天下无处不乒乓

在消息传递（Message Passing）领域，PingPong是最常见的测试之一。它的功能简单的有些无聊，一个Ping Actor和一个Pong Actor之间互相传递消息，你Ping过来我Pong过去。也正因为如此简单，PingPong的目标仅仅是测试纯粹的消息传递机制的效率。也正因为如此，各Actor模型往往都将其作为展示自己功能的第一个示例。老赵从互联网上收集了一些最为常见的，不同语言/平台下Actor模型实现PingPong的示例，可作“观赏”之用。

由于语言特性不同，有的Actor模型内置于语言之中的（如Erlang），其他大都由框架进行补充。有的Actor模型使用字符串作为消息，有的却通过语言功能而可以传递强类型的消息。简单的罗列各种实现，把这些实现从表面上进行简单的对比，也可以说颇有趣味。当然，这些实现虽然都使用了PingPong作为演示，但是其中细节还是略有不同的。例如有些会让Ping/Pong过程永远持续下去，而有些会让它们在进行一段时间过后就中止；有些会对命令进行识别，而有些对此并不在意。老赵认为这些区别无伤大雅，对此也未作任何修改。

本文会涉及以下Actor模型的实现（不少语言缺少语法着色，如果您有好用的HTML高亮方案请不吝指明）：

• Erlang (source)
• Scala (source)
• Haskell (source)
• Ruby (source)
• Python (source)
• Axum with Channel (source)
• Axum with Ordered Interaction Points (source)
• F# (source)
• F# with ActorLite (source)

在这些示例中，您会发现缺少了C#下的Actor——因为老赵打算下次单独开篇对此进行说明和介绍。:)

Erlang

-module(tut15).

-export([start/0, ping/2, pong/0]).

ping(0, Pong_PID) ->
    Pong_PID ! finished,
    io:format("ping finished~n", []);

ping(N, Pong_PID) ->
    Pong_PID ! {ping, self()},
    receive
        pong ->
            io:format("Ping received pong~n", [])
    end,
    ping(N - 1, Pong_PID).

pong() ->
    receive
        finished ->
            io:format("Pong finished~n", []);
        {ping, Ping_PID} ->
            io:format("Pong received ping~n", []),
            Ping_PID ! pong,
            pong()
    end.

start() ->
    Pong_PID = spawn(tut15, pong, []),
    spawn(tut15, ping, [3, Pong_PID]).

Scala

import scala.actors.Actor
import scala.actors.Actor._

class Ping(count: int, pong: Actor) extends Actor {
  def act() {
    var pingsLeft = count - 1
    pong ! Ping
    loop {
      react {
        case Pong =>
          if (pingsLeft % 1000 == 0)
            Console.println("Ping: pong")
          if (pingsLeft > 0) {
            pong ! Ping
            pingsLeft -= 1
          } else {
            Console.println("Ping: stop")
            pong ! Stop
            exit()
          }
      }
    }
  }
}

class Pong extends Actor {
  def act() {
    var pongCount = 0
    loop {
      react {
        case Ping =>
          if (pongCount % 1000 == 0)
            Console.println("Pong: ping "+pongCount)
          sender ! Pong
          pongCount = pongCount + 1
        case Stop =>
          Console.println("Pong: stop")
          exit()
      }
    }
  }
}

object pingpong extends Application {
  val pong = new Pong
  val ping = new Ping(100000, pong)
  ping.start
  pong.start
}

Ruby

require 'concurrent/actors'
include Concurrent::Actors

Message = Struct.new :ping, :pong

ping_thread = Actor.spawn do
  loop do
    Actor.receive do |f|
      f.when Message do |m|
        puts "PING" 
        sleep(1)
        m.pong << Message.new(m.ping, m.pong)
      end
    end
  end
end

pong_thread = Actor.spawn do
  loop do
    Actor.receive do |f|
      f.when Message do |m|
        puts "PONG" 
        sleep(1)
        m.ping << Message.new(m.ping, m.pong)
      end
    end
  end
end

ping_thread << Message.new(ping_thread, pong_thread)
while(true)
  puts("WAITING...")
  sleep(5)
end

Python

#
# pingpong_stackless.py
#

import stackless

ping_channel = stackless.channel()
pong_channel = stackless.channel()

def ping():
    while ping_channel.receive(): #blocks here
        print "PING"
        pong_channel.send("from ping")

def pong():
    while pong_channel.receive():
        print "PONG"
        ping_channel.send("from pong")



stackless.tasklet(ping)()
stackless.tasklet(pong)()

# we need to 'prime' the game by sending a start message
# if not, both tasklets will block
stackless.tasklet(ping_channel.send)('startup')

stackless.run()

Axum with Channel

using System;
using System.Concurrency;
using System.Collections.Generic;
using Microsoft.Axum;
using System.Concurrency.Messaging;


public channel PingPongStatus
{
    input int HowMany;
    output int Done;
    
    Start: { HowMany, Done -> End; }
}

public agent Ping : channel PingPongStatus
{
    public Ping()
    {
        // How many are we supposed to do?
        var iters = receive(PrimaryChannel::HowMany);
        
        // Create pong and send how many to do
        var chan = Pong.CreateInNewDomain();      
        chan::HowMany <-- iters;
        
        // Send pings and receive pongs
        for (int i = 0; i < iters; i++)
        {
            chan::Ping <-- Signal.Value;
            receive(chan::Pong);
            Console.WriteLine("Ping received Pong");
        }
        
        // How many did Pong process?
        int pongIters = receive(chan::Done);

        PrimaryChannel::Done <-- 0;
    }  
}

public channel PingPong
{
    input int HowMany;
    output int Done;
    
    input Signal Ping;
    output Signal Pong;
}

public agent Pong : channel PingPong
{
    public Pong()
    {
        // Get how many we're supposed to do
        var iters = receive(PrimaryChannel::HowMany);

        int i = 0;

        // Receive our ping and send back our pong
        for (; i < iters; i++)
        {
            receive(PrimaryChannel::Ping);
            Console.WriteLine("Pong received Ping");
            PrimaryChannel::Pong <-- Signal.Value;    
        }
    
        PrimaryChannel::Done <-- i;
    }
}

public agent Program : channel Application
{
    public Program()
    {
        // Wait for our command args
        var cmdArgs = receive(PrimaryChannel::CommandLine);
        
        // Set the iterations to be some number
        var iters = 3;
        
        // Create instance of ping and send msg
        var chan = Ping.CreateInNewDomain();
        chan::HowMany <-- iters;
        
        // Receive how many done
        receive(chan::Done);
        
        PrimaryChannel::Done <-- Signal.Value;
    }
}

F#

open System

type message = Finished | Msg of int * MailboxProcessor<message>

let ping iters (outbox : MailboxProcessor<message>) =
    MailboxProcessor.Start(fun inbox -> 
        let rec loop n = async { 
            if n > 0 then
                outbox.Post( Msg(n, inbox) )
                let! msg = inbox.Receive()
                Console.WriteLine("ping received pong")
                return! loop(n-1)
            else
                outbox.Post(Finished)
                Console.WriteLine("ping finished")
                return ()}
        loop iters)
            
let pong () =
    MailboxProcessor.Start(fun inbox -> 
        let rec loop () = async { 
            let! msg = inbox.Receive()
            match msg with
            | Finished -> 
                Console.WriteLine("pong finished")
                return ()
            | Msg(n, outbox) -> 
                Console.WriteLine("pong received ping")
                outbox.Post(Msg(n, inbox)
                return! loop() }
                    
        loop())

let ponger = pong()
do (ping 10 ponger) |> ignore

F# with ActorLite

#light

open System
open ActorLite

let (<=) (m:_ Actor) msg = m.Post msg

type message = string * message Actor

let pong = 
    { new message Actor() with
        override self.Receive(message) =
            let msg, ping = message
            Console.WriteLine(msg)
            ping <= ("Pong", self) }
            
let ping = 
    { new message Actor() with
        override self.Receive(message) =
            let msg, pong = message
            Console.WriteLine(msg)
            pong <= ("Ping", self) }

ping <= ("Start", pong)
Console.ReadLine |> ignore