关于cluster-singleton我在前面的博文已经介绍过,在这篇我想回顾一下它的作用和使用方法。首先,cluster-singleton就是集群某个节点上的一个actor。任何时间在集群内保证只会有一个这种actor的实例。它可以是在任何节点上,具体位置由akka-cluster系统的leader节点根据一定规则选定。当cluster-singleton所处的节点停止运作时leader会选择另一个节点,然后系统会将cluster-singleton迁移到新的节点上来保证集群中一定有一个活着的cluster-singleton实例,不过值得注意的是迁移的actor会丢失它的内部状态。在编程实践中常常会需要保证一项程序功能只能由唯一的actor来运行的情况,比如我们需要保证某种运算的顺序,这时在集群环境里就可以使用cluster-singleton了。下面是cluster-singleton可能的一些使用场景:
1、在集群中的一个单点运算决策角色,或者是集群各节点交互运算的协调角色
2、集群与外界软件唯一的接口点
3、单一主角,多个从属
4、中央命名机制,或者中央路由逻辑
cluster-singleton的工作原理是:在集群的所有节点上都部署一个能产生、启动某singleton类型的ClusterSingletonManager,这样可以保证singleton可以迁移到任何节点。集群中的leader节点动态决定singleton的具体生存节点并指示该节点上的ClusterSingletonManager创建singleton实例。其它actor与singleton的交互是通过这个singleton类型的ClusterSingletonProxy进行的,这是cluster系统提供的一项与singleton交互的渠道服务,在需要接触singleton时可以创建ClusterSingletonProxy实例,然后把它当作目标发送操作消息,这样就可以保证cluster-singleton的位置透明特性了。
下面我们就用个简单的例子来示范cluster-singleton的使用看看它的唯一性和自动迁移特点:
构建一个简单的actor:
- class SingletonActor extends Actor with ActorLogging {
- import SingletonActor._
- import akka.cluster._
- override def receive: Receive = {
- case Greeting(msg) =>
- log.info("*********got {} from {}********", msg, sender().path.address)
- case Relocate =>
- log.info("*********I'll move from {}********", self.path.address)
- val cluster = Cluster(context.system)
- cluster.leave(cluster.selfAddress)
- case Die =>
- log.info("*******I'm shutting down ... ********")
- self ! PoisonPill
- }
- }
把SingletonActor包嵌在ClusterSingletonManager里:
- bject SingletonActor {
- trait SingletonMsg {}
- case class Greeting(msg: String) extends SingletonMsg
- case object Relocate extends SingletonMsg
- case object Die extends SingletonMsg
- def props = Props(new SingletonActor)
- def createSingleton(port: Int) = {
- val config = ConfigFactory.parseString(s"akka.remote.netty.tcp.port=$port")
- .withFallback(ConfigFactory.parseString("akka.cluster.roles=[singleton]"))
- .withFallback(ConfigFactory.load())
- val singletonSystem = ActorSystem("ClusterSingletonSystem",config)
- val singletonManager = singletonSystem.actorOf(ClusterSingletonManager.props(
- singletonProps = props,
- terminationMessage = Die,
- settings = ClusterSingletonManagerSettings(singletonSystem)
- .withRole(Some("singleton")) //只部署在角色是singleton的节点上
- ),
- name = "singletonManager"
- )
- }
- }
注意:singletonManager就是一个actor,所以是用actorOf(...)来构建的。现在这个singletonManager只能部署在singleton角色的节点上。
调用SingletonActor是通过ClusterSingletonProxy进行的:
- object SingletonUser {
- import SingletonActor._
- def sendToSingleton(msg: SingletonMsg) = {
- val config = ConfigFactory.parseString("akka.cluster.roles=[greeter]")
- .withFallback(ConfigFactory.load())
- val system = ActorSystem("ClusterSingletonSystem",config)
- val singletonProxy = system.actorOf(ClusterSingletonProxy.props(
- "user/singletonManager",
- ClusterSingletonProxySettings(system).withRole(None)
- ))
- singletonProxy ! msg
- }
- }
withRole(None)代表singletonProxy可以部署在任何节点上。下面是测试代码:
- import SingletonActor._
- import SingletonUser._
- object SingletonDemo extends App {
- createSingleton(2551) //seednode
- createSingleton(2552)
- createSingleton(2553)
- createSingleton(2554)
- scala.io.StdIn.readLine()
- sendToSingleton(Greeting("hello from tiger"))
- scala.io.StdIn.readLine()
- sendToSingleton(Relocate)
- scala.io.StdIn.readLine()
- sendToSingleton(Greeting("hello from cat"))
- scala.io.StdIn.readLine()
- sendToSingleton(Die)
- scala.io.StdIn.readLine()
- }
检验一下输出简要:
- [INFO] [10/25/2018 13:41:25.642] [ClusterSingletonSystem-akka.actor.default-dispatcher-3] [akka.tcp://ClusterSingletonSystem@127.0.0.1:51660/user/$a] Singleton identified at [akka.tcp://ClusterSingletonSystem@127.0.0.1:2551/user/singletonManager/singleton]
- [INFO] [10/25/2018 13:41:25.654] [ClusterSingletonSystem-akka.actor.default-dispatcher-20] [akka.tcp://ClusterSingletonSystem@127.0.0.1:2551/user/singletonManager/singleton] *********got hello from tiger from akka.tcp://ClusterSingletonSystem@127.0.0.1:51660********
- [INFO] [10/25/2018 13:43:10.839] [ClusterSingletonSystem-akka.actor.default-dispatcher-3] [akka.tcp://ClusterSingletonSystem@127.0.0.1:2551/user/singletonManager/singleton] *********I'll move from akka://ClusterSingletonSystem********
- INFO] [10/25/2018 13:43:18.885] [ClusterSingletonSystem-akka.actor.default-dispatcher-2] [akka.tcp://ClusterSingletonSystem@127.0.0.1:51670/user/$a] Singleton identified at [akka.tcp://ClusterSingletonSystem@127.0.0.1:2552/user/singletonManager/singleton]
- [INFO] [10/25/2018 13:43:18.886] [ClusterSingletonSystem-akka.actor.default-dispatcher-3] [akka.tcp://ClusterSingletonSystem@127.0.0.1:2552/user/singletonManager/singleton] *********got hello from cat from akka.tcp://ClusterSingletonSystem@127.0.0.1:51670********
- [INFO] [10/25/2018 13:43:18.156] [ClusterSingletonSystem-akka.actor.default-dispatcher-16] [akka.tcp://ClusterSingletonSystem@127.0.0.1:2551/user/singletonManager/singleton] *******I'm shutting down ... ********
- [INFO] [10/25/2018 13:43:18.177] [ClusterSingletonSystem-akka.remote.default-remote-dispatcher-18] [akka.tcp://ClusterSingletonSystem@127.0.0.1:2551/system/remoting-terminator] Shutting down remote daemon.
- [INFO] [10/25/2018 13:43:18.178] [ClusterSingletonSystem-akka.remote.default-remote-dispatcher-18] [akka.tcp://ClusterSingletonSystem@127.0.0.1:2551/system/remoting-terminator] Remote daemon shut down; proceeding with flushing remote transports.
- [INFO] [10/25/2018 13:43:18.215] [ClusterSingletonSystem-akka.remote.default-remote-dispatcher-14] [akka.tcp://ClusterSingletonSystem@127.0.0.1:2551/system/remoting-terminator] Remoting shut down.