DAG的生成
概述
spark作为一套高效的分布式运算框架，但是想要更深入的学习它，就要通过分析spark的源码，不但可以更好的帮助理解spark的工作过程，还可以提高对集群的排错能力，本文主要关注的是Spark的Stage任务的执行流程的流程。
DAG(Directed Acyclic Graph)叫做有向无环图，原始的RDD通过一系列的转换就就形成了DAG，根据RDD之间的依赖关系的不同将DAG划分成不同的Stage，对于窄依赖，partition的转换处理在Stage中完成计算。对于宽依赖，由于有Shuffle的存在，只能在parent RDD处理完成后，才能开始接下来的计算，因此宽依赖是划分Stage的依据。
窄依赖 指的是每一个父RDD的Partition最多被子RDD的一个Partition使用
宽依赖 指的是多个子RDD的Partition会依赖同一个父RDD的Partition
DAGScheduler调度队列
当我们看完Executor的创建与启动流程后，我们继续在SparkContext的构造方法中继续查看
class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationClient {
。。。。。。
private[spark] def createSparkEnv(
conf: SparkConf,
isLocal: Boolean,
listenerBus: LiveListenerBus): SparkEnv = { //通过SparkEnv来创建createDriverEnv
SparkEnv.createDriverEnv(conf, isLocal, listenerBus)
} //在这里调用了createSparkEnv，返回一个SparkEnv对象，这个对象里面有很多重要属性，最重要的ActorSystem
private[spark] val env = createSparkEnv(conf, isLocal, listenerBus) SparkEnv.set(env) //创建taskScheduler
// Create and start the scheduler
private[spark] var (schedulerBackend, taskScheduler) = SparkContext.createTaskScheduler(this, master) //创建DAGScheduler
dagScheduler = new DAGScheduler(this) //启动TaksScheduler
taskScheduler.start()
。。。。。
}在构造方法中还创建了一个DAGScheduler对象，这个类的任务就是用来划分Stage任务的，构造方法中初始化了 private[scheduler] val eventProcessLoop = new DAGSchedulerEventProcessLoop(this)
DAGSchedulerEventProcessLoop是一个事件总线对象，用来负责任务的分发，在构造方法eventProcessLoop.start()被调用，该方法是父类EventLoop的start
def start(): Unit = { if (stopped.get) { throw new IllegalStateException(name + " has already been stopped")
} // Call onStart before starting the event thread to make sure it happens before onReceive
onStart()
eventThread.start()
}调用了eventThread的start方法，开启了一个线程
private val eventThread = new Thread(name) {
setDaemon(true) override def run(): Unit = { try { while (!stopped.get) { val event = eventQueue.take() try {
onReceive(event)
} catch { case NonFatal(e) => { try {
onError(e)
} catch { case NonFatal(e) => logError("Unexpected error in " + name, e)
}
}
}
}
} catch { case ie: InterruptedException => // exit even if eventQueue is not empty
case NonFatal(e) => logError("Unexpected error in " + name, e)
}
}
}run方法中不断的从LinkedBlockingDeque阻塞队列中取消息，然后调用onReceive(event)方法，该方法是由子类DAGSchedulerEventProcessLoop实现的
override def onReceive(event: DAGSchedulerEvent): Unit = event match { case JobSubmitted(jobId, rdd, func, partitions, allowLocal, callSite, listener, properties) => //调用dagScheduler来出来提交任务
dagScheduler.handleJobSubmitted(jobId, rdd, func, partitions, allowLocal, callSite,
listener, properties) case StageCancelled(stageId) =>
dagScheduler.handleStageCancellation(stageId) case JobCancelled(jobId) =>
dagScheduler.handleJobCancellation(jobId) case JobGroupCancelled(groupId) =>
dagScheduler.handleJobGroupCancelled(groupId) case AllJobsCancelled =>
dagScheduler.doCancelAllJobs() case ExecutorAdded(execId, host) =>
dagScheduler.handleExecutorAdded(execId, host) case ExecutorLost(execId) =>
dagScheduler.handleExecutorLost(execId, fetchFailed = false) case BeginEvent(task, taskInfo) =>
dagScheduler.handleBeginEvent(task, taskInfo) case GettingResultEvent(taskInfo) =>
dagScheduler.handleGetTaskResult(taskInfo) case completion @ CompletionEvent(task, reason, _, _, taskInfo, taskMetrics) =>
dagScheduler.handleTaskCompletion(completion) case TaskSetFailed(taskSet, reason) =>
dagScheduler.handleTaskSetFailed(taskSet, reason) case ResubmitFailedStages =>
dagScheduler.resubmitFailedStages()
}onReceive中会匹配到传入的任务类型，执行相应的逻辑。到此DAGScheduler的调度队列会一直挂起，不断轮询队列中的任务。
DAG提交Task任务流程
当RDD经过一系列的转换Transformation方法后，最终要执行Action动作方法，这里比如WordCount程序中最后调用collect()方法时会将数据提交到Master上运行，任务真正的被执行，这里的方法执行过程如下
/**
* Return an array that contains all of the elements in this RDD.
*/
def collect(): Array[T] = { val results = sc.runJob(this, (iter: Iterator[T]) => iter.toArray) Array.concat(results: _*)
}sc 是SparkContext对象，这里调用 一个runJob该方法调用多次重载的方法后,该方法最终会调用 dagScheduler.runJob
def runJob[T, U: ClassTag](
rdd: RDD[T],
func: (TaskContext, Iterator[T]) => U,
partitions: Seq[Int],
allowLocal: Boolean,
resultHandler: (Int, U) => Unit) { if (stopped) { throw new IllegalStateException("SparkContext has been shutdown")
} val callSite = getCallSite val cleanedFunc = clean(func)
logInfo("Starting job: " + callSite.shortForm) if (conf.getBoolean("spark.logLineage", false)) {
logInfo("RDD's recursive dependencies:\n" + rdd.toDebugString)
} //dagScheduler出现了，可以切分stage
dagScheduler.runJob(rdd, cleanedFunc, partitions, callSite, allowLocal,
resultHandler, localProperties.get)
progressBar.foreach(_.finishAll())
rdd.doCheckpoint()
}dagScheduler的runJob是我们比较关心的
def runJob[T, U: ClassTag](
。。。。。 val waiter = submitJob(rdd, func, partitions, callSite, allowLocal, resultHandler, properties)
waiter.awaitResult() match { case JobSucceeded => {
logInfo("Job %d finished: %s, took %f s".format
(waiter.jobId, callSite.shortForm, (System.nanoTime - start) / 1e9))
} case JobFailed(exception: Exception) =>
logInfo("Job %d failed: %s, took %f s".format
(waiter.jobId, callSite.shortForm, (System.nanoTime - start) / 1e9)) throw exception
}
}这里面的我们主要看的是submitJob(rdd, func, partitions, callSite, allowLocal, resultHandler, properties)提交任务
def submitJob[T, U](
rdd: RDD[T],
func: (TaskContext, Iterator[T]) => U,
partitions: Seq[Int],
callSite: CallSite,
allowLocal: Boolean,
resultHandler: (Int, U) => Unit,
properties: Properties): JobWaiter[U] = {
。。。。。。 //把job加入到任务队列里面
eventProcessLoop.post(JobSubmitted(
jobId, rdd, func2, partitions.toArray, allowLocal, callSite, waiter, properties))
waiter
}这里比较关键的地方是eventProcessLoop.post往任务队列中加入一个JobSubmitted类型的任务，eventProcessLoop是在构造方法中就初始化好的事件总线对象，内部有一个线程不断的轮询队列里的任务
轮询到任务后调用onReceive方法匹配任务类型，在这里我们提交的任务是JobSubmitted类型
case JobSubmitted(jobId, rdd, func, partitions, allowLocal, callSite, listener, properties) => //调用dagScheduler来出来提交任务 dagScheduler.handleJobSubmitted(jobId, rdd, func, partitions, allowLocal, callSite, listener, properties)
调用了handleJobSubmitted方法，接下来查看该方法
private[scheduler] def handleJobSubmitted(jobId: Int,
finalRDD: RDD[_],
func: (TaskContext, Iterator[_]) => _,
partitions: Array[Int],
allowLocal: Boolean,
callSite: CallSite,
listener: JobListener,
properties: Properties) { var finalStage: Stage = null
try { // New stage creation may throw an exception if, for example, jobs are run on a
// HadoopRDD whose underlying HDFS files have been deleted.
//最终的stage
finalStage = newStage(finalRDD, partitions.size, None, jobId, callSite)
} catch { case e: Exception =>
logWarning("Creating new stage failed due to exception - job: " + jobId, e)
listener.jobFailed(e) return
}
。。。。
submitStage(finalStage)
}上面的代码中，调用了newStage进行任务的划分，该方法是划分任务的核心方法，划分任务的根据最后一个依赖关系作为开始，通过递归，将每个宽依赖做为切分Stage的依据，切分Stage的过程是流程中的一环，但在这里不详细阐述，当任务切分完毕后，代码继续执行来到submitStage(finalStage)这里开始进行任务提交
这里以递归的方式进行任务的提交
//递归的方式提交stage
private def submitStage(stage: Stage) { val jobId = activeJobForStage(stage) if (jobId.isDefined) {
logDebug("submitStage(" + stage + ")") if (!waitingStages(stage) && !runningStages(stage) && !failedStages(stage)) { val missing = getMissingParentStages(stage).sortBy(_.id)
logDebug("missing: " + missing) if (missing == Nil) {
logInfo("Submitting " + stage + " (" + stage.rdd + "), which has no missing parents") //提交任务
submitMissingTasks(stage, jobId.get)
} else { for (parent <- missing) {
submitStage(parent)
}
waitingStages += stage
}
}
} else {
abortStage(stage, "No active job for stage " + stage.id)
}
}调用submitMissingTasks(stage, jobId.get)提交任务，将每一个Stage和jobId传入
private def submitMissingTasks(stage: Stage, jobId: Int) {
。。。。。 if (tasks.size > 0) {
logInfo("Submitting " + tasks.size + " missing tasks from " + stage + " (" + stage.rdd + ")")
stage.pendingTasks ++= tasks
logDebug("New pending tasks: " + stage.pendingTasks) //taskScheduler提交task
taskScheduler.submitTasks( new TaskSet(tasks.toArray, stage.id, stage.newAttemptId(), stage.jobId, properties))
stage.latestInfo.submissionTime = Some(clock.getTimeMillis())
} else { // Because we posted SparkListenerStageSubmitted earlier, we should mark
// the stage as completed here in case there are no tasks to run
markStageAsFinished(stage, None)
logDebug("Stage " + stage + " is actually done; %b %d %d".format(
stage.isAvailable, stage.numAvailableOutputs, stage.numPartitions))
}
}这里的代码我们需要关注的是 taskScheduler.submitTasks( new TaskSet(tasks.toArray, stage.id, stage.newAttemptId(), stage.jobId, properties))
创建了一个TaskSet对象，将所有任务的信息封装，包括task任务列表，stageId,任务id,分区数参数等
Task任务调度
override def submitTasks(taskSet: TaskSet) { val tasks = taskSet.tasks
logInfo("Adding task set " + taskSet.id + " with " + tasks.length + " tasks") this.synchronized { //创建TaskSetManager保存了taskSet任务列表
val manager = createTaskSetManager(taskSet, maxTaskFailures)
activeTaskSets(taskSet.id) = manager //将任务加入调度池
schedulableBuilder.addTaskSetManager(manager, manager.taskSet.properties) if (!isLocal && !hasReceivedTask) {
starvationTimer.scheduleAtFixedRate(new TimerTask() { override def run() { if (!hasLaunchedTask) {
logWarning("Initial job has not accepted any resources; " + "check your cluster UI to ensure that workers are registered " + "and have sufficient resources")
} else { this.cancel()
}
}
}, STARVATION_TIMEOUT, STARVATION_TIMEOUT)
}
hasReceivedTask = true
} //接受任务
backend.reviveOffers()
}该方法比较重要，主要将任务加入调度池，最后调用了backend.reviveOffers()这里的backend是CoarseGrainedSchedulerBackend一个Executor任务调度对象
override def reviveOffers() { //自己给自己发消息
driverActor ! ReviveOffers
}这里用了内部的DriverActor对象发送了一个内部消息给自己，接下来查看receiver方法接受的消息
case ReviveOffers => makeOffers()
收到消息后调用了makeOffers()方法
def makeOffers() {
launchTasks(scheduler.resourceOffers(executorDataMap.map { case (id, executorData) => new WorkerOffer(id, executorData.executorHost, executorData.freeCores)
}.toSeq))
}makeOffers方法中，将Executor的信息集合与调度池中的Tasks封装成WokerOffers列表传给了launchTasks
def launchTasks(tasks: Seq[Seq[TaskDescription]]) { for (task <- tasks.flatten) {
。。。。。。 //把task序列化
val serializedTask = ser.serialize(task)
。。。。。 val executorData = executorDataMap(task.executorId)
executorData.freeCores -= scheduler.CPUS_PER_TASK
//把序列化好的task发送给Executor
executorData.executorActor ! LaunchTask(new SerializableBuffer(serializedTask))
}
}
}launchTasks方法将遍历Tasks集合,每个Task任务序列化，发送启动Task执行消息的给Executor
Executor的onReceive方法
//DriverActor发送给Executor的启动Task的消息
case LaunchTask(data) => if (executor == null) {
logError("Received LaunchTask command but executor was null") System.exit(1)
} else { val ser = env.closureSerializer.newInstance() //把Task反序列化
val taskDesc = ser.deserialize[TaskDescription](data.value)
logInfo("Got assigned task " + taskDesc.taskId) //启动task
executor.launchTask(this, taskId = taskDesc.taskId, attemptNumber = taskDesc.attemptNumber,
taskDesc.name, taskDesc.serializedTask)
}Executor收到DriverActor发送的启动Task的消息，这里才开始真正执行任务了，将收到的Task序列化信息反序列化,调用Executor的launchTask方法执行任务
def launchTask(
context: ExecutorBackend,
taskId: Long,
attemptNumber: Int,
taskName: String,
serializedTask: ByteBuffer) { //把task的描述信息放到了一份TaskRunner
val tr = new TaskRunner(context, taskId = taskId, attemptNumber = attemptNumber, taskName,
serializedTask)
runningTasks.put(taskId, tr) //然后把TaskRunner丢到线程池里面
threadPool.execute(tr)
}launchTask内将Task提交到线程池去运行，TaskRunner是Runnable对象，里面的run方法执行了我们app生成的每一个RDD的链上的逻辑。 到此，RDD的整个作业方式就结束了。
作者：那年的坏人
链接：https://www.jianshu.com/p/a95454e35022