概述
Spark Application只有遇到action操作时才会真正的提交任务并进行计算，DAGScheduler 会根据各个RDD之间的依赖关系形成一个DAG，并根据ShuffleDependency来进行stage的划分，stage包含多个tasks，个数由该stage的finalRDD决定，stage里面的task完全相同，DAGScheduler 完成stage的划分后基于每个Stage生成TaskSet，并提交给TaskScheduler，TaskScheduler负责具体的task的调度，在Worker节点上启动task。
Job的提交
以count为例，直接看源码都有哪些步骤：
def count(): Long = sc.runJob(this, Utils.getIteratorSize _).sum DAGScheduler#runJob DAGScheduler#runJob DAGScheduler#runJob DAGScheduler#dagScheduler.runJob DAGScheduler#submitJob eventProcessLoop.post(JobSubmitted(**))
eventProcessLoop是一个DAGSchedulerEventProcessLoop(this)对象，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。
eventProcessLoop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 { 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)
......
}当提交的是JobSubmitted，便会通过 dagScheduler.handleJobSubmitted处理此事件。
Stage的划分
在handleJobSubmitted方法中第一件事情就是通过finalRDD向前追溯对Stage的划分。
private[scheduler] def handleJobSubmitted(jobId: Int, finalRDD: RDD[_], func: (TaskContext, Iterator[_]) => _, partitions: Array[Int], callSite: CallSite, listener: JobListener, properties: Properties) { var finalStage: ResultStage = null
try {
//Stage划分过程是从最后一个Stage开始往前执行的，最后一个Stage的类型是ResultStage
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
} //为此job生成一个ActiveJob对象
val job = new ActiveJob(jobId, finalStage, callSite, listener, properties)
clearCacheLocs()
logInfo("Got job %s (%s) with %d output partitions".format(
job.jobId, callSite.shortForm, partitions.length))
logInfo("Final stage: " + finalStage + " (" + finalStage.name + ")")
logInfo("Parents of final stage: " + finalStage.parents)
logInfo("Missing parents: " + getMissingParentStages(finalStage))
val jobSubmissionTime = clock.getTimeMillis()
jobIdToActiveJob(jobId) = job //记录该job处于active状态
activeJobs += job
finalStage.setActiveJob(job)
val stageIds = jobIdToStageIds(jobId).toArray
val stageInfos = stageIds.flatMap(id => stageIdToStage.get(id).map(_.latestInfo))
listenerBus.post( //向LiveListenerBus发送Job提交事件
SparkListenerJobStart(job.jobId, jobSubmissionTime, stageInfos, properties))
submitStage(finalStage) //提交Stage
submitWaitingStages()
}跟进newResultStage方法：
private def newResultStage(
rdd: RDD[_],
func: (TaskContext, Iterator[_]) => _,
partitions: Array[Int],
jobId: Int,
callSite: CallSite): ResultStage = {
val (parentStages: List[Stage], id: Int) = getParentStagesAndId(rdd, jobId) //获取stage的parentstage
val stage = new ResultStage(id, rdd, func, partitions, parentStages, jobId, callSite)
stageIdToStage(id) = stage //将Stage和stage_id关联
updateJobIdStageIdMaps(jobId, stage) //跟新job所包含的stage
stage
}直接实例化一个ResultStage，但需要parentStages作为参数，我们看看getParentStagesAndId做了什么：
private def getParentStagesAndId(rdd: RDD[_], firstJobId: Int): (List[Stage], Int) = {
val parentStages = getParentStages(rdd, firstJobId)
val id = nextStageId.getAndIncrement()
(parentStages, id)
}获取parentStages，并返回一个与stage关联的唯一id，由于是递归的向前生成stage，所以最先生成的stage是最前面的stage，越往前的stageId就越小，即父Stage的id最小。继续跟进getParentStages：
private def getParentStages(rdd: RDD[_], firstJobId: Int): List[Stage] = {
val parents = new HashSet[Stage] // 当前Stage的所有parent Stage
val visited = new HashSet[RDD[_]] // 已经访问过的RDD
// We are manually maintaining a stack here to prevent StackOverflowError
// caused by recursively visiting
val waitingForVisit = new Stack[RDD[_]] //等待访问的RDD
def visit(r: RDD[_]) { if (!visited(r)) { //若未访问过
visited += r //标记已被访问
// Kind of ugly: need to register RDDs with the cache here since
// we can't do it in its constructor because # of partitions is unknown
for (dep <- r.dependencies) { //遍历其所有依赖
dep match { case shufDep: ShuffleDependency[_, _, _] => //若为宽依赖，则生成新的Stage，shuffleMapstage
parents += getShuffleMapStage(shufDep, firstJobId) case _ => //若为窄依赖（归为当前Stage），压入栈，继续向前循环，直到遇到宽依赖或者无依赖
waitingForVisit.push(dep.rdd)
}
}
}
}
waitingForVisit.push(rdd) //将当前rdd压入栈
while (waitingForVisit.nonEmpty) { //等待访问的rdd不为空时继续访问
visit(waitingForVisit.pop())
}
parents.toList
}通过给定的RDD返回其依赖的Stage集合。通过RDD每一个依赖进行遍历，遇到窄依赖就继续往前遍历，遇到ShuffleDependency便通过getShuffleMapStage返回一个ShuffleMapStage对象添加到父Stage列表中。可见，这里的parentStage是Stage直接依赖的父stages（parentStage也有自己的parentStage），而不是整个DAG的所有stages。继续跟进getShuffleMapStage的实现：
private def getShuffleMapStage(
shuffleDep: ShuffleDependency[_, _, _],
firstJobId: Int): ShuffleMapStage = {
shuffleToMapStage.get(shuffleDep.shuffleId) match { case Some(stage) => stage //若已经在shuffleToMapStage存在直接返回Stage
case None => //不存在需要生成新的Stage
//为当前shuffle的父shuffle都生成一个ShuffleMapStage
getAncestorShuffleDependencies(shuffleDep.rdd).foreach { dep => if (!shuffleToMapStage.contains(dep.shuffleId)) {
shuffleToMapStage(dep.shuffleId) = newOrUsedShuffleStage(dep, firstJobId) //跟新shuffleToMapStage映射
}
} // 为当前shuffle生成新的Stage
val stage = newOrUsedShuffleStage(shuffleDep, firstJobId)
shuffleToMapStage(shuffleDep.shuffleId) = stage
stage
}
}先从shuffleToMapStage根据shuffleid获取Stage，若未获取到再去计算，第一次都肯定为None，我们先看getAncestorShuffleDependencies干了什么：
private def getAncestorShuffleDependencies(rdd: RDD[_]): Stack[ShuffleDependency[_, _, _]] = {
val parents = new Stack[ShuffleDependency[_, _, _]] // 当前shuffleDependency所有的祖先ShuffleDependency（不是直接ShuffleDependency）
val visited = new HashSet[RDD[_]] // 已经被访问过的RDD
// 等待被访问的RDD
val waitingForVisit = new Stack[RDD[_]] def visit(r: RDD[_]) { if (!visited(r)) { //未被访问过
visited += r //标记已被访问
for (dep <- r.dependencies) { //遍历直接依赖
dep match { case shufDep: ShuffleDependency[_, _, _] =>
if (!shuffleToMapStage.contains(shufDep.shuffleId)) { // 若为shuffleDependency并且还没有映射，则添加到parents
parents.push(shufDep)
} case _ =>
}
waitingForVisit.push(dep.rdd) //即使是shuffleDependency的rdd也要继续遍历
}
}
}
waitingForVisit.push(rdd) while (waitingForVisit.nonEmpty) {
visit(waitingForVisit.pop())
}
parents
}貌似和getParentStages方法很像，区别是这里获取的所有祖先ShuffleDependency，而不是直接父ShuffleDependency。
为当前shuffle的父shuffle都生成一个ShuffleMapStage后再通过newOrUsedShuffleStage获取当前依赖的shuffleStage，再和shuffleid关联起来，看newOrUsedShuffleStage的实现：
private def newOrUsedShuffleStage(
shuffleDep: ShuffleDependency[_, _, _],
firstJobId: Int): ShuffleMapStage = {
val rdd = shuffleDep.rdd //依赖对应的rdd
val numTasks = rdd.partitions.length //分区个数
val stage = newShuffleMapStage(rdd, numTasks, shuffleDep, firstJobId, rdd.creationSite) //返回当前rdd的shufflestage
if (mapOutputTracker.containsShuffle(shuffleDep.shuffleId)) { //如果当前shuffle已经在MapOutputTracker中注册过，也就是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 => // 更新Shuffle的Shuffle Write路径
if (locs(i) ne null) { // locs(i) will be null if missing
stage.addOutputLoc(i, locs(i))
}
}
} else { //还没有被注册计算过
// 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) //注册
}
stage
}继续看newShuffleMapStage：
private def newShuffleMapStage(
rdd: RDD[_],
numTasks: Int,
shuffleDep: ShuffleDependency[_, _, _],
firstJobId: Int,
callSite: CallSite): ShuffleMapStage = {
val (parentStages: List[Stage], id: Int) = getParentStagesAndId(rdd, firstJobId) //获取parentstages即stageid
val stage: ShuffleMapStage = new ShuffleMapStage(id, rdd, numTasks, parentStages,
firstJobId, callSite, shuffleDep) //实例化一个shuffleStage对象
stageIdToStage(id) = stage //Stage和id关联
updateJobIdStageIdMaps(firstJobId, stage) //跟新job所有的Stage
stage
}怎么和newResultStage极其的相似？是的没错，这里会生成ShuffleStage，getParentStagesAndId里面的实现就是一个递归调用。
由finalRDD往前追溯递归生成Stage，最前面的ShuffleStage先生成，最终生成ResultStage，至此，DAGScheduler对Stage的划分已经完成。
作者：BIGUFO
链接：https://www.jianshu.com/p/a1ec03185b82