Spark技术内幕：Client，Master和Worker 通信源码解析
Spark技术内幕：Client，Master和Worker 通信源码解析
Spark的Cluster Manager可以有几种部署模式：
在向集群提交计算任务后，系统的运算模型就是Driver Program定义的SparkContext向APP Master提交，有APP Master进行计算资源的调度并最终完成计算。具体阐述可以阅读《Spark：大数据的电花火石！ 》。
那么Standalone模式下，Client，Master和Worker是如何进行通信，注册并开启服务的呢？
1. node之间的RPC - akka
模块间通信有很多成熟的实现，现在很多成熟的Framework已经早已经让我们摆脱原始的Socket编程了。简单归类，可以归纳为基于消息的传递和基于资源共享的同步机制。
基于消息的传递的机制应用比较广泛的有Message Queue。Message Queue, 是一种应用程序对应用程序的通信方法。应用程序通过读写出入队列的消息（针对应用程序的数据）来通信，而无需专用连接来链接它们。消 息传递指的是程序之间通过在消息中发送数据进行通信，而不是通过直接调用彼此来通信，直接调用通常是用于诸如远程过程调用的技术。排队指的是应用程序通过 队列来通信。队列的使用除去了接收和发送应用程序同时执行的要求。其中较为成熟的MQ产品有IBM WEBSPHERE MQ和RabbitMQ（AMQP的开源实现，现在由Pivotal维护）。
还有不得不提的是ZeroMQ，一个致力于进入Linux内核的基于Socket的编程框架。官方的说法： “ZeroMQ是一个简单好用的传输层，像框架一样的一个socket library，它使得Socket编程更加简单、简洁和性能更高。是一个消息处理队列库，可在多个线程、内核和主机盒之间弹性伸缩。ZMQ的明确目标是“成为标准网络协议栈的一部分，之后进入Linux内核”。
Spark在很多模块之间的通信选择是Scala原生支持的akka，一个用 Scala 编写的库，用于简化编写容错的、高可伸缩性的 Java 和 Scala 的 Actor 模型应用。akka有以下5个特性：
在Spark中的Client，Master和Worker实际上都是一个actor，拿Client来说：
import akka.actor._
import akka.pattern.ask
import akka.remote.{AssociationErrorEvent, DisassociatedEvent, RemotingLifecycleEvent}
private class ClientActor(driverArgs: ClientArguments, conf: SparkConf) extends Actor with Logging {
var masterActor: ActorSelection = _
val timeout = AkkaUtils.askTimeout(conf)
override def preStart() = {
masterActor = context.actorSelection(Master.toAkkaUrl(driverArgs.master))
context.system.eventStream.subscribe(self, classOf[RemotingLifecycleEvent])
println(s"Sending ${driverArgs.cmd} command to ${driverArgs.master}")
driverArgs.cmd match {
case "launch" =>
...
masterActor ! RequestSubmitDriver(driverDescription)
case "kill" =>
val driverId = driverArgs.driverId
val killFuture = masterActor ! RequestKillDriver(driverId)
}
}
override def receive = {
case SubmitDriverResponse(success, driverId, message) =>
println(message)
if (success) pollAndReportStatus(driverId.get) else System.exit(-1)
case KillDriverResponse(driverId, success, message) =>
println(message)
if (success) pollAndReportStatus(driverId) else System.exit(-1)
case DisassociatedEvent(_, remoteAddress, _) =>
println(s"Error connecting to master ${driverArgs.master} ($remoteAddress), exiting.")
System.exit(-1)
case AssociationErrorEvent(cause, _, remoteAddress, _) =>
println(s"Error connecting to master ${driverArgs.master} ($remoteAddress), exiting.")
println(s"Cause was: $cause")
System.exit(-1)
}
}
/**
* Executable utility for starting and terminating drivers inside of a standalone cluster.
*/
object Client {
def main(args: Array[String]) {
println("WARNING: This client is deprecated and will be removed in a future version of Spark.")
println("Use ./bin/spark-submit with \"--master spark://host:port\"")
val conf = new SparkConf()
val driverArgs = new ClientArguments(args)
if (!driverArgs.logLevel.isGreaterOrEqual(Level.WARN)) {
conf.set("spark.akka.logLifecycleEvents", "true")
}
conf.set("spark.akka.askTimeout", "10")
conf.set("akka.loglevel", driverArgs.logLevel.toString.replace("WARN", "WARNING"))
Logger.getRootLogger.setLevel(driverArgs.logLevel)
// TODO: See if we can initialize akka so return messages are sent back using the same TCP
// flow. Else, this (sadly) requires the DriverClient be routable from the Master.
val (actorSystem, _) = AkkaUtils.createActorSystem(
"driverClient", Utils.localHostName(), 0, conf, new SecurityManager(conf))
actorSystem.actorOf(Props(classOf[ClientActor], driverArgs, conf))
actorSystem.awaitTermination()
}
}其中第19行的含义就是向Master提交Driver的请求，
masterActor ! RequestSubmitDriver(driverDescription)
而Master将在receive里处理这个请求。当然了27行到44行的是处理Client Actor收到的消息。
可以看出，通过akka，可以非常简单高效的处理模块间的通信，这可以说是Spark RPC的一大特色。
2. Client，Master和Workerq启动通信详解
源码位置：spark-1.0.0\core\src\main\scala\org\apache\spark\deploy。主要涉及的类：Client.scala, Master.scala和Worker.scala。这三大模块之间的通信框架如下图。
Standalone模式下存在的角色：
实际上，Master和Worker要处理的消息要比这多得多，本图只是反映了集群启动和向集群提交运算时候的主要消息处理。
接下来将分别走读这三大角色的源码。
2.1 Client源码解析
Client启动：
object Client {
def main(args: Array[String]) {
println("WARNING: This client is deprecated and will be removed in a future version of Spark.")
println("Use ./bin/spark-submit with \"--master spark://host:port\"")
val conf = new SparkConf()
val driverArgs = new ClientArguments(args)
if (!driverArgs.logLevel.isGreaterOrEqual(Level.WARN)) {
conf.set("spark.akka.logLifecycleEvents", "true")
}
conf.set("spark.akka.askTimeout", "10")
conf.set("akka.loglevel", driverArgs.logLevel.toString.replace("WARN", "WARNING"))
Logger.getRootLogger.setLevel(driverArgs.logLevel)
// TODO: See if we can initialize akka so return messages are sent back using the same TCP
// flow. Else, this (sadly) requires the DriverClient be routable from the Master.
val (actorSystem, _) = AkkaUtils.createActorSystem(
"driverClient", Utils.localHostName(), 0, conf, new SecurityManager(conf))
// 使用ClientActor初始化actorSystem
actorSystem.actorOf(Props(classOf[ClientActor], driverArgs, conf))
//启动并等待actorSystem的结束
actorSystem.awaitTermination()
}
}从行21可以看出，核心实现是由ClientActor实现的。Client的Actor是akka.Actor的一个扩展。对于Actor，从它对recevie的override就可以看出它需要处理的消息。
override def receive = {
case SubmitDriverResponse(success, driverId, message) =>
println(message)
if (success) pollAndReportStatus(driverId.get) else System.exit(-1)
case KillDriverResponse(driverId, success, message) =>
println(message)
if (success) pollAndReportStatus(driverId) else System.exit(-1)
case DisassociatedEvent(_, remoteAddress, _) =>
println(s"Error connecting to master ${driverArgs.master} ($remoteAddress), exiting.")
System.exit(-1)
case AssociationErrorEvent(cause, _, remoteAddress, _) =>
println(s"Error connecting to master ${driverArgs.master} ($remoteAddress), exiting.")
println(s"Cause was: $cause")
System.exit(-1)
}2.2 Master的源码分析
源码分析详见注释。
override def receive = {
case ElectedLeader => {
// 被选为Master，首先判断是否该Master原来为active，如果是那么进行Recovery。
}
case CompleteRecovery => completeRecovery() // 删除没有响应的worker和app，并且将所有没有worker的Driver分配worker
case RevokedLeadership => {
// Master将关闭。
}
case RegisterWorker(id, workerHost, workerPort, cores, memory, workerUiPort, publicAddress) =>
{
// 如果该Master不是active，不做任何操作，返回
// 如果注册过该worker id，向sender返回错误
sender ! RegisterWorkerFailed("Duplicate worker ID")
// 注册worker，如果worker注册成功则返回成功的消息并且进行调度
sender ! RegisteredWorker(masterUrl, masterWebUiUrl)
schedule()
// 如果worker注册失败，发送消息到sender
sender ! RegisterWorkerFailed("Attempted to re-register worker at same address: " + workerAddress)
}
case RequestSubmitDriver(description) => {
// 如果master不是active，返回错误
sender ! SubmitDriverResponse(false, None, msg)
// 否则创建driver，返回成功的消息
sender ! SubmitDriverResponse(true, Some(driver.id), s"Driver successfully submitted as ${driver.id}")
}
}
case RequestKillDriver(driverId) => {
if (state != RecoveryState.ALIVE) {
// 如果master不是active，返回错误
val msg = s"Can only kill drivers in ALIVE state. Current state: $state."
sender ! KillDriverResponse(driverId, success = false, msg)
} else {
logInfo("Asked to kill driver " + driverId)
val driver = drivers.find(_.id == driverId)
driver match {
case Some(d) =>
//如果driver仍然在等待队列，从等待队列删除并且更新driver状态为KILLED
} else {
// 通知worker kill driver id的driver。结果会由workder发消息给master ! DriverStateChanged
d.worker.foreach { w => w.actor ! KillDriver(driverId) }
}
// 注意，此时driver不一定被kill，master只是通知了worker去kill driver。
sender ! KillDriverResponse(driverId, success = true, msg)
case None =>
// driver已经被kill，直接返回结果
sender ! KillDriverResponse(driverId, success = false, msg)
}
}
}
case RequestDriverStatus(driverId) => {
// 查找请求的driver，如果找到则返回driver的状态
(drivers ++ completedDrivers).find(_.id == driverId) match {
case Some(driver) =>
sender ! DriverStatusResponse(found = true, Some(driver.state),
driver.worker.map(_.id), driver.worker.map(_.hostPort), driver.exception)
case None =>
sender ! DriverStatusResponse(found = false, None, None, None, None)
}
}
case RegisterApplication(description) => {
//如果是standby，那么忽略这个消息
//否则注册application；返回结果并且开始调度
}
case ExecutorStateChanged(appId, execId, state, message, exitStatus) => {
// 通过idToApp获得app，然后通过app获得executors，从而通过execId获得executor
val execOption = idToApp.get(appId).flatMap(app => app.executors.get(execId))
execOption match {
case Some(exec) => {
exec.state = state
exec.application.driver ! ExecutorUpdated(execId, state, message, exitStatus)
if (ExecutorState.isFinished(state)) {
val appInfo = idToApp(appId)
// Remove this executor from the worker and app
logInfo("Removing executor " + exec.fullId + " because it is " + state)
appInfo.removeExecutor(exec)
exec.worker.removeExecutor(exec)
}
｝
}
case DriverStateChanged(driverId, state, exception) => {
// 如果Driver的state为ERROR | FINISHED | KILLED | FAILED， 删除它。
}
case Heartbeat(workerId) => {
// 更新worker的时间戳 workerInfo.lastHeartbeat = System.currentTimeMillis()
}
case MasterChangeAcknowledged(appId) => {
// 将appId的app的状态置为WAITING，为切换Master做准备。
}
case WorkerSchedulerStateResponse(workerId, executors, driverIds) => {
// 通过workerId查找到worker，那么worker的state置为ALIVE，
// 并且查找状态为idDefined的executors，并且将这些executors都加入到app中，
// 然后保存这些app到worker中。可以理解为Worker在Master端的Recovery
idToWorker.get(workerId) match {
case Some(worker) =>
logInfo("Worker has been re-registered: " + workerId)
worker.state = WorkerState.ALIVE
val validExecutors = executors.filter(exec => idToApp.get(exec.appId).isDefined)
for (exec <- validExecutors) {
val app = idToApp.get(exec.appId).get
val execInfo = app.addExecutor(worker, exec.cores, Some(exec.execId))
worker.addExecutor(execInfo)
execInfo.copyState(exec)
}
// 将所有的driver设置为RUNNING然后加入到worker中。
for (driverId <- driverIds) {
drivers.find(_.id == driverId).foreach { driver =>
driver.worker = Some(worker)
driver.state = DriverState.RUNNING
worker.drivers(driverId) = driver
}
}
}
}
case DisassociatedEvent(_, address, _) => {
// 这个请求是Worker或者是App发送的。删除address对应的Worker和App
// 如果Recovery可以结束，那么结束Recovery
}
case RequestMasterState => {
//向sender返回master的状态
sender ! MasterStateResponse(host, port, workers.toArray, apps.toArray, completedApps.toArray, drivers.toArray, completedDrivers.toArray, state)
}
case CheckForWorkerTimeOut => {
//删除超时的Worker
}
case RequestWebUIPort => {
//向sender返回web ui的端口号
sender ! WebUIPortResponse(webUi.boundPort)
}
}2.3 Worker 源码解析
通过对Client和Master的源码解析，相信你也知道如何去分析Worker是如何和Master进行通信的了，没错，答案就在下面：
override def receive
参考资料：
Spark源码1.0.0。
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