關於cluster-singleton我在前面的博文已經介紹過,在這篇我想回顧一下它的作用和使用方法。首先,cluster-singleton就是集羣某個節點上的一個actor。任何時間在集羣內保證只會有一個這種actor的實例。它可以是在任何節點上,具體位置由akka-cluster系統的leader節點根據一定規則選定。當cluster-singleton所處的節點停止運作時leader會選擇另一個節點,然後系統會將cluster-singleton遷移到新的節點上來保證集羣中一定有一個活着的cluster-singleton實例,不過值得注意的是遷移的actor會丟失它的內部狀態。在編程實踐中常常會需要保證一項程序功能只能由唯一的actor來運行的情況,比如我們需要保證某種運算的順序,這時在集羣環境裏就可以使用cluster-singleton了。下面是cluster-singleton可能的一些使用場景:
1、在集羣中的一個單點運算決策角色,或者是集羣各節點交互運算的協調角色
2、集羣與外界軟件唯一的接口點
3、單一主角,多個從屬
4、中央命名機制,或者中央路由邏輯
cluster-singleton的工作原理是:在集羣的所有節點上都部署一個能產生、啓動某singleton類型的ClusterSingletonManager,這樣可以保證singleton可以遷移到任何節點。集羣中的leader節點動態決定singleton的具體生存節點並指示該節點上的ClusterSingletonManager創建singleton實例。其它actor與singleton的交互是通過這個singleton類型的ClusterSingletonProxy進行的,這是cluster系統提供的一項與singleton交互的渠道服務,在需要接觸singleton時可以創建ClusterSingletonProxy實例,然後把它當作目標發送操作消息,這樣就可以保證cluster-singleton的位置透明特性了。
下面我們就用個簡單的例子來示範cluster-singleton的使用看看它的唯一性和自動遷移特點:
構建一個簡單的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裏:
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=$port") .withFallback(ConfigFactory.parseString("akka.cluster.roles=[singleton]")) .withFallback(ConfigFactory.load()) val singletonSystem = ActorSystem("ClusterSingletonSystem",config) val singletonManager = singletonSystem.actorOf(ClusterSingletonManager.props( singletonProps = props, terminationMessage = Die, settings = ClusterSingletonManagerSettings(singletonSystem) .withRole(Some("singleton")) //只部署在角色是singleton的節點上 ), name = "singletonManager" ) } }
注意:singletonManager就是一個actor,所以是用actorOf(...)來構建的。現在這個singletonManager只能部署在singleton角色的節點上。
調用SingletonActor是通過ClusterSingletonProxy進行的:
object SingletonUser { import SingletonActor._ def sendToSingleton(msg: SingletonMsg) = { val config = ConfigFactory.parseString("akka.cluster.roles=[greeter]") .withFallback(ConfigFactory.load()) val system = ActorSystem("ClusterSingletonSystem",config) val singletonProxy = system.actorOf(ClusterSingletonProxy.props( "user/singletonManager", ClusterSingletonProxySettings(system).withRole(None) )) singletonProxy ! msg } }
withRole(None)代表singletonProxy可以部署在任何節點上。下面是測試代碼:
import SingletonActor._ import SingletonUser._ object SingletonDemo extends App { createSingleton(2551) //seednode createSingleton(2552) createSingleton(2553) createSingleton(2554) scala.io.StdIn.readLine() sendToSingleton(Greeting("hello from tiger")) scala.io.StdIn.readLine() sendToSingleton(Relocate) scala.io.StdIn.readLine() sendToSingleton(Greeting("hello from cat")) scala.io.StdIn.readLine() sendToSingleton(Die) scala.io.StdIn.readLine() }
檢驗一下輸出簡要:
[INFO] [10/25/2018 13:41:25.642] [ClusterSingletonSystem-akka.actor.default-dispatcher-3] [akka.tcp://[email protected]:51660/user/$a] Singleton identified at [akka.tcp://[email protected]:2551/user/singletonManager/singleton] [INFO] [10/25/2018 13:41:25.654] [ClusterSingletonSystem-akka.actor.default-dispatcher-20] [akka.tcp://[email protected]:2551/user/singletonManager/singleton] *********got hello from tiger from akka.tcp://[email protected]:51660******** [INFO] [10/25/2018 13:43:10.839] [ClusterSingletonSystem-akka.actor.default-dispatcher-3] [akka.tcp://[email protected]: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://[email protected]:51670/user/$a] Singleton identified at [akka.tcp://[email protected]:2552/user/singletonManager/singleton] [INFO] [10/25/2018 13:43:18.886] [ClusterSingletonSystem-akka.actor.default-dispatcher-3] [akka.tcp://[email protected]:2552/user/singletonManager/singleton] *********got hello from cat from akka.tcp://[email protected]:51670******** [INFO] [10/25/2018 13:43:18.156] [ClusterSingletonSystem-akka.actor.default-dispatcher-16] [akka.tcp://[email protected]: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://[email protected]: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://[email protected]: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://[email protected]:2551/system/remoting-terminator] Remoting shut down.