1、为什么引入Backpressure

默认情况下，Spark Streaming通过Receiver以生产者生产数据的速率接收数据，计算过程中会出现batch processing time > batch interval的情况，其中batch processing time 为实际计算一个批次花费时间， batch interval为Streaming应用设置的批处理间隔。这意味着Spark Streaming的数据接收速率高于Spark从队列中移除数据的速率，也就是数据处理能力低，在设置间隔内不能完全处理当前接收速率接收的数据。如果这种情况持续过长的时间，会造成数据在内存中堆积，导致Receiver所在Executor内存溢出等问题（如果设置StorageLevel包含disk, 则内存存放不下的数据会溢写至disk, 加大延迟）。Spark 1.5以前版本，用户如果要限制Receiver的数据接收速率，可以通过设置静态配制参数“spark.streaming.receiver.maxRate”的值来实现，此举虽然可以通过限制接收速率，来适配当前的处理能力，防止内存溢出，但也会引入其它问题。比如：producer数据生产高于maxRate，当前集群处理能力也高于maxRate，这就会造成资源利用率下降等问题。为了更好的协调数据接收速率与资源处理能力，Spark Streaming 从v1.5开始引入反压机制（back-pressure）,通过动态控制数据接收速率来适配集群数据处理能力。

2、Backpressure

Spark Streaming Backpressure: 根据JobScheduler反馈作业的执行信息来动态调整Receiver数据接收率。通过属性spark.streaming.backpressure.enabled来控制是否启用backpressure机制，默认值false，即不启用。

2.1 Streaming架构如下图所示（详见Streaming数据接收过程文档和Streaming 源码解析）

2.2 BackPressure执行过程如下图所示:

在原架构的基础上加上一个新的组件RateController,这个组件负责监听“OnBatchCompleted”事件，然后从中抽取processingDelay 及schedulingDelay信息. Estimator依据这些信息估算出最大处理速度（rate），最后由基于Receiver的Input Stream将rate通过ReceiverTracker与ReceiverSupervisorImpl转发给BlockGenerator（继承自RateLimiter）.

3、BackPressure 源码解析

3.1 RateController类体系

RatenController 继承自StreamingListener. 用于处理BatchCompleted事件。核心代码为：

**
 * A StreamingListener that receives batch completion updates, and maintains
 * an estimate of the speed at which this stream should ingest messages,
 * given an estimate computation from a `RateEstimator`
 */
private[streaming] abstract class RateController(val streamUID: Int, rateEstimator: RateEstimator)
extends StreamingListener with Serializable {
……
……  /**
   * Compute the new rate limit and publish it asynchronously.
   */
  private def computeAndPublish(time: Long, elems: Long, workDelay: Long, waitDelay: Long): Unit =
    Future[Unit] {
      val newRate =  elems, workDelay, waitDelay)
      newRate.foreach { s =>
        rateLimit.set(s.toLong)
        publish(getLatestRate())
      }
    }
  def getLatestRate(): Long = rateLimit.get()
 
  override def onBatchCompleted(batchCompleted: StreamingListenerBatchCompleted) {
    val elements = batchCompleted.batchInfo.streamIdToInputInfo
    for {
      processingEnd <- batchCompleted.batchInfo.processingEndTime
      workDelay <- batchCompleted.batchInfo.processingDelay
      waitDelay <- batchCompleted.batchInfo.schedulingDelay
      elems <- elements.get(streamUID).map(_.numRecords)
    } computeAndPublish(processingEnd, elems, workDelay, waitDelay)
  }
}  

3.2 RateController的注册

JobScheduler启动时会抽取在DStreamGraph中注册的所有InputDstream中的rateController，并向ListenerBus注册监听. 此部分代码如下：

def start(): Unit = synchronized {
   if (eventLoop != null) return // scheduler has already been started
 
   logDebug("Starting JobScheduler")
   eventLoop = new EventLoop[JobSchedulerEvent]("JobScheduler") {
     override protected def onReceive(event: JobSchedulerEvent): Unit = processEvent(event)
 
     override protected def onError(e: Throwable): Unit = reportError("Error in job scheduler", e)
   }
   eventLoop.start()
 
   // attach rate controllers of input streams to receive batch completion updates
   for {
     inputDStream <- ssc.graph.getInputStreams
     rateController <- inputDStream.rateController
   } ssc.addStreamingListener(rateController)
 
   listenerBus.start()
   receiverTracker = new ReceiverTracker(ssc)
   inputInfoTracker = new InputInfoTracker(ssc)
   receiverTracker.start()
   jobGenerator.start()
   logInfo("Started JobScheduler")
 }

3.3 BackPressure执行过程分析

BackPressure 执行过程分为BatchCompleted事件触发时机和事件处理两个过程

3.3.1 BatchCompleted触发过程

对BatchedCompleted的分析，应该从JobGenerator入手，因为BatchedCompleted是批次处理结束的标志，也就是JobGenerator产生的作业执行完成时触发的，因此进行作业执行分析。
Streaming 应用中JobGenerator每个Batch Interval都会为应用中的每个Output Stream建立一个Job, 该批次中的所有Job组成一个Job Set.使用JobScheduler的submitJobSet进行批量Job提交。此部分代码结构如下所示

/** Generate jobs and perform checkpoint for the given `time`.  */
private def generateJobs(time: Time) {
  // Set the SparkEnv in this thread, so that job generation code can access the environment
  // Example: BlockRDDs are created in this thread, and it needs to access BlockManager
  // Update: This is probably redundant after threadlocal stuff in SparkEnv has been removed.
  SparkEnv.set(ssc.env)
 
  // Checkpoint all RDDs marked for checkpointing to ensure their lineages are
  // truncated periodically. Otherwise, we may run into stack overflows (SPARK-6847).
  ssc.sparkContext.setLocalProperty(RDD.CHECKPOINT_ALL_MARKED_ANCESTORS, "true")
  Try {
    jobScheduler.receiverTracker.allocateBlocksToBatch(time) // allocate received blocks to batch
    graph.generateJobs(time) // generate jobs using allocated block
  } match {
    case Success(jobs) =>
      val streamIdToInputInfos = jobScheduler.inputInfoTracker.getInfo(time)
      jobScheduler.submitJobSet(JobSet(time, jobs, streamIdToInputInfos))
    case Failure(e) =>
      jobScheduler.reportError("Error generating jobs for time " + time, e)
  }
  eventLoop.post(DoCheckpoint(time, clearCheckpointDataLater = false))
}

其中，sumitJobSet会创建固定数量的后台线程（具体由spark.streaming.concurrentJobs指定），去处理Job Set中的Job. 具体实现逻辑为：

def submitJobSet(jobSet: JobSet) {
  if (jobSet.jobs.isEmpty) {
    logInfo("No jobs added for time " + jobSet.time)
  } else {
    listenerBus.post(StreamingListenerBatchSubmitted(jobSet.toBatchInfo))
    jobSets.put(jobSet.time, jobSet)
    jobSet.jobs.foreach(job => jobExecutor.execute(new JobHandler(job)))
    logInfo("Added jobs for time " + jobSet.time)
  }
}

其中JobHandler用于执行Job及处理Job执行结果信息。当Job执行完成时会产生JobCompleted事件. JobHandler的具体逻辑如下面代码所示：

private class JobHandler(job: Job) extends Runnable with Logging {
    import JobScheduler._
 
    def run() {
      try {
        val formattedTime = UIUtils.formatBatchTime(
          job.time.milliseconds, ssc.graph.batchDuration.milliseconds, showYYYYMMSS = false)
        val batchUrl = s"/streaming/batch/?id=${job.time.milliseconds}"
        val batchLinkText = s"[output operation ${job.outputOpId}, batch time ${formattedTime}]"
 
        ssc.sc.setJobDescription(
          s"""Streaming job from <a href="$batchUrl">$batchLinkText</a>""")
        ssc.sc.setLocalProperty(BATCH_TIME_PROPERTY_KEY, job.time.milliseconds.toString)
        ssc.sc.setLocalProperty(OUTPUT_OP_ID_PROPERTY_KEY, job.outputOpId.toString)
        // Checkpoint all RDDs marked for checkpointing to ensure their lineages are
        // truncated periodically. Otherwise, we may run into stack overflows (SPARK-6847).
        ssc.sparkContext.setLocalProperty(RDD.CHECKPOINT_ALL_MARKED_ANCESTORS, "true")
 
        // We need to assign `eventLoop` to a temp variable. Otherwise, because
        // `JobScheduler.stop(false)` may set `eventLoop` to null when this method is running, then
        // it's possible that when `post` is called, `eventLoop` happens to null.
        var _eventLoop = eventLoop
        if (_eventLoop != null) {
          _eventLoop.post(JobStarted(job, clock.getTimeMillis()))
          // Disable checks for existing output directories in jobs launched by the streaming
          // scheduler, since we may need to write output to an existing directory during checkpoint
          // recovery; see SPARK-4835 for more details.
          PairRDDFunctions.disableOutputSpecValidation.withValue(true) {
            job.run()
          }
          _eventLoop = eventLoop
          if (_eventLoop != null) {
            _eventLoop.post(JobCompleted(job, clock.getTimeMillis()))
          }
        } else {
          // JobScheduler has been stopped.
        }
      } finally {
        ssc.sc.setLocalProperty(JobScheduler.BATCH_TIME_PROPERTY_KEY, null)
        ssc.sc.setLocalProperty(JobScheduler.OUTPUT_OP_ID_PROPERTY_KEY, null)
      }
    }
  }
}

当Job执行完成时，向eventLoop发送JobCompleted事件。EventLoop事件处理器接到JobCompleted事件后将调用handleJobCompletion 来处理Job完成事件。handleJobCompletion使用Job执行信息创建StreamingListenerBatchCompleted事件并通过StreamingListenerBus向监听器发送。实现如下：

private def handleJobCompletion(job: Job, completedTime: Long) {
   val jobSet = jobSets.get(job.time)
   jobSet.handleJobCompletion(job)
   job.setEndTime(completedTime)
   listenerBus.post(StreamingListenerOutputOperationCompleted(job.toOutputOperationInfo))
   logInfo("Finished job " + job.id + " from job set of time " + jobSet.time)
   if (jobSet.hasCompleted) {
     jobSets.remove(jobSet.time)
     jobGenerator.onBatchCompletion(jobSet.time)
     logInfo("Total delay: %.3f s for time %s (execution: %.3f s)".format(
       jobSet.totalDelay / 1000.0, jobSet.time.toString,
       jobSet.processingDelay / 1000.0
     ))
     listenerBus.post(StreamingListenerBatchCompleted(jobSet.toBatchInfo))
   }
   job.result match {
     case Failure(e) =>
       reportError("Error running job " + job, e)
     case _ =>
   }
 }

3.3.2、BatchCompleted事件处理过程

StreamingListenerBus将事件转交给具体的StreamingListener，因此BatchCompleted将交由RateController进行处理。RateController接到BatchCompleted事件后将调用onBatchCompleted对事件进行处理。

override def onBatchCompleted(batchCompleted: StreamingListenerBatchCompleted) {
  val elements = batchCompleted.batchInfo.streamIdToInputInfo
 
  for {
    processingEnd <- batchCompleted.batchInfo.processingEndTime
    workDelay <- batchCompleted.batchInfo.processingDelay
    waitDelay <- batchCompleted.batchInfo.schedulingDelay
    elems <- elements.get(streamUID).map(_.numRecords)
  } computeAndPublish(processingEnd, elems, workDelay, waitDelay)
}

  /**
   * Compute the new rate limit and publish it asynchronously.
   */
  private def computeAndPublish(time: Long, elems: Long, workDelay: Long, waitDelay: Long): Unit =
    Future[Unit] {
      val newRate =  elems, workDelay, waitDelay)
      newRate.foreach { s =>
        rateLimit.set(s.toLong)
        publish(getLatestRate())
      }
    }

其中publish（）由RateController的子类ReceiverRateController来定义。具体逻辑如下（ReceiverInputDStream中定义）：

  /**
   * A RateController that sends the new rate to receivers, via the receiver tracker.
   */
  private[streaming] class ReceiverRateController(id: Int, estimator: RateEstimator)
      extends RateController(id, estimator) {
    override def publish(rate: Long): Unit =
      ssc.scheduler.receiverTracker.sendRateUpdate(id, rate)
  }

publish的功能为新生成的rate 借助ReceiverTracker进行转发。ReceiverTracker将rate包装成UpdateReceiverRateLimit事交ReceiverTrackerEndpoint

/** Update a receiver's maximum ingestion rate */
def sendRateUpdate(streamUID: Int, newRate: Long): Unit = synchronized {
  if (isTrackerStarted) {
    endpoint.send(UpdateReceiverRateLimit(streamUID, newRate))
  }
}

ReceiverTrackerEndpoint接到消息后，其将会从receiverTrackingInfos列表中获取Receiver注册时使用的endpoint(实为ReceiverSupervisorImpl)，再将rate包装成UpdateLimit发送至endpoint.其接到信息后，使用updateRate更新BlockGenerators(RateLimiter子类),来计算出一个固定的令牌间隔。

/** RpcEndpointRef for receiving messages from the ReceiverTracker in the driver */
private val endpoint = env.rpcEnv.setupEndpoint(
  "Receiver-" + streamId + "-" + System.currentTimeMillis(), new ThreadSafeRpcEndpoint {
    override val rpcEnv: RpcEnv = env.rpcEnv
 
    override def receive: PartialFunction[Any, Unit] = {
      case StopReceiver =>
        logInfo("Received stop signal")
        ReceiverSupervisorImpl.this.stop("Stopped by driver", None)
      case CleanupOldBlocks(threshTime) =>
        logDebug("Received delete old batch signal")
        cleanupOldBlocks(threshTime)
      case UpdateRateLimit(eps) =>
        logInfo(s"Received a new rate limit: $eps.")
        registeredBlockGenerators.asScala.foreach { bg =>
          bg.updateRate(eps)
        }
    }
  })

其中RateLimiter的updateRate实现如下：

/**
  * Set the rate limit to `newRate`. The new rate will not exceed the maximum rate configured by
  * {{{spark.streaming.receiver.maxRate}}}, even if `newRate` is higher than that.
  *
  * @param newRate A new rate in events per second. It has no effect if it's 0 or negative.
  */
 private[receiver] def updateRate(newRate: Long): Unit =
   if (newRate > 0) {
     if (maxRateLimit > 0) {
       rateLimiter.setRate(newRate.min(maxRateLimit))
     } else {
       rateLimiter.setRate(newRate)
     }
   }

setRate的实现 如下

public final void setRate(double permitsPerSecond) {
    Preconditions.checkArgument(permitsPerSecond > 0.0
        && !Double.isNaN(permitsPerSecond), "rate must be positive");
    synchronized (mutex) {
      resync(readSafeMicros());
      double stableIntervalMicros = TimeUnit.SECONDS.toMicros(1L) / permitsPerSecond;  //固定间隔
      this.stableIntervalMicros = stableIntervalMicros;
      doSetRate(permitsPerSecond, stableIntervalMicros);
    }
  }

到此，backpressure反压机制调整rate结束。

4．流量控制点

当Receiver开始接收数据时，会通过supervisor.pushSingle()方法将接收的数据存入currentBuffer等待BlockGenerator定时将数据取走，包装成block. 在将数据存放入currentBuffer之时，要获取许可（令牌）。如果获取到许可就可以将数据存入buffer, 否则将被阻塞，进而阻塞Receiver从数据源拉取数据。

/**
 * Push a single data item into the buffer.
 */
def addData(data: Any): Unit = {
  if (state == Active) {
    waitToPush()  //获取令牌
    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")
  }
}

其令牌投放采用令牌桶机制进行， 原理如下图所示:

令牌桶机制： 大小固定的令牌桶可自行以恒定的速率源源不断地产生令牌。如果令牌不被消耗，或者被消耗的速度小于产生的速度，令牌就会不断地增多，直到把桶填满。后面再产生的令牌就会从桶中溢出。最后桶中可以保存的最大令牌数永远不会超过桶的大小。当进行某操作时需要令牌时会从令牌桶中取出相应的令牌数，如果获取到则继续操作，否则阻塞。用完之后不用放回。

Streaming 数据流被Receiver接收后，按行解析后存入iterator中。然后逐个存入Buffer，在存入buffer时会先获取token，如果没有token存在，则阻塞；如果获取到则将数据存入buffer. 然后等价后续生成block操作。