注：本文是为了配合《Spark内核设计的艺术 架构设计与实现》一书的内容而编写，目的是为了节省成本、方便读者查阅。书中附录B的内容都在本文呈现。

Akka是一款提供了用于构建高并发的、分布式的、可伸缩的、基于Java虚拟机的消息驱动应用的工具集和运行时环境。从下面Akka官网提供的一段代码示例，可以看出Akka并发编程的简约。

case class Greeting(who: String)
class GreetingActor extends Actor with ActorLogging {
def receive = {
case Greeting(who) ⇒ log.info("Hello " + who)
  	}
}
val system = ActorSystem("MySystem")
val greeter = system.actorOf(Props[GreetingActor], name = "greeter")
greeter ! Greeting("Charlie Parker")

// config on all machines
akka {
 actor {
   provider = akka.remote.RemoteActorRefProvider
   deployment {
     /greeter {
       remote = akka.tcp://MySystem@machine1:2552
     }
   }
 }
}
// ------------------------------
// define the greeting actor and the greeting message
case class Greeting(who: String) extends Serializable
class GreetingActor extends Actor with ActorLogging {
  def receive = {
    case Greeting(who) ⇒ log.info("Hello " + who)
  }
}
// ------------------------------
// on machine 1: empty system, target for deployment from machine 2
val system = ActorSystem("MySystem")
// ------------------------------
// on machine 2: Remote Deployment - deploying on machine1
val system = ActorSystem("MySystem")
val greeter = system.actorOf(Props[GreetingActor], name = "greeter")
// ------------------------------
// on machine 3: Remote Lookup (logical home of “greeter” is machine2, remote deployment is transparent)
val system = ActorSystem("MySystem")
val greeter = system.actorSelection("akka.tcp://MySystem@machine2:2552/user/greeter")
greeter ! Greeting("Sonny Rollins")

Actor之间最终会构成一棵树，作为父亲的Actor应当对所有儿子的异常失败进行处理（监管）Akka给出了简单的示例，代码如下。

class Supervisor extends Actor {
  override val supervisorStrategy =
    OneForOneStrategy(maxNrOfRetries = 10, withinTimeRange = 1 minute) {
      case _: ArithmeticException      ⇒ Resume
      case _: NullPointerException     ⇒ Restart
      case _: Exception                ⇒ Escalate
    }
  val worker = context.actorOf(Props[Worker])
  def receive = {
    case n: Int => worker forward n
  }
}

其它Akka应用的博文如下：

1. 《Spring与Akka的集成》；
2. 《使用Akka的远程调用》；
   
3. 《使用Akka构建集群（一）》；
4. 《使用Akka构建集群（二）》；
5. 《使用Akka持久化——持久化与快照》；
6. 《使用Akka持久化——消息发送与接收》；

Akka的更多信息请访问官方网站：http://akka.io/