akka actor中FSM有限状态机的设计原则

akka actor中的FSM有限状态机的设计原则参考了erlang中的FSM（http://www.erlang.org/documentation/doc-4.8.2/doc/design_principles/fsm.html）

许多应用程序可以利用FSMs来设计，Protocol stacks是其中的一个例子。

我们可以用一个关系集合来描述一个FSM，形式如下：

State(S) x Event(E) -> Actions (A), State(S')...

这些关系可以被解释为如下含义：

       If we are in state S and the event E occurs, we should perform the actions A and make a transition to the state S'.       如果当前状态处于S，且事件E发生了的话，我们应该执行行为A，并把状态转移到S'

如果用Erlang的gen_fsm行为来编程的话，上述转换规则可以用如下代码来表示：

StateName(Event, StateData) ->    .. code for actions here ...    {next_state, StateName', StateData'}

下图用一个简单的FSM来描述”Plain Ordinary Telephony Service“（POTS， 一般的电话服务）

电话服务POTS FSM可以用以下gen_fsm行为来描述：

init(A) ->    {ok, idle, A}.idle({off_hook, A}, A) ->    {next_state, getting_number, {A,[]}};idle({seize, A}, B) when A /= B ->    {next_state, ringing_b_side, {B, A});idle(_, A) ->    {next_state, idle, A}.getting_number({digit, D}, {A, Seq}) ->    case ... of        ... ->            ...                {next_state, ringing_a_side, {A, B}};        ... ->            ...                {next_state, getting_number, {A, Seq1}};        ... ->            ...                {next_state, wait_on_hook, A}    end;getting_number({on_hook, A}, {A,_}) ->    {next_state, idle, A}.ringing_a_side({on_hook, A}, {A, B}) ->    {next_state, idle, A};ringing_a_side({answered, B}, {A, B}) ->    {next_state, speech, {A,B}}.ringing_b_side({on_hook, A}, {B, A}) ->    {next_state, idle, B};ringing_b_side({off_hook, B}, {B, A}) ->    {next_state, speech, {B, A}}.speech({on_hook, A}, {A, B}) ->    {next_state, idle, A};speech({on_hook, B}, {A, B}) ->    {next_state, wait_on_hook, A}.wait_on_hook({on_hook, A}, A) ->    {next_state, idle, A}.

上述代码只是描述了状态转移。为了增加行为，需要增加以下代码：

getting_number({digit, D}, {A, Seq}) ->    Seq1 = Seq ++ [D],    case number_analyser:analyse(Seq1) of        {user, B} ->            hw:seize(B, A),            {next_state, ringing_a_side, {A, B}};        get_more_digits ->            {next_state, getting_number, {A, Seq1}};        invalid_number ->            hw:send_nasty_tone(A, bad_number_tone),            {next_state, wait_on_hook, A}    end;getting_number({on_hook, A}, {A,_}) ->    hw:stop_codec(A),    {next_state, idle, A}.

为了完成这个例子，我们需要把FSM和时间生成器程序打包成behaviour模型，然后增加FSM相关的代码，如下所示：

-module(pots).-behaviour(gen_fsm).-export([...]).start() -> gen_fsm:start(...)stop() -> gen_fsm:send_all_state_event(...)on_hook(A) -> gen_fsm:send_event(..., {off_hook, A})....

一个FSM的例子：

上面的POTS例子并没有包含全部的细节。下面的小例子是完整的：

-module(test1_fsm).-behaviour(gen_fsm).%% interface routines%% start us upstart() -> gen_fsm:start({local, hello}, test1_fsm, [], []).%% send a hello -- this will end up in the FSM routineshello() -> gen_fsm:send_event(hello, {hello, self()}).%% send a stop this will end up in "handle_event"stop()  -> gen_fsm:send_all_state_event(hello, stopit).%% -- interface end%% This initialisation routine gets calledinit(_) ->    {ok, idle, []}.%% The state machineidle({hello, A}, []) ->    {next_state, one, A}.one({hello, B}, A) ->    A ! {hello, B},    B ! {hello, A},    {next_state, idle, []}.%% this handles events in *any* statehandle_event(stopit, AnyState, TheStateData) ->    {stop, i_shall_stop, []}.   %% tell it to stop%% This is the finalisation routine - it gets called%% When we have to stopterminate(i_shall_stop, TheStateIwasIn, TheStateData) ->    ok.

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上面主要讲的是Erlang中的FSM的使用，下面来正式介绍Akka 中的FSM。

为了展示Akka  FSM特质的大部分特性，我们假设一个Actor接收和存队列大量的爆发式传过来的消息，在消息爆发过后，转发或者清除消息。

to be continued...........