Programming Erlang读书笔记8: Concurrency Programming

在Erlang里:
1，创建和销毁进程非常快
2，进程间发送消息非常快
3，在所有的操作系统间进程行为一致
4，可以有大量的进程
5，进程不共享内存并且完全独立
6，与进程交互的唯一途径是发送消息

Concurrency Primitives

Pid = spawn(Fun)

Pid!Message

receive
  Pattern1 [when Guard1] ->
    Expressions1;
  Pattern2 [when Guard2] ->
    Expressions2;
  ...
  after Time ->
    Expressions
end

Example

-module(area_server).
-export([loop/0]).

loop() ->
  receive
    {rectangle, Width, Ht} ->
      io:format("Area of rectangle is ~p~n", [Width * Ht]),
      loop();
    {circle, R} ->
      io:format("Area of circle is ~p~n", [3.14 * R * R]),
      loop();
    Other ->
      io:format("I don't know what the area of a ~p is. ~n", [Other]),
      loop()
  end.

Client-Server通信时需要通过self()传递client端PID

-module(area_server).
-export([start/0, area/2).

start() -> spawn(fun loop/0).

area(Pid, What) ->
  rpc(Pid, What).

rpc(Pid, Request) ->
  Pid ! {self(), Request},
  receive
    {Pid, Response} ->
      Response
  end.

loop() ->
  receive
    {From, {rectangle, Width, Ht}} ->
      From ! {self(), Width * Ht},
      loop();
    {From, {circle, R}} ->
      loop();
    {From, Other} ->
      From ! {self(), {error, Other}},
      loop()
  end.
%%%%%%%%%%%%%
1> Pid = area_server:start().
<0.36.0>
2> area_server:area(Pid, {rectangle, 10, 8}).
80
3> area_server:area(Pid, {circle, 4}).
50.2645

自定义让当前进程sleep T毫秒的方法

sleep(T) ->
  receive
  after T ->
    true
  end.

自定义Timer

-module(stimer).
-export([start/2, cancel/1]).

start(Time, Fun) -> spawn(fun() -> timer(Time, Fun) end).

cancel(Pid) -> Pid ! cancel.

timer(Time, Fun) ->
  receive
    cancel ->
      void
  after Time ->
      Fun()
  end.

每个进程都有一个mailbox
发送消息到该进程时，消息被放入mailbox
当程序运行到receive语句时，启动一个timer
读取mailbox中的第一条消息，匹配Pattern1，2，...，如果消息匹配了，则从mailbox中删除掉，并执行该Pattern后的表达式
如果receive语句中没有匹配的Pattern，则将该消息从mailbox中删除并放入save queue，然后下一条消息进来匹配
如果mailbox中没有一条消息是匹配成功的，则进程suspend并等待下一次新的消息进来，下次有新消息时save queue里的消息不会再匹配
一旦有消息匹配，则已经放入save queue的消息按到达进程的顺序重新进入mailbox，如果设置了timer，则清空save queue
如果在等待消息时timer到点，则执行ExpressionsTimeout表达式并将保存的消息按到达进程的顺序放入mailbox

publishing一个process identifier，这样系统里所有process都可以和这个process交互，发布的process称为registered process
BIFs:

register(AnAtom, Pid)
unregister(AnAtom)
whereis(AnAtom) -> Pid | undefined
registered() -> [AnAtom::atom()]

Example

1> Pid = spawn(fun area_server:loop/0).
<0.51.0>
2> register(area, Pid).
true
3> area ! {rectangle, 4, 5}.
Area of rectangle is 20
{rectangle,4,5}

Example2

-module(clock).
-export([start/2, stop/0]).

start(Time, Fun) ->
  register(clock, spawn(fun() -> tick(Time, Fun) end)).

stop() -> clock ! stop.

tick(Time, Fun) ->
  receive
    stop ->
      void
  after Time ->
    Fun(),
    tick(Time, Fun)
  end.

%%%%%%%%%%%%%%%%%%
1> clock:start(5000, fun() -> io:format("TICK ~p~n", [erlang:now()]) end).
2> clock:stop().

Concurrent Program Template

-module(ctemplate).
-compile(export_all).

start() ->
  spawn(fun() -> loop([]) end).

rpc(Pid, Request) ->
  Pid ! {self(), Request},
  receive
    {Pid, Response} ->
      Response
  end.

loop(X) ->
  receive
    Any ->
      io:format("Received:~p~n", [Any]),
      loop(X)
  end.

在loop里receive之后马上又调用loop，则loop称为tail-recursive方法
如果我们写一个方法F，F never returns，则需要确保在调用F之后不会调用其他方法，也不要将F用于List或Tuple的constructor，否则内存溢出

spawn with MFA:

spawn(Mod, FuncName, Args)

使用Mod名、Func名和Args列表(成为一个MFA)来显式的调用spawn是确保系统动态加载代码(热修改)的正确方式
spawn with funs则不会动态加载代码

作业
Erlang Ring Benchmark