Spark2.4.0 SparkContext 原始碼分析

更多資源

• github: https://github.com/opensourceteams/spark-scala-maven-2.4.0

時序圖

前置條件

• Hadoop版本: hadoop-2.9.2
• Spark版本: spark-2.4.0-bin-hadoop2.7
• JDK.1.8.0_191
• scala2.11.12

主要內容描述

• createSparkEnv
• Started SparkUI
• 註冊端點HeartbeatReceiver
• createTaskScheduler
• 啟動任務排程器，指定預設任務排程模式FIFO,構建排程池
• new DAGScheduler
• 註冊DriverEndpoint端點:CoarseGrainedScheduler
• new StandaloneAppClient
• 註冊端點:AppClient, ClientEndpoint
• 回撥ClientEndpoint.onStart()方法,該方法向所有master註冊
• 傳送訊息:RegisterApplication
• ClientEndpoint.receive()函式接收master回覆的訊息： RegisteredApplication

SparkContext

類函造方法

• 完成對SparkContext的構造
• createSparkEnv
• Started SparkUI
• 註冊端點HeartbeatReceiver
• createTaskScheduler
• new new DAGScheduler

try {
    _conf = config.clone()
    _conf.validateSettings()

    if (!_conf.contains("spark.master")) {
      throw new SparkException("A master URL must be set in your configuration")
    }
    if (!_conf.contains("spark.app.name")) {
      throw new SparkException("An application name must be set in your configuration")
    }

    // log out spark.app.name in the Spark driver logs
    logInfo(s"Submitted application: $appName")

    // System property spark.yarn.app.id must be set if user code ran by AM on a YARN cluster
    if (master == "yarn" && deployMode == "cluster" && !_conf.contains("spark.yarn.app.id")) {
      throw new SparkException("Detected yarn cluster mode, but isn't running on a cluster. " +
        "Deployment to YARN is not supported directly by SparkContext. Please use spark-submit.")
    }

    if (_conf.getBoolean("spark.logConf", false)) {
      logInfo("Spark configuration:\n" + _conf.toDebugString)
    }

    // Set Spark driver host and port system properties. This explicitly sets the configuration
    // instead of relying on the default value of the config constant.
    _conf.set(DRIVER_HOST_ADDRESS, _conf.get(DRIVER_HOST_ADDRESS))
    _conf.setIfMissing("spark.driver.port", "0")

    _conf.set("spark.executor.id", SparkContext.DRIVER_IDENTIFIER)

    _jars = Utils.getUserJars(_conf)
    _files = _conf.getOption("spark.files").map(_.split(",")).map(_.filter(_.nonEmpty))
      .toSeq.flatten

    _eventLogDir =
      if (isEventLogEnabled) {
        val unresolvedDir = conf.get("spark.eventLog.dir", EventLoggingListener.DEFAULT_LOG_DIR)
          .stripSuffix("/")
        Some(Utils.resolveURI(unresolvedDir))
      } else {
        None
      }

    _eventLogCodec = {
      val compress = _conf.getBoolean("spark.eventLog.compress", false)
      if (compress && isEventLogEnabled) {
        Some(CompressionCodec.getCodecName(_conf)).map(CompressionCodec.getShortName)
      } else {
        None
      }
    }

    _listenerBus = new LiveListenerBus(_conf)

    // Initialize the app status store and listener before SparkEnv is created so that it gets
    // all events.
    _statusStore = AppStatusStore.createLiveStore(conf)
    listenerBus.addToStatusQueue(_statusStore.listener.get)

    // Create the Spark execution environment (cache, map output tracker, etc)
    _env = createSparkEnv(_conf, isLocal, listenerBus)
    SparkEnv.set(_env)

    // If running the REPL, register the repl's output dir with the file server.
    _conf.getOption("spark.repl.class.outputDir").foreach { path =>
      val replUri = _env.rpcEnv.fileServer.addDirectory("/classes", new File(path))
      _conf.set("spark.repl.class.uri", replUri)
    }

    _statusTracker = new SparkStatusTracker(this, _statusStore)

    _progressBar =
      if (_conf.get(UI_SHOW_CONSOLE_PROGRESS) && !log.isInfoEnabled) {
        Some(new ConsoleProgressBar(this))
      } else {
        None
      }

    _ui =
      if (conf.getBoolean("spark.ui.enabled", true)) {
        Some(SparkUI.create(Some(this), _statusStore, _conf, _env.securityManager, appName, "",
          startTime))
      } else {
        // For tests, do not enable the UI
        None
      }
    // Bind the UI before starting the task scheduler to communicate
    // the bound port to the cluster manager properly
    _ui.foreach(_.bind())

    _hadoopConfiguration = SparkHadoopUtil.get.newConfiguration(_conf)

    // Add each JAR given through the constructor
    if (jars != null) {
      jars.foreach(addJar)
    }

    if (files != null) {
      files.foreach(addFile)
    }

    _executorMemory = _conf.getOption("spark.executor.memory")
      .orElse(Option(System.getenv("SPARK_EXECUTOR_MEMORY")))
      .orElse(Option(System.getenv("SPARK_MEM"))
      .map(warnSparkMem))
      .map(Utils.memoryStringToMb)
      .getOrElse(1024)

    // Convert java options to env vars as a work around
    // since we can't set env vars directly in sbt.
    for { (envKey, propKey) <- Seq(("SPARK_TESTING", "spark.testing"))
      value <- Option(System.getenv(envKey)).orElse(Option(System.getProperty(propKey)))} {
      executorEnvs(envKey) = value
    }
    Option(System.getenv("SPARK_PREPEND_CLASSES")).foreach { v =>
      executorEnvs("SPARK_PREPEND_CLASSES") = v
    }
    // The Mesos scheduler backend relies on this environment variable to set executor memory.
    // TODO: Set this only in the Mesos scheduler.
    executorEnvs("SPARK_EXECUTOR_MEMORY") = executorMemory + "m"
    executorEnvs ++= _conf.getExecutorEnv
    executorEnvs("SPARK_USER") = sparkUser

    // We need to register "HeartbeatReceiver" before "createTaskScheduler" because Executor will
    // retrieve "HeartbeatReceiver" in the constructor. (SPARK-6640)
    _heartbeatReceiver = env.rpcEnv.setupEndpoint(
      HeartbeatReceiver.ENDPOINT_NAME, new HeartbeatReceiver(this))

    // Create and start the scheduler
    val (sched, ts) = SparkContext.createTaskScheduler(this, master, deployMode)
    _schedulerBackend = sched
    _taskScheduler = ts
    _dagScheduler = new DAGScheduler(this)
    _heartbeatReceiver.ask[Boolean](TaskSchedulerIsSet)

    // start TaskScheduler after taskScheduler sets DAGScheduler reference in DAGScheduler's
    // constructor
    _taskScheduler.start()

    _applicationId = _taskScheduler.applicationId()
    _applicationAttemptId = taskScheduler.applicationAttemptId()
    _conf.set("spark.app.id", _applicationId)
    if (_conf.getBoolean("spark.ui.reverseProxy", false)) {
      System.setProperty("spark.ui.proxyBase", "/proxy/" + _applicationId)
    }
    _ui.foreach(_.setAppId(_applicationId))
    _env.blockManager.initialize(_applicationId)

    // The metrics system for Driver need to be set spark.app.id to app ID.
    // So it should start after we get app ID from the task scheduler and set spark.app.id.
    _env.metricsSystem.start()
    // Attach the driver metrics servlet handler to the web ui after the metrics system is started.
    _env.metricsSystem.getServletHandlers.foreach(handler => ui.foreach(_.attachHandler(handler)))

    _eventLogger =
      if (isEventLogEnabled) {
        val logger =
          new EventLoggingListener(_applicationId, _applicationAttemptId, _eventLogDir.get,
            _conf, _hadoopConfiguration)
        logger.start()
        listenerBus.addToEventLogQueue(logger)
        Some(logger)
      } else {
        None
      }

    // Optionally scale number of executors dynamically based on workload. Exposed for testing.
    val dynamicAllocationEnabled = Utils.isDynamicAllocationEnabled(_conf)
    _executorAllocationManager =
      if (dynamicAllocationEnabled) {
        schedulerBackend match {
          case b: ExecutorAllocationClient =>
            Some(new ExecutorAllocationManager(
              schedulerBackend.asInstanceOf[ExecutorAllocationClient], listenerBus, _conf,
              _env.blockManager.master))
          case _ =>
            None
        }
      } else {
        None
      }
    _executorAllocationManager.foreach(_.start())

    _cleaner =
      if (_conf.getBoolean("spark.cleaner.referenceTracking", true)) {
        Some(new ContextCleaner(this))
      } else {
        None
      }
    _cleaner.foreach(_.start())

    setupAndStartListenerBus()
    postEnvironmentUpdate()
    postApplicationStart()

    // Post init
    _taskScheduler.postStartHook()
    _env.metricsSystem.registerSource(_dagScheduler.metricsSource)
    _env.metricsSystem.registerSource(new BlockManagerSource(_env.blockManager))
    _executorAllocationManager.foreach { e =>
      _env.metricsSystem.registerSource(e.executorAllocationManagerSource)
    }

    // Make sure the context is stopped if the user forgets about it. This avoids leaving
    // unfinished event logs around after the JVM exits cleanly. It doesn't help if the JVM
    // is killed, though.
    logDebug("Adding shutdown hook") // force eager creation of logger
    _shutdownHookRef = ShutdownHookManager.addShutdownHook(
      ShutdownHookManager.SPARK_CONTEXT_SHUTDOWN_PRIORITY) { () =>
      logInfo("Invoking stop() from shutdown hook")
      try {
        stop()
      } catch {
        case e: Throwable =>
          logWarning("Ignoring Exception while stopping SparkContext from shutdown hook", e)
      }
    }
  } catch {
    case NonFatal(e) =>
      logError("Error initializing SparkContext.", e)
      try {
        stop()
      } catch {
        case NonFatal(inner) =>
          logError("Error stopping SparkContext after init error.", inner)
      } finally {
        throw e
      }
  }
 

建立SparkEnv

• ).SparkEnv物件是在這個方法中構造的
• ).new SecurityManager()
• ).new NettyRpcEnvFactory()
• ).建立NettyRpcEnv
• ).Utils.startServiceOnPort(啟動sparkDriver)
• ). new BroadcastManager
• ).註冊端點MapOutputTracker
• ).ShuffleManager:SortShuffleManager
• ).預設記憶體管理器：UnifiedMemoryManager
• ).註冊端點MapOutputTracker
• ).SortShuffleManager
• ).UnifiedMemoryManager
• ).註冊端點BlockManagerMaster
• ).new BlockManager
• ).註冊端點OutputCommitCoordinator

    // Create the Spark execution environment (cache, map output tracker, etc)
    _env = createSparkEnv(_conf, isLocal, listenerBus)
    SparkEnv.set(_env)

建立SparkUI

    _ui =
      if (conf.getBoolean("spark.ui.enabled", true)) {
        Some(SparkUI.create(Some(this), _statusStore, _conf, _env.securityManager, appName, "",
          startTime))
      } else {
        // For tests, do not enable the UI
        None
      }
    // Bind the UI before starting the task scheduler to communicate
    // the bound port to the cluster manager properly
    _ui.foreach(_.bind())

建立任務排程器

 // Create and start the scheduler
    val (sched, ts) = SparkContext.createTaskScheduler(this, master, deployMode)

SparkContext.createTaskScheduler

• 根據master配置匹配對應的任務排程器
• 本地模式 local
• 本地模式n個執行緒 LOCAL_N_REGEX
• standalone模式 SPARK_REGEX
• 本文分析standalone模式的處理方式

/**
   * Create a task scheduler based on a given master URL.
   * Return a 2-tuple of the scheduler backend and the task scheduler.
   */
  private def createTaskScheduler(
      sc: SparkContext,
      master: String,
      deployMode: String): (SchedulerBackend, TaskScheduler) = {
    import SparkMasterRegex._

    // When running locally, don't try to re-execute tasks on failure.
    val MAX_LOCAL_TASK_FAILURES = 1

    master match {
      case "local" =>
        val scheduler = new TaskSchedulerImpl(sc, MAX_LOCAL_TASK_FAILURES, isLocal = true)
        val backend = new LocalSchedulerBackend(sc.getConf, scheduler, 1)
        scheduler.initialize(backend)
        (backend, scheduler)

      case LOCAL_N_REGEX(threads) =>
        def localCpuCount: Int = Runtime.getRuntime.availableProcessors()
        // local[*] estimates the number of cores on the machine; local[N] uses exactly N threads.
        val threadCount = if (threads == "*") localCpuCount else threads.toInt
        if (threadCount <= 0) {
          throw new SparkException(s"Asked to run locally with $threadCount threads")
        }
        val scheduler = new TaskSchedulerImpl(sc, MAX_LOCAL_TASK_FAILURES, isLocal = true)
        val backend = new LocalSchedulerBackend(sc.getConf, scheduler, threadCount)
        scheduler.initialize(backend)
        (backend, scheduler)

      case LOCAL_N_FAILURES_REGEX(threads, maxFailures) =>
        def localCpuCount: Int = Runtime.getRuntime.availableProcessors()
        // local[*, M] means the number of cores on the computer with M failures
        // local[N, M] means exactly N threads with M failures
        val threadCount = if (threads == "*") localCpuCount else threads.toInt
        val scheduler = new TaskSchedulerImpl(sc, maxFailures.toInt, isLocal = true)
        val backend = new LocalSchedulerBackend(sc.getConf, scheduler, threadCount)
        scheduler.initialize(backend)
        (backend, scheduler)

      case SPARK_REGEX(sparkUrl) =>
        val scheduler = new TaskSchedulerImpl(sc)
        val masterUrls = sparkUrl.split(",").map("spark://" + _)
        val backend = new StandaloneSchedulerBackend(scheduler, sc, masterUrls)
        scheduler.initialize(backend)
        (backend, scheduler)

      case LOCAL_CLUSTER_REGEX(numSlaves, coresPerSlave, memoryPerSlave) =>
        // Check to make sure memory requested <= memoryPerSlave. Otherwise Spark will just hang.
        val memoryPerSlaveInt = memoryPerSlave.toInt
        if (sc.executorMemory > memoryPerSlaveInt) {
          throw new SparkException(
            "Asked to launch cluster with %d MB RAM / worker but requested %d MB/worker".format(
              memoryPerSlaveInt, sc.executorMemory))
        }

        val scheduler = new TaskSchedulerImpl(sc)
        val localCluster = new LocalSparkCluster(
          numSlaves.toInt, coresPerSlave.toInt, memoryPerSlaveInt, sc.conf)
        val masterUrls = localCluster.start()
        val backend = new StandaloneSchedulerBackend(scheduler, sc, masterUrls)
        scheduler.initialize(backend)
        backend.shutdownCallback = (backend: StandaloneSchedulerBackend) => {
          localCluster.stop()
        }
        (backend, scheduler)

      case masterUrl =>
        val cm = getClusterManager(masterUrl) match {
          case Some(clusterMgr) => clusterMgr
          case None => throw new SparkException("Could not parse Master URL: '" + master + "'")
        }
        try {
          val scheduler = cm.createTaskScheduler(sc, masterUrl)
          val backend = cm.createSchedulerBackend(sc, masterUrl, scheduler)
          cm.initialize(scheduler, backend)
          (backend, scheduler)
        } catch {
          case se: SparkException => throw se
          case NonFatal(e) =>
            throw new SparkException("External scheduler cannot be instantiated", e)
        }
    }
  }

SparkContext.createTaskScheduler standalone模式

• 任務排程器 val scheduler = new TaskSchedulerImpl(sc)
• standalone後端排程器 val backend = new StandaloneSchedulerBackend(scheduler, sc, masterUrls)
• 呼叫任務排程器的 initialize(backend) 方法
• 返回(backend, scheduler)

      case SPARK_REGEX(sparkUrl) =>
        val scheduler = new TaskSchedulerImpl(sc)
        val masterUrls = sparkUrl.split(",").map("spark://" + _)
        val backend = new StandaloneSchedulerBackend(scheduler, sc, masterUrls)
        scheduler.initialize(backend)
        (backend, scheduler)

TaskSchedulerImpl.initialize

• 給變數backend 賦值: StandaloneSchedulerBackend
• 匹配排程模式，用schedulableBuilder構建排程池

var backend: SchedulerBackend = null

  def initialize(backend: SchedulerBackend) {
    this.backend = backend
    schedulableBuilder = {
      schedulingMode match {
        case SchedulingMode.FIFO =>
          new FIFOSchedulableBuilder(rootPool)
        case SchedulingMode.FAIR =>
          new FairSchedulableBuilder(rootPool, conf)
        case _ =>
          throw new IllegalArgumentException(s"Unsupported $SCHEDULER_MODE_PROPERTY: " +
          s"$schedulingMode")
      }
    }
    schedulableBuilder.buildPools()
  }

• 預設任務排程器排程方式FIFO

private val schedulingModeConf = conf.get(SCHEDULER_MODE_PROPERTY, SchedulingMode.FIFO.toString)
  val schedulingMode: SchedulingMode =
    try {
      SchedulingMode.withName(schedulingModeConf.toUpperCase(Locale.ROOT))
    } catch {
      case e: java.util.NoSuchElementException =>
        throw new SparkException(s"Unrecognized $SCHEDULER_MODE_PROPERTY: $schedulingModeConf")
    }

  val rootPool: Pool = new Pool("", schedulingMode, 0, 0)

SparkContext.DAGScheduler

_dagScheduler = new DAGScheduler(this)

SparkContext 起動任務排程器

• 呼叫TaskSchedulerImpl.start()函式

_taskScheduler.start()

TaskSchedulerImpl.start()

• 呼叫StandaloneSchedulerBackend.start()函式

  override def start() {
    backend.start()

    if (!isLocal && conf.getBoolean("spark.speculation", false)) {
      logInfo("Starting speculative execution thread")
      speculationScheduler.scheduleWithFixedDelay(new Runnable {
        override def run(): Unit = Utils.tryOrStopSparkContext(sc) {
          checkSpeculatableTasks()
        }
      }, SPECULATION_INTERVAL_MS, SPECULATION_INTERVAL_MS, TimeUnit.MILLISECONDS)
    }
  }

StandaloneSchedulerBackend.start()

• StandaloneSchedulerBackend extends CoarseGrainedSchedulerBackend
• super.start()呼叫CoarseGrainedSchedulerBackend.start()函式
• client = new StandaloneAppClient() 例項化StandaloneAppClinet
• client.start() 函式排程

override def start() {
    super.start()

    // SPARK-21159. The scheduler backend should only try to connect to the launcher when in client
    // mode. In cluster mode, the code that submits the application to the Master needs to connect
    // to the launcher instead.
    if (sc.deployMode == "client") {
      launcherBackend.connect()
    }

    // The endpoint for executors to talk to us
    val driverUrl = RpcEndpointAddress(
      sc.conf.get("spark.driver.host"),
      sc.conf.get("spark.driver.port").toInt,
      CoarseGrainedSchedulerBackend.ENDPOINT_NAME).toString
    val args = Seq(
      "--driver-url", driverUrl,
      "--executor-id", "{{EXECUTOR_ID}}",
      "--hostname", "{{HOSTNAME}}",
      "--cores", "{{CORES}}",
      "--app-id", "{{APP_ID}}",
      "--worker-url", "{{WORKER_URL}}")
    val extraJavaOpts = sc.conf.getOption("spark.executor.extraJavaOptions")
      .map(Utils.splitCommandString).getOrElse(Seq.empty)
    val classPathEntries = sc.conf.getOption("spark.executor.extraClassPath")
      .map(_.split(java.io.File.pathSeparator).toSeq).getOrElse(Nil)
    val libraryPathEntries = sc.conf.getOption("spark.executor.extraLibraryPath")
      .map(_.split(java.io.File.pathSeparator).toSeq).getOrElse(Nil)

    // When testing, expose the parent class path to the child. This is processed by
    // compute-classpath.{cmd,sh} and makes all needed jars available to child processes
    // when the assembly is built with the "*-provided" profiles enabled.
    val testingClassPath =
      if (sys.props.contains("spark.testing")) {
        sys.props("java.class.path").split(java.io.File.pathSeparator).toSeq
      } else {
        Nil
      }

    // Start executors with a few necessary configs for registering with the scheduler
    val sparkJavaOpts = Utils.sparkJavaOpts(conf, SparkConf.isExecutorStartupConf)
    val javaOpts = sparkJavaOpts ++ extraJavaOpts
    val command = Command("org.apache.spark.executor.CoarseGrainedExecutorBackend",
      args, sc.executorEnvs, classPathEntries ++ testingClassPath, libraryPathEntries, javaOpts)
    val webUrl = sc.ui.map(_.webUrl).getOrElse("")
    val coresPerExecutor = conf.getOption("spark.executor.cores").map(_.toInt)
    // If we're using dynamic allocation, set our initial executor limit to 0 for now.
    // ExecutorAllocationManager will send the real initial limit to the Master later.
    val initialExecutorLimit =
      if (Utils.isDynamicAllocationEnabled(conf)) {
        Some(0)
      } else {
        None
      }
    val appDesc = ApplicationDescription(sc.appName, maxCores, sc.executorMemory, command,
      webUrl, sc.eventLogDir, sc.eventLogCodec, coresPerExecutor, initialExecutorLimit)
    client = new StandaloneAppClient(sc.env.rpcEnv, masters, appDesc, this, conf)
    client.start()
    launcherBackend.setState(SparkAppHandle.State.SUBMITTED)
    waitForRegistration()
    launcherBackend.setState(SparkAppHandle.State.RUNNING)
  }

CoarseGrainedSchedulerBackend.start()

• 註冊DriverEndpoint: CoarseGrainedScheduler
• 首先呼叫DriverEndpoint.OnStart()函式

  override def start() {
    val properties = new ArrayBuffer[(String, String)]
    for ((key, value) <- scheduler.sc.conf.getAll) {
      if (key.startsWith("spark.")) {
        properties += ((key, value))
      }
    }

    // TODO (prashant) send conf instead of properties
    driverEndpoint = createDriverEndpointRef(properties)
  }

DriverEndpoint.OnStart()函式

• 呼叫執行緒池定時任務，預設每隔一秒傳送訊息:ReviveOffers
• DriverEndpoint.receive()函式對ReviveOffers訊息進行處理
• 呼叫CoarseGrainedSchedulerBackend.DriverEndpoint.makeOffers()函式,為所有的executor分配資源

    override def onStart() {
      // Periodically revive offers to allow delay scheduling to work
      val reviveIntervalMs = conf.getTimeAsMs("spark.scheduler.revive.interval", "1s")

      reviveThread.scheduleAtFixedRate(new Runnable {
        override def run(): Unit = Utils.tryLogNonFatalError {
          Option(self).foreach(_.send(ReviveOffers))
        }
      }, 0, reviveIntervalMs, TimeUnit.MILLISECONDS)
    }

StandaloneAppClient.start()

• 註冊通訊端點: AppClient
• 注意，通訊端點首先呼叫OnStart()函式，即呼叫ClientEndpoint.OnStart()函式,該函式會向master註冊應用程式

  def start() {
    // Just launch an rpcEndpoint; it will call back into the listener.
    endpoint.set(rpcEnv.setupEndpoint("AppClient", new ClientEndpoint(rpcEnv)))
  }

ClientEndpoint.OnStart()

• 呼叫函式 registerWithMaster(1)向master註冊應用程式

    override def onStart(): Unit = {
      try {
        registerWithMaster(1)
      } catch {
        case e: Exception =>
          logWarning("Failed to connect to master", e)
          markDisconnected()
          stop()
      }
    }

ClientEndpoint.registerWithMaster()

• 呼叫函式 tryRegisterAllMasters() 向所有master註冊應用程式

    /**
     * Register with all masters asynchronously. It will call `registerWithMaster` every
     * REGISTRATION_TIMEOUT_SECONDS seconds until exceeding REGISTRATION_RETRIES times.
     * Once we connect to a master successfully, all scheduling work and Futures will be cancelled.
     *
     * nthRetry means this is the nth attempt to register with master.
     */
    private def registerWithMaster(nthRetry: Int) {
      registerMasterFutures.set(tryRegisterAllMasters())
      registrationRetryTimer.set(registrationRetryThread.schedule(new Runnable {
        override def run(): Unit = {
          if (registered.get) {
            registerMasterFutures.get.foreach(_.cancel(true))
            registerMasterThreadPool.shutdownNow()
          } else if (nthRetry >= REGISTRATION_RETRIES) {
            markDead("All masters are unresponsive! Giving up.")
          } else {
            registerMasterFutures.get.foreach(_.cancel(true))
            registerWithMaster(nthRetry + 1)
          }
        }
      }, REGISTRATION_TIMEOUT_SECONDS, TimeUnit.SECONDS))
    }

ClientEndpoint.tryRegisterAllMasters()

• 向所有master傳送訊息: RegisterApplication()
• 注意 ClientEndpoint 是一個通訊端點，函式receive*()也可以接收訊息,如接收master發過來的訊息：RegisteredApplication

    /**
     *  Register with all masters asynchronously and returns an array `Future`s for cancellation.
     */
    private def tryRegisterAllMasters(): Array[JFuture[_]] = {
      for (masterAddress <- masterRpcAddresses) yield {
        registerMasterThreadPool.submit(new Runnable {
          override def run(): Unit = try {
            if (registered.get) {
              return
            }
            logInfo("Connecting to master " + masterAddress.toSparkURL + "...")
            val masterRef = rpcEnv.setupEndpointRef(masterAddress, Master.ENDPOINT_NAME)
            masterRef.send(RegisterApplication(appDescription, self))
          } catch {
            case ie: InterruptedException => // Cancelled
            case NonFatal(e) => logWarning(s"Failed to connect to master $masterAddress", e)
          }
        })
      }
    }

ClientEndpoint.receive()

• 接收master發過來的註冊應用程式完成訊息: RegisteredApplication

override def receive: PartialFunction[Any, Unit] = {
      case RegisteredApplication(appId_, masterRef) =>
        // FIXME How to handle the following cases?
        // 1. A master receives multiple registrations and sends back multiple
        // RegisteredApplications due to an unstable network.
        // 2. Receive multiple RegisteredApplication from different masters because the master is
        // changing.
        appId.set(appId_)
        registered.set(true)
        master = Some(masterRef)
        listener.connected(appId.get)

      case ApplicationRemoved(message) =>
        markDead("Master removed our application: %s".format(message))
        stop()

      case ExecutorAdded(id: Int, workerId: String, hostPort: String, cores: Int, memory: Int) =>
        val fullId = appId + "/" + id
        logInfo("Executor added: %s on %s (%s) with %d core(s)".format(fullId, workerId, hostPort,
          cores))
        listener.executorAdded(fullId, workerId, hostPort, cores, memory)

      case ExecutorUpdated(id, state, message, exitStatus, workerLost) =>
        val fullId = appId + "/" + id
        val messageText = message.map(s => " (" + s + ")").getOrElse("")
        logInfo("Executor updated: %s is now %s%s".format(fullId, state, messageText))
        if (ExecutorState.isFinished(state)) {
          listener.executorRemoved(fullId, message.getOrElse(""), exitStatus, workerLost)
        }

      case WorkerRemoved(id, host, message) =>
        logInfo("Master removed worker %s: %s".format(id, message))
        listener.workerRemoved(id, host, message)

      case MasterChanged(masterRef, masterWebUiUrl) =>
        logInfo("Master has changed, new master is at " + masterRef.address.toSparkURL)
        master = Some(masterRef)
        alreadyDisconnected = false
        masterRef.send(MasterChangeAcknowledged(appId.get))
    }

end