【Spark九十九】Spark Streaming的batch interval时间内的数据流转源码分析

 

以如下代码为例（SocketInputDStream）：

Spark Streaming从Socket读取数据的代码是在SocketReceiver的receive方法中，撇开异常情况不谈(Receiver有重连机制，restart方法，默认情况下在Receiver挂了之后，间隔两秒钟重新建立Socket连接)，读取到的数据通过调用store(textRead)方法进行存储。数据的流转需要关注如下几个问题：

1. 数据存储到什么位置了

2. 数据存储的结构如何？

3. 数据什么时候被读取

4. 读取到的数据(batch interval)如何转换为RDD

 

1. SocketReceiver#receive

  /** Create a socket connection and receive data until receiver is stopped */
  def receive() {
    var socket: Socket = null
    try {
      logInfo("Connecting to " + host + ":" + port)
      socket = new Socket(host, port)
      logInfo("Connected to " + host + ":" + port)
      val iterator = bytesToObjects(socket.getInputStream())
      while(!isStopped && iterator.hasNext) {
        store(iterator.next)
      }
      logInfo("Stopped receiving")
      restart("Retrying connecting to " + host + ":" + port)
    } catch {
      case e: java.net.ConnectException =>
        restart("Error connecting to " + host + ":" + port, e)
      case t: Throwable =>
        restart("Error receiving data", t)
    } finally {
      if (socket != null) {
        socket.close()
        logInfo("Closed socket to " + host + ":" + port)
      }
    }
  }

 

2. SocketReceiver#receive=>SocketReceiver#store

 

  /**
   * Store a single item of received data to Spark's memory.
   * These single items will be aggregated together into data blocks before
   * being pushed into Spark's memory.
   */
  def store(dataItem: T) {
    executor.pushSingle(dataItem)
  }

 

数据存储作为Executor功能之一，store方法调用了executor中的pushSingle操作，此时的Single可以理解为一次数据读取，而dataItem就是一次读取的数据对象

 

 

3. SocketReceiver#store=>executor.pushSingle（ReceiverSupervisorImpl.pushSingle）

 

  /** Push a single record of received data into block generator. */
  def pushSingle(data: Any) {
    blockGenerator.addData(data)
  }

 

数据放入到了blockGenerator数据结构中了，blockGenerator，类型为BlockGenerator，顾名思义是一个block生成器，所谓的block生成器，是指Spark Streaming每隔一段时间(默认200毫秒,   private val blockInterval = conf.getLong("spark.streaming.blockInterval", 200)）将接收到的数据合并成一个block，然后将这个block写入到BlockManager，继续沿着个思路分析

 

 

4. executor.pushSingle=>BlockGenerator.addData

  /**
   * Push a single data item into the buffer. All received data items
   * will be periodically pushed into BlockManager.
   */
  def addData (data: Any): Unit = synchronized {
    waitToPush() ///通过阻塞控制Push的速度
    currentBuffer += data，将数据追加到currentBuffer中
  }

 

 当数据写入到currentBuffer中之后，似乎线索已经断了。事实上是BlockGenerator内部开启的两个线程（BlockIntervalTimer和BlockPushingThread）在背后继续处理currentBuffer

 

BlockIntervalTimer默认每200毫秒执行一次updateCurrentBufferer，该函数的功能是将类型为ArrayBuffer的currentBuffer合并成一个小的Block

  private val blockInterval = conf.getLong("spark.streaming.blockInterval", 200)
  private val blockIntervalTimer =
    new RecurringTimer(clock, blockInterval, updateCurrentBuffer, "BlockGenerator")

 

 

 BlockPushingThread是通过循环调用keepPushingBlocks将BlockIntervalTimer创建的各个Block写入到BlockManager中,

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

 

 

 上面说到的两个线程的同步是通过ArrayBlockQueue实现的

  private val blockQueueSize = conf.getInt("spark.streaming.blockQueueSize", 10)
  private val blocksForPushing = new ArrayBlockingQueue[Block](blockQueueSize)

 

 

 

5. BlockGenerator#updateCurrentBuffer

updateCurrentBuffer由BlockIntervalTimer线程执行

 

  /** Change the buffer to which single records are added to. */
  private def updateCurrentBuffer(time: Long): Unit = synchronized {
    try {
      val newBlockBuffer = currentBuffer 
      currentBuffer = new ArrayBuffer[Any] //这两句对currentBuffere这样的操作，是否有线程安全问题？没有，因为currentBuffer已经标注为@volatile类型的变量
      if (newBlockBuffer.size > 0) {
        val blockId = StreamBlockId(receiverId, time - blockInterval) //构造StreamBlockId
        val newBlock = new Block(blockId, newBlockBuffer) //创建出一个Block
        listener.onGenerateBlock(blockId) //通知谁？空实现，listener是作为BlockGenerator的构造函数传入的，这是一个所有通知时间的空实现
        blocksForPushing.put(newBlock)  //添加到阻塞队列中，等待BlockPushingThread读取
        logDebug("Last element in " + blockId + " is " + newBlockBuffer.last)
      }
    } catch {
      case ie: InterruptedException =>
        logInfo("Block updating timer thread was interrupted")
      case e: Exception =>
        reportError("Error in block updating thread", e)
    }
  }

 

 

6. BlockGenerator#keepPushingBlocks

keepPushingBlocks由BlockPushingThread执行

 

 

  /** Keep pushing blocks to the BlockManager. */
  private def keepPushingBlocks() {
    logInfo("Started block pushing thread")
    try {
      while(!stopped) {
        //poll是阻塞队列的非阻塞方法，但是如果队列中没有元素，则等待100ms，poll是取一个元素操作
        Option(blocksForPushing.poll(100, TimeUnit.MILLISECONDS)) match {
          case Some(block) => pushBlock(block)
          case None =>
        }
      }
      // Push out the blocks that are still left
      logInfo("Pushing out the last " + blocksForPushing.size() + " blocks")
      while (!blocksForPushing.isEmpty) {
        logDebug("Getting block ")
        val block = blocksForPushing.take()
        pushBlock(block)
        logInfo("Blocks left to push " + blocksForPushing.size())
      }
      logInfo("Stopped block pushing thread")
    } catch {
      case ie: InterruptedException =>
        logInfo("Block pushing thread was interrupted")
      case e: Exception =>
        reportError("Error in block pushing thread", e)
    }
  }

 

 

7. BlockGenerator#pushBlock

这个方法是针对一个Block进行push，而不是一次从队列中把所有的Block取出来，一次进行push。

 

 

  private def pushBlock(block: Block) {
    listener.onPushBlock(block.id, block.buffer)
    logInfo("Pushed block " + block.id)
  }

 

 

8. BlockGeneratorListener#onPushBlock

pushBlock是通过Observer模式，通知listener，这个liestener是BlockGenerator的构造函数传入的(其实是作为内部类，在构造时创建的实例）

 

 

  /** Divides received data records into data blocks for pushing in BlockManager. */
  private val blockGenerator = new BlockGenerator(new BlockGeneratorListener {
    def onAddData(data: Any, metadata: Any): Unit = { }

    def onGenerateBlock(blockId: StreamBlockId): Unit = { }

    def onError(message: String, throwable: Throwable) {
      reportError(message, throwable)
    }

    def onPushBlock(blockId: StreamBlockId, arrayBuffer: ArrayBuffer[_]) {
      pushArrayBuffer(arrayBuffer, None, Some(blockId))
    }
  }, streamId, env.conf)

 

 

9. BlockGenerator#pushArrayBuffer

 

  /** Store an ArrayBuffer of received data as a data block into Spark's memory. */
  def pushArrayBuffer(
      arrayBuffer: ArrayBuffer[_],
      metadataOption: Option[Any],
      blockIdOption: Option[StreamBlockId]
    ) {
    pushAndReportBlock(ArrayBufferBlock(arrayBuffer), metadataOption, blockIdOption)
  }

 

 

10. BlockGenerator#pushAndReportBlock

 

 

  /** Store block and report it to driver */
  def pushAndReportBlock(
      receivedBlock: ReceivedBlock,
      metadataOption: Option[Any],
      blockIdOption: Option[StreamBlockId]
    ) {
    val blockId = blockIdOption.getOrElse(nextBlockId)
    val numRecords = receivedBlock match {
      case ArrayBufferBlock(arrayBuffer) => arrayBuffer.size
      case _ => -1
    }

    val time = System.currentTimeMillis
    val blockStoreResult = receivedBlockHandler.storeBlock(blockId, receivedBlock)
    logDebug(s"Pushed block $blockId in ${(System.currentTimeMillis - time)} ms")

    val blockInfo = ReceivedBlockInfo(streamId, numRecords, blockStoreResult)
    val future = trackerActor.ask(AddBlock(blockInfo))(askTimeout)
    Await.result(future, askTimeout)
    logDebug(s"Reported block $blockId")
  }

 

 

pushAndReportBlock做了两件事，一是Store Block，而是想Tracker汇报有Block加入

 

10.1 receivedBlockHandler.storeBlock（BlockManagerBasedBlockHandler#storeBlock）

 

 

  def storeBlock(blockId: StreamBlockId, block: ReceivedBlock): ReceivedBlockStoreResult = {
    val putResult: Seq[(BlockId, BlockStatus)] = block match {
      case ArrayBufferBlock(arrayBuffer) =>
        blockManager.putIterator(blockId, arrayBuffer.iterator, storageLevel, tellMaster = true)
      case IteratorBlock(iterator) =>
        blockManager.putIterator(blockId, iterator, storageLevel, tellMaster = true)
      case ByteBufferBlock(byteBuffer) =>
        blockManager.putBytes(blockId, byteBuffer, storageLevel, tellMaster = true)
      case o =>
        throw new SparkException(
          s"Could not store $blockId to block manager, unexpected block type ${o.getClass.getName}")
    }
    if (!putResult.map { _._1 }.contains(blockId)) {
      throw new SparkException(
        s"Could not store $blockId to block manager with storage level $storageLevel")
    }
    BlockManagerBasedStoreResult(blockId)
  }

 

 

其中，blockManager是BlockManager类型的变量，定义于org.apache.spark.storage包中，实现向BlockManager写入数据，具体调用putIterator，putBytes，这是Spark存储子系统的内容，此处不赘述，重要的是，在此处写入进了BlockManager

 

10.2 ReceiverTracker#AddBlock

通过下面两个语句，将写入到BlockManager的信息汇报给TrackActor，这是一个进程间的同步调用(ask语法）

    val blockInfo = ReceivedBlockInfo(streamId, numRecords, blockStoreResult)
    val future = trackerActor.ask(AddBlock(blockInfo))(askTimeout)
    Await.result(future, askTimeout)

 

trackerActor对应的实体是ReceiverTracker，AddBlock消息将触发ReceiverTracker.addBlock,进而调用ReceivedBlockTracker.addBlock

 

 

  /** Add new blocks for the given stream */
  private def addBlock(receivedBlockInfo: ReceivedBlockInfo): Boolean = {
    receivedBlockTracker.addBlock(receivedBlockInfo)
  }

 

 

11. ReceivedBlockTracker.addBlock

 

 

  /** Add received block. This event will get written to the write ahead log (if enabled). */
  def addBlock(receivedBlockInfo: ReceivedBlockInfo): Boolean = synchronized {
    try {
      writeToLog(BlockAdditionEvent(receivedBlockInfo))//写WAL
      getReceivedBlockQueue(receivedBlockInfo.streamId) += receivedBlockInfo //getReceivedBlockQueue从Map<streamId,streamReceivedBlockQueue>中获取相应的streamReceivedBlockQueue
      logDebug(s"Stream ${receivedBlockInfo.streamId} received " +
        s"block ${receivedBlockInfo.blockStoreResult.blockId}")
      true
    } catch {
      case e: Exception =>
        logError(s"Error adding block $receivedBlockInfo", e)
        false
    }
  }