前言

在Spark Streaming里，总体负责任务的动态调度是JobScheduler，而JobScheduler有两个很重要的成员：JobGenerator 和 ReceiverTracker。JobGenerator 负责将每个 batch 生成具体的 RDD DAG ，而ReceiverTracker负责数据的来源。

需要在executor上运行的receiver接收数据的InputDStream都需要继承ReceiverInputDStream，ReceiverInputDStream有一个def getReceiver(): Receiver[T]方法，子类都需要实现这个方法。如KafkaInputDStream对应KafkaReceiver，FlumeInputDStream对应FlumeReceiver，TwitterInputDStream对应TwitterReceiver等。

流程概述：

• ReceiverTracker 启动，获取所有InputDStreams对应的receivers
• 根据调度策略确定每个Receiver的优先位置（能在哪些executor上执行）
• 将Receiver包装成RDD并通过sc提交一个job，执行函数为创建supervisor实例，调用start()方法，也即调用了Receiver的onStart()方法
• Receiver的onStart不断接收数据，通过store方法最终调用supervisor来存储块
• 存储后通知ReceiverTracker此Block的信息
• ReceiverTracker将Block消息交给ReceivedBlockTracker管理

启动 Receiver

先看看receiverTracker的启动过程：

ssc.start()
    scheduler.start()
        receiverTracker.start()
        jobGenerator.start()
----
 def start(): Unit = synchronized {
    if (isTrackerStarted) {
      throw new SparkException("ReceiverTracker already started")
    }

    if (!receiverInputStreams.isEmpty) {
      endpoint = ssc.env.rpcEnv.setupEndpoint(
        "ReceiverTracker", new ReceiverTrackerEndpoint(ssc.env.rpcEnv))
      if (!skipReceiverLaunch) launchReceivers()
      logInfo("ReceiverTracker started")
      trackerState = Started
    }
  }

在start方法中先创建了ReceiverTracker的Endpoint，接着调用launchReceivers()方法来启动Recivers：

 private def launchReceivers(): Unit = {
    val receivers = receiverInputStreams.map { nis =>
      val rcvr = nis.getReceiver()
      rcvr.setReceiverId(nis.id)
      rcvr
    }

    runDummySparkJob()

    logInfo("Starting " + receivers.length + " receivers")
    endpoint.send(StartAllReceivers(receivers))
  }

遍历所有的InputStream，并得到所对应的Receiver集合receivers。并向ReceiverTrackerEndpoint发送了StartAllReceivers消息，看看接收到该消息后是如何处理的：

 case StartAllReceivers(receivers) =>
        val scheduledLocations = schedulingPolicy.scheduleReceivers(receivers, getExecutors)
        for (receiver <- receivers) {
          val executors = scheduledLocations(receiver.streamId)
          updateReceiverScheduledExecutors(receiver.streamId, executors)
          receiverPreferredLocations(receiver.streamId) = receiver.preferredLocation
          startReceiver(receiver, executors)
        }

通过调度策略来计算决定每个receiver的一组优先位置，即一个Receiver改在哪个executor节点上启动，调度的主要原则是：

• 满足Receiver的preferredLocation。
• 其次保证将Receiver分布的尽量均匀。

接着遍历所有receivers调用了startReceiver(receiver, executors)方法来启动receiver：

 private def startReceiver(
        receiver: Receiver[_],
        scheduledLocations: Seq[TaskLocation]): Unit = {
      def shouldStartReceiver: Boolean = {
        // It's okay to start when trackerState is Initialized or Started
        !(isTrackerStopping || isTrackerStopped)
      }

      val receiverId = receiver.streamId
      if (!shouldStartReceiver) {
        onReceiverJobFinish(receiverId)
        return
      }

      val checkpointDirOption = Option(ssc.checkpointDir)
      val serializableHadoopConf =
        new SerializableConfiguration(ssc.sparkContext.hadoopConfiguration)

      // Function to start the receiver on the worker node
      val startReceiverFunc: Iterator[Receiver[_]] => Unit =
        (iterator: Iterator[Receiver[_]]) => {
          if (!iterator.hasNext) {
            throw new SparkException(
              "Could not start receiver as object not found.")
          }
          if (TaskContext.get().attemptNumber() == 0) {
            val receiver = iterator.next()
            assert(iterator.hasNext == false)
            val supervisor = new ReceiverSupervisorImpl(
              receiver, SparkEnv.get, serializableHadoopConf.value, checkpointDirOption)
            supervisor.start()
            supervisor.awaitTermination()
          } else {
            // It's restarted by TaskScheduler, but we want to reschedule it again. So exit it.
          }
        }

      // Create the RDD using the scheduledLocations to run the receiver in a Spark job
      val receiverRDD: RDD[Receiver[_]] =
        if (scheduledLocations.isEmpty) {
          ssc.sc.makeRDD(Seq(receiver), 1)
        } else {
          val preferredLocations = scheduledLocations.map(_.toString).distinct
          ssc.sc.makeRDD(Seq(receiver -> preferredLocations))
        }
      receiverRDD.setName(s"Receiver $receiverId")
      ssc.sparkContext.setJobDescription(s"Streaming job running receiver $receiverId")
      ssc.sparkContext.setCallSite(Option(ssc.getStartSite()).getOrElse(Utils.getCallSite()))

      val future = ssc.sparkContext.submitJob[Receiver[_], Unit, Unit](
        receiverRDD, startReceiverFunc, Seq(0), (_, _) => Unit, ())
      // We will keep restarting the receiver job until ReceiverTracker is stopped
      future.onComplete {
        case Success(_) =>
          if (!shouldStartReceiver) {
            onReceiverJobFinish(receiverId)
          } else {
            logInfo(s"Restarting Receiver $receiverId")
            self.send(RestartReceiver(receiver))
          }
        case Failure(e) =>
          if (!shouldStartReceiver) {
            onReceiverJobFinish(receiverId)
          } else {
            logError("Receiver has been stopped. Try to restart it.", e)
            logInfo(s"Restarting Receiver $receiverId")
            self.send(RestartReceiver(receiver))
          }
      }(ThreadUtils.sameThread)
      logInfo(s"Receiver ${receiver.streamId} started")
    }

注意这里巧妙的将receiver包装成了RDD，并把scheduledLocations作为RDD的优先位置locationPrefs。

然后通过sc提交了一个Spark Core Job，执行函数是startReceiverFunc（也就是要在executor上执行的），在该方法中创建一个ReceiverSupervisorImpl对象，并调用了start()方法，在该方法中会调用 receiver的onStart 后立即返回。

receiver的onStart 方法一般会新建线程或线程池来接收数据，比如在 KafkaReceiver 中，就新建了线程池，在线程池中接收 topics 的数据。

supervisor.start() 返回后，由 supervisor.awaitTermination() 阻塞住线程，以让这个 task 一直不退出，从而可以源源不断接收数据。

Receiver 数据处理

前面提到receiver的onStart()方法会新建线程或线程池来接收数据，那接收的数据怎么处理的呢？都会调用receiver的store()，而store方法又调用了supervisor的方法。对应的store方法有多种形式：

• pushSingle: 对应单条小数据，需要通过BlockGenerator聚集多条数据后再成块的存储
• pushArrayBuffer: 对应数组形式的数据
• pushIterator: 对应 iterator 形式数据
• pushBytes: 对应 ByteBuffer 形式的块数据

除了pushSingle需要通过BlockGenerator将数据聚集成一个块的时候再存储，其他方法都是直接成块存储。

看看pushSingle是怎么通过聚集的方式存储块的：

def pushSingle(data: Any) {
    defaultBlockGenerator.addData(data)
  }
------
def addData(data: Any): Unit = {
    if (state == Active) {
      waitToPush()
      synchronized {
        if (state == Active) {
          currentBuffer += data
        } else {
          throw new SparkException(
            "Cannot add data as BlockGenerator has not been started or has been stopped")
        }
      }
    } else {
      throw new SparkException(
        "Cannot add data as BlockGenerator has not been started or has been stopped")
    }
  }

这里的先调用waitToPush()，会有rateLimiter检查速率，防止加入过快，如果过快会block住等到下一秒再添加，一秒能添加的条数受spark.streaming.receiver.maxRate控制，即一个Receiver每秒能添加的条数。
检查完后会将数据添加到一个变长数组currentBuffer中。

另外，BlockGenerator被初始化的时候就创建了一个定时器：

private val blockIntervalMs = conf.getTimeAsMs("spark.streaming.blockInterval", "200ms")
  require(blockIntervalMs > 0, s"'spark.streaming.blockInterval' should be a positive value")

  private val blockIntervalTimer =
    new RecurringTimer(clock, blockIntervalMs, updateCurrentBuffer, "BlockGenerator")

定时间隔默认200ms，可通过spark.streaming.blockInterval配置，每次定时执行的是updateCurrentBuffer方法：

private def updateCurrentBuffer(time: Long): Unit = {
    try {
      var newBlock: Block = null
      synchronized {
        if (currentBuffer.nonEmpty) {
          val newBlockBuffer = currentBuffer
          currentBuffer = new ArrayBuffer[Any]
          val blockId = StreamBlockId(receiverId, time - blockIntervalMs)
          listener.onGenerateBlock(blockId)
          newBlock = new Block(blockId, newBlockBuffer)
        }
      }

      if (newBlock != null) {
        blocksForPushing.put(newBlock)  // put is blocking when queue is full
      }
    } catch {
      case ie: InterruptedException =>
        logInfo("Block updating timer thread was interrupted")
      case e: Exception =>
        reportError("Error in block updating thread", e)
    }
  }

• 将 currentBuffer 赋值给 newBlockBuffer
• 重新为currentBuffer分配一个新对象，以供存储新的数据
• 将currentBuffer封装为Block后添加到blocksForPushing中，blocksForPushing是一个默认长度为10的Queue，可通过spark.streaming.blockQueueSize配置

BlockGenerator初始化的时候还启动了一个线程来从blocksForPushing队列中取出Block通过supervisor来存储块的：

private val blockPushingThread = new Thread() { override def run() { keepPushingBlocks() } }

supervisor 存储数据块

先存储再向上报告：

#pushAndReportBlock
val blockStoreResult = receivedBlockHandler.storeBlock(blockId, receivedBlock)
logDebug(s"Pushed block $blockId in ${(System.currentTimeMillis - time)} ms")
val numRecords = blockStoreResult.numRecords
val blockInfo = ReceivedBlockInfo(streamId, numRecords, metadataOption, blockStoreResult) trackerEndpoint.askWithRetry[Boolean](AddBlock(blockInfo))

存储数据块有对应的receivedBlockHandler，在启用了WAL（spark.streaming.receiver.writeAheadLog.enable为true）的情况下对应的是WriteAheadLogBasedBlockHandler（启用了WAL的情况下在应用程序挂掉后可以从WAL恢复数据），未启用的情况下对应的是BlockManagerBasedBlockHandler。

private val receivedBlockHandler: ReceivedBlockHandler = {
    if (WriteAheadLogUtils.enableReceiverLog(env.conf)) {
      if (checkpointDirOption.isEmpty) {
        throw new SparkException(
          "Cannot enable receiver write-ahead log without checkpoint directory set. " +
            "Please use streamingContext.checkpoint() to set the checkpoint directory. " +
            "See documentation for more details.")
      }
      new WriteAheadLogBasedBlockHandler(env.blockManager, env.serializerManager, receiver.streamId,
        receiver.storageLevel, env.conf, hadoopConf, checkpointDirOption.get)
    } else {
      new BlockManagerBasedBlockHandler(env.blockManager, receiver.storageLevel)
    }

storeBlock方法部分代码：

case ArrayBufferBlock(arrayBuffer) =>
    numRecords = Some(arrayBuffer.size.toLong)
    blockManager.putIterator(blockId, arrayBuffer.iterator, storageLevel,tellMaster = true)
case IteratorBlock(iterator) =>
    val countIterator = new CountingIterator(iterator)
    val putResult = blockManager.putIterator(blockId, countIterator, storageLevel,tellMaster = true)
    numRecords = countIterator.count
    putResult
case ByteBufferBlock(byteBuffer) =>
    blockManager.putBytes(blockId, new ChunkedByteBuffer(byteBuffer.duplicate()), storageLevel, tellMaster = true)

两种handler都是通过blockManager来存储block到内存或者磁盘，存储的细节可见BlockManager 解析。

通知 ReceiverTracker

存储了block后，接着创建了ReceivedBlockInfo实例，对应该block的一些信息，包括streamId（一个InputDStream对应一个Receiver，一个Receiver对应一个streamId）、block中数据的条数、storeResult等信息。

接着将receivedBlockInfo作为参数和ReceiverTracker通信发送AddBlock消息，ReceiverTracker收到消息后的处理如下：

 case AddBlock(receivedBlockInfo) =>
        if (WriteAheadLogUtils.isBatchingEnabled(ssc.conf, isDriver = true)) {
          walBatchingThreadPool.execute(new Runnable {
            override def run(): Unit = Utils.tryLogNonFatalError {
              if (active) {
                context.reply(addBlock(receivedBlockInfo))
              } else {
                throw new IllegalStateException("ReceiverTracker RpcEndpoint shut down.")
              }
            }
          })
        } else {
          context.reply(addBlock(receivedBlockInfo))
        }

都会调用addBlock(receivedBlockInfo)方法：

private def addBlock(receivedBlockInfo: ReceivedBlockInfo): Boolean = {
    receivedBlockTracker.addBlock(receivedBlockInfo)
  }

ReceiverTracker有个专门管理block的成员receivedBlockTracker，通过addBlock(receivedBlockInfo)来添加block信息：

def addBlock(receivedBlockInfo: ReceivedBlockInfo): Boolean = {
    try {
      val writeResult = writeToLog(BlockAdditionEvent(receivedBlockInfo))
      if (writeResult) {
        synchronized {
          getReceivedBlockQueue(receivedBlockInfo.streamId) += receivedBlockInfo
        }
        logDebug(s"Stream ${receivedBlockInfo.streamId} received " +
          s"block ${receivedBlockInfo.blockStoreResult.blockId}")
      } else {
        logDebug(s"Failed to acknowledge stream ${receivedBlockInfo.streamId} receiving " +
          s"block ${receivedBlockInfo.blockStoreResult.blockId} in the Write Ahead Log.")
      }
      writeResult
    } catch {
      case NonFatal(e) =>
        logError(s"Error adding block $receivedBlockInfo", e)
        false
    }
  }

若启用WAL则会先将block信息以WAL保存，之后都会将block信息保存到streamIdToUnallocatedBlockQueuesmutable.HashMap[Int, ReceivedBlockQueue]中，其中key为InputDStream唯一id，value为已存储但未分配的block信息。之后为 batch 分配blocks，会访问该结构来获取每个 InputDStream 对应的未消费的 blocks。