一、Spark 运行架构
Spark 运行架构如下图：
各个RDD之间存在着依赖关系，这些依赖关系形成有向无环图DAG，DAGScheduler对这些依赖关系形成的DAG，进行Stage划分，划分的规则很简单，从后往前回溯，遇到窄依赖加入本stage，遇见宽依赖进行Stage切分。完成了Stage的划分,DAGScheduler基于每个Stage生成TaskSet,并将TaskSet提交给TaskScheduler。TaskScheduler 负责具体的task调度,在Worker节点上启动task。
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二、源码解析：DAGScheduler中的DAG划分
当RDD触发一个Action操作（如：colllect）后，导致SparkContext.runJob的执行。而在SparkContext的run方法中会调用DAGScheduler的run方法最终调用了DAGScheduler的submit方法：
def submitJob[T, U](
rdd: RDD[T],
func: (TaskContext, Iterator[T]) => U,
partitions: Seq[Int],
callSite: CallSite,
resultHandler: (Int, U) => Unit,
properties: Properties): JobWaiter[U] = {
// Check to make sure we are not launching a task on a partition that does not exist.
val maxPartitions = rdd.partitions.length
partitions.find(p => p >= maxPartitions || p < 0).foreach { p =>
throw new IllegalArgumentException(
"Attempting to access a non-existent partition: " + p + ". " +
"Total number of partitions: " + maxPartitions)
}
val jobId = nextJobId.getAndIncrement()
if (partitions.size == 0) {
// Return immediately if the job is running 0 tasks
return new JobWaiter[U](this, jobId, 0, resultHandler)
}
assert(partitions.size > 0)
val func2 = func.asInstanceOf[(TaskContext, Iterator[_]) => _]
val waiter = new JobWaiter(this, jobId, partitions.size, resultHandler)
//给eventProcessLoop发送JobSubmitted消息
eventProcessLoop.post(JobSubmitted(
jobId, rdd, func2, partitions.toArray, callSite, waiter,
SerializationUtils.clone(properties)))
waiter
}
DAGScheduler的submit方法中，像eventProcessLoop对象发送了JobSubmitted消息。eventProcessLoop是DAGSchedulerEventProcessLoop类的对象
private[scheduler] val eventProcessLoop = new DAGSchedulerEventProcessLoop(this)
DAGSchedulerEventProcessLoop，接收各种消息并进行处理，处理的逻辑在其doOnReceive方法中：
private def doOnReceive(event: DAGSchedulerEvent): Unit = event match {
//Job提交
case JobSubmitted(jobId, rdd, func, partitions, callSite, listener, properties) =>
dagScheduler.handleJobSubmitted(jobId, rdd, func, partitions, callSite, listener, properties)
case MapStageSubmitted(jobId, dependency, callSite, listener, properties) =>
dagScheduler.handleMapStageSubmitted(jobId, dependency, 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 =>
dagScheduler.handleTaskCompletion(completion)
case TaskSetFailed(taskSet, reason, exception) =>
dagScheduler.handleTaskSetFailed(taskSet, reason, exception)
case ResubmitFailedStages =>
dagScheduler.resubmitFailedStages()
}
可以把DAGSchedulerEventProcessLoop理解成DAGScheduler的对外的功能接口。它对外隐藏了自己内部实现的细节。无论是内部还是外部消息，DAGScheduler可以共用同一消息处理代码，逻辑清晰，处理方式统一。
接下来分析DAGScheduler的Stage划分，handleJobSubmitted方法首先创建ResultStage
try {
//创建新stage可能出现异常，比如job运行依赖hdfs文文件被删除
finalStage = newResultStage(finalRDD, func, partitions, jobId, callSite)
} catch {
case e: Exception =>
logWarning("Creating new stage failed due to exception - job: " + jobId, e)
listener.jobFailed(e)
return
}
然后调用submitStage方法，进行stage的划分。
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首先由finalRDD获取它的父RDD依赖，判断依赖类型，如果是窄依赖，则将父RDD压入栈中，如果是宽依赖，则作为父Stage。
看一下源码的具体过程:
private def getMissingParentStages(stage: Stage): List[Stage] = {
val missing = new HashSet[Stage] //存储需要返回的父Stage
val visited = new HashSet[RDD[_]] //存储访问过的RDD
//自己建立栈，以免函数的递归调用导致
val waitingForVisit = new Stack[RDD[_]]
def visit(rdd: RDD[_]) {
if (!visited(rdd)) {
visited += rdd
val rddHasUncachedPartitions = getCacheLocs(rdd).contains(Nil)
if (rddHasUncachedPartitions) {
for (dep <- rdd.dependencies) {
dep match {
case shufDep: ShuffleDependency[_, _, _] =>
val mapStage = getShuffleMapStage(shufDep, stage.firstJobId)
if (!mapStage.isAvailable) {
missing += mapStage //遇到宽依赖，加入父stage
}
case narrowDep: NarrowDependency[_] =>
waitingForVisit.push(narrowDep.rdd) //窄依赖入栈,
}
}
}
}
}
//回溯的起始RDD入栈
waitingForVisit.push(stage.rdd)
while (waitingForVisit.nonEmpty) {
visit(waitingForVisit.pop())
}
missing.toList
}
getMissingParentStages方法是由当前stage，返回他的父stage，父stage的创建由getShuffleMapStage返回，最终会调用newOrUsedShuffleStage方法返回ShuffleMapStage
private def newOrUsedShuffleStage(
shuffleDep: ShuffleDependency[_, _, _],
firstJobId: Int): ShuffleMapStage = {
val rdd = shuffleDep.rdd
val numTasks = rdd.partitions.length
val stage = newShuffleMapStage(rdd, numTasks, shuffleDep, firstJobId, rdd.creationSite)
if (mapOutputTracker.containsShuffle(shuffleDep.shuffleId)) {
//Stage已经被计算过，从MapOutputTracker中获取计算结果
val serLocs = mapOutputTracker.getSerializedMapOutputStatuses(shuffleDep.shuffleId)
val locs = MapOutputTracker.deserializeMapStatuses(serLocs)
(0 until locs.length).foreach { i =>
if (locs(i) ne null) {
// locs(i) will be null if missing
stage.addOutputLoc(i, locs(i))
}
}
} else {
// Kind of ugly: need to register RDDs with the cache and map output tracker here
// since we can't do it in the RDD constructor because # of partitions is unknown
logInfo("Registering RDD " + rdd.id + " (" + rdd.getCreationSite + ")")
mapOutputTracker.registerShuffle(shuffleDep.shuffleId, rdd.partitions.length)
}
stage
}
现在父Stage已经划分好，下面看看你Stage的提交逻辑
/** Submits stage, but first recursively submits any missing parents. */
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.isEmpty) {
logInfo("Submitting " + stage + " (" + stage.rdd + "), which has no missing parents")
//如果没有父stage，则提交当前stage
submitMissingTasks(stage, jobId.get)
} else {
for (parent <- missing) {
//如果有父stage，则递归提交父stage
submitStage(parent)
}
waitingStages += stage
}
}
} else {
abortStage(stage, "No active job for stage " + stage.id, None)
}
}
提交的过程很简单，首先当前stage获取父stage，如果父stage为空，则当前Stage为起始stage，交给submitMissingTasks处理，如果当前stage不为空，则递归调用submitStage进行提交。
到这里，DAGScheduler中的DAG划分与提交就讲完了，下次解析这些stage是如果封装成TaskSet交给TaskScheduler以及TaskSchedule的调度过程。
