Akka了解

概念
Akka是编译Java和scala开发的高并发、分布式、弹性的消息驱动应用程序的工具。

一个工具为什么能通过编译实现高并发、分布式？
1）使用actor和stream使得编译的系统能做简单并发和分布式系统。
2）可恢复性。使用了reactive模型，具有对错误自愈功能。
3）高性能。支持5000万消息/秒单机并发。较小的内存脚印，每GB能记录250万actors。
4）可伸缩性和去中心化。通过负载均衡路由到合适的工作节点。使用CQRS架构是的集群分片处理。使用CRDT解决最终一致性算法。
5）反应器（reactive）的流数据处理。采用异步非阻塞模型实现。

怎么使用
官方一个简单示例，地址：Get Started with Lightbend Technologies


点击下载，使用Idea打开。我使用的scala版本的，也可以选择java版本。
下面是示例的代码：
//#full-example
package com.example

import akka.actor.{ Actor, ActorLogging, ActorRef, ActorSystem, Props }

//#greeter-companion
//#greeter-messages
object Greeter {
  //#greeter-messages
  def props(message: String, printerActor: ActorRef): Props = Props(new Greeter(message, printerActor))
  //#greeter-messages
  final case class WhoToGreet(who: String)
  case object Greet
}
//#greeter-messages
//#greeter-companion

//#greeter-actor
class Greeter(message: String, printerActor: ActorRef) extends Actor {
  import Greeter._
  import Printer._

  var greeting = ""

  def receive = {
    case WhoToGreet(who) =>
      greeting = message + ", " + who
    case Greet           =>
      //#greeter-send-message
      printerActor ! Greeting(greeting)
      //#greeter-send-message
  }
}
//#greeter-actor

//#printer-companion
//#printer-messages
object Printer {
  //#printer-messages
  def props: Props = Props[Printer]
  //#printer-messages
  final case class Greeting(greeting: String)
}
//#printer-messages
//#printer-companion

//#printer-actor
class Printer extends Actor with ActorLogging {
  import Printer._

  def receive = {
    case Greeting(greeting) =>
      log.info("Greeting received (from " + sender() + "): " + greeting)
  }
}
//#printer-actor

//#main-class
object AkkaQuickstart extends App {
  import Greeter._

  // Create the 'helloAkka' actor system
  val system: ActorSystem = ActorSystem("helloAkka")

  //#create-actors
  // Create the printer actor
  val printer: ActorRef = system.actorOf(Printer.props, "printerActor")

  // Create the 'greeter' actors
  val howdyGreeter: ActorRef =
    system.actorOf(Greeter.props("Howdy", printer), "howdyGreeter")
  val helloGreeter: ActorRef =
    system.actorOf(Greeter.props("Hello", printer), "helloGreeter")
  val goodDayGreeter: ActorRef =
    system.actorOf(Greeter.props("Good day", printer), "goodDayGreeter")
  //#create-actors

  //#main-send-messages
  howdyGreeter ! WhoToGreet("Akka")
  howdyGreeter ! Greet

  howdyGreeter ! WhoToGreet("Lightbend")
  howdyGreeter ! Greet

  helloGreeter ! WhoToGreet("Scala")
  helloGreeter ! Greet

  goodDayGreeter ! WhoToGreet("Play")
  goodDayGreeter ! Greet
  //#main-send-messages
}
//#main-class
//#full-example

运行结果：
[INFO] [01/19/2019 09:41:55.524] [helloAkka-akka.actor.default-dispatcher-5] [akka://helloAkka/user/printerActor] Greeting received (from Actor[akka://helloAkka/user/helloGreeter#1507446]): Hello, Scala
[INFO] [01/19/2019 09:41:55.525] [helloAkka-akka.actor.default-dispatcher-5] [akka://helloAkka/user/printerActor] Greeting received (from Actor[akka://helloAkka/user/howdyGreeter#641219299]): Howdy, Akka
[INFO] [01/19/2019 09:41:55.525] [helloAkka-akka.actor.default-dispatcher-5] [akka://helloAkka/user/printerActor] Greeting received (from Actor[akka://helloAkka/user/goodDayGreeter#1523963167]): Good day, Play
[INFO] [01/19/2019 09:41:55.525] [helloAkka-akka.actor.default-dispatcher-5] [akka://helloAkka/user/printerActor] Greeting received (from Actor[akka://helloAkka/user/howdyGreeter#641219299]): Howdy, Lightbend
上面结果表示：程序是并发运行的。如最后一行  goodDayGreeter ! WhoToGreet("Play")在结果中是第二行就已经执行。

总结
上面从表象上已经大概能了解Akka是做什么事情的了。通过Akka提供的Api并且合理的使用它就可以简单的实现分布式、高性能的Java或者scala程序了。
如果需要深入对其工作原理进行了解，需要研究actor运作机制，后面有时间再学习。