StateData#data.socket, Data) of
        ok ->
            {keep_state_and_data, [{reply, ok}]};
        error ->
            {next_state, disconnected, StateData, [{reply, {error, send_failed}}]}
    end;

connected(call, disconnect, StateData) ->
    close_connection(StateData#data.socket),
    {next_state, disconnected, StateData#data{socket = undefined}, [{reply, ok}]}.

terminate(_Reason, _State, Data) ->
    case Data#data.socket of
        undefined -> ok;
        Socket -> close_connection(Socket)
    end.

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

%% Helper functions
connect_to_server() ->
    {ok, socket}.

send_data(_Socket, _Data) ->
    ok.

close_connection(_Socket) ->
    ok.

-module(my_supervisor).
-behaviour(supervisor).

-export([start_link/0]).
-export([init/1]).

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

init([]) ->
    SupFlags = #{
        strategy => one_for_one,
        intensity => 5,
        period => 60
    },

    ChildSpecs = [
        #{
            id => counter_server,
            start => {counter_server, start_link, []},
            restart => permanent,
            shutdown => 5000,
            type => worker,
            modules => [counter_server]
        },
        #{
            id => cache_server,
            start => {cache_server, start_link, [1000]},
            restart => permanent,
            shutdown => 5000,
            type => worker,
            modules => [cache_server]
        }
    ],

    {ok, {SupFlags, ChildSpecs}}.

%%%===================================================================
%%% Supervisor strategies
%%%===================================================================

%% one_for_one: Restart only failed child
init_one_for_one([]) ->
    SupFlags = #{strategy => one_for_one},
    Children = [worker_spec(worker1), worker_spec(worker2)],
    {ok, {SupFlags, Children}}.

%% one_for_all: Restart all children if any fails
init_one_for_all([]) ->
    SupFlags = #{strategy => one_for_all},
    Children = [worker_spec(worker1), worker_spec(worker2)],
    {ok, {SupFlags, Children}}.

%% rest_for_one: Restart failed child and all started after it
init_rest_for_one([]) ->
    SupFlags = #{strategy => rest_for_one},
    Children = [
        worker_spec(database),
        worker_spec(cache),  % Depends on database
        worker_spec(api)     % Depends on cache
    ],
    {ok, {SupFlags, Children}}.

worker_spec(Name) ->
    #{
        id => Name,
        start => {Name, start_link, []},
        restart => permanent,
        shutdown => 5000,
        type => worker
    }.

%%%===================================================================
%%% Nested supervisors (supervision tree)
%%%===================================================================

-module(app_supervisor).
-behaviour(supervisor).

-export([start_link/0, init/1]).

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

init([]) ->
    SupFlags = #{strategy => one_for_one},

    ChildSpecs = [
        #{
            id => database_sup,
            start => {database_supervisor, start_link, []},
            restart => permanent,
            type => supervisor
        },
        #{
            id => api_sup,
            start => {api_supervisor, start_link, []},
            restart => permanent,
            type => supervisor
        },
        #{
            id => worker_sup,
            start => {worker_supervisor, start_link, []},
            restart => permanent,
            type => supervisor
        }
    ],

    {ok, {SupFlags, ChildSpecs}}.

%%%===================================================================
%%% Dynamic supervision
%%%===================================================================

-module(dynamic_sup).
-behaviour(supervisor).

-export([start_link/0, start_child/1, stop_child/1]).
-export([init/1]).

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

start_child(Args) ->
    supervisor:start_child(?MODULE, [Args]).

stop_child(Pid) ->
    supervisor:terminate_child(?MODULE, Pid).

init([]) ->
    SupFlags = #{
        strategy => simple_one_for_one,
        intensity => 5,
        period => 60
    },

    ChildSpec = #{
        id => worker,
        start => {worker, start_link, []},
        restart => temporary,
        shutdown => 5000,
        type => worker
    },

    {ok, {SupFlags, [ChildSpec]}}.