背景介紹
當正在悠閑敲著代碼的時候，業務方兄弟反饋接收到大量線上運行的spark streaming任務的告警短信，查看應用的web頁面信息，發現spark應用已經退出了，第一時間拉起線上的應用，再慢慢的定位故障原因。本文代碼基于spark 1.6.1。
問題定位
登陸到線上機器，查看錯誤日志，發現系統一直報Cannot call methods on a stopped SparkContext.，全部日志如下
[ERROR][JobScheduler][2017-03-08+15:56:00.067][org.apache.spark.streaming.scheduler.JobScheduler]Error running job streaming job 1488959760000 ms.0
java.lang.IllegalStateException: Cannot call methods on a stopped SparkContext.
This stopped SparkContext was created at:
org.apache.spark.SparkContext.<init>(SparkContext.scala:82)
org.apache.spark.streaming.StreamingContext$.createNewSparkContext(StreamingContext.scala:874)
org.apache.spark.streaming.StreamingContext.<init>(StreamingContext.scala:81)
com.xxxx.xxxx.MainApp$.createStreamingContext(MainApp.scala:46)
com.xxxx.xxxx.MainApp$$anonfun$15.apply(MainApp.scala:126)
com.xxxx.xxxx.MainApp$$anonfun$15.apply(MainApp.scala:126)
scala.Option.getOrElse(Option.scala:120)
org.apache.spark.streaming.StreamingContext$.getOrCreate(StreamingContext.scala:864)
com.xxxx.xxxx.MainApp$.main(MainApp.scala:125)
com.xxxx.xxxx.MainApp.main(MainApp.scala)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
java.lang.reflect.Method.invoke(Method.java:498)
org.apache.spark.deploy.SparkSubmit$.org$apache$spark$deploy$SparkSubmit$$runMain(SparkSubmit.scala:731)
org.apache.spark.deploy.SparkSubmit$.doRunMain$1(SparkSubmit.scala:181)
org.apache.spark.deploy.SparkSubmit$.submit(SparkSubmit.scala:206)
org.apache.spark.deploy.SparkSubmit$.main(SparkSubmit.scala:121)
org.apache.spark.deploy.SparkSubmit.main(SparkSubmit.scala)
看到此處應該很清楚了，是SparkContext已經停止了，接下來我們分析下是什么原因導致了SparkContext的停止，首先找到關閉的日志；分析SparkContext的代碼可知，在關閉結束后會打印一個成功關閉的詳情日志。
logInfo("Successfully stopped SparkContext")
通過grep命令找到相應的日志的位置，如下所示
[INFO][dag-scheduler-event-loop][2017-03-03+22:16:30.841][org.apache.spark.SparkContext]Successfully stopped SparkContext
從日志中可以看出是dag-scheduler-event-loop線程關閉了SparkContext，查看該線程的日志信息，顯示如下
java.lang.IllegalStateException: more than one active taskSet for stage 4571114: 4571114.2,4571114.1
at org.apache.spark.scheduler.TaskSchedulerImpl.submitTasks(TaskSchedulerImpl.scala:173)
at org.apache.spark.scheduler.DAGScheduler.submitMissingTasks(DAGScheduler.scala:1052)
at org.apache.spark.scheduler.DAGScheduler.org$apache$spark$scheduler$DAGScheduler$$submitStage(DAGScheduler.scala:921)
at org.apache.spark.scheduler.DAGScheduler.handleTaskCompletion(DAGScheduler.scala:1214)
at org.apache.spark.scheduler.DAGSchedulerEventProcessLoop.doOnReceive(DAGScheduler.scala:1637)
at org.apache.spark.scheduler.DAGSchedulerEventProcessLoop.onReceive(DAGScheduler.scala:1599)
at org.apache.spark.scheduler.DAGSchedulerEventProcessLoop.onReceive(DAGScheduler.scala:1588)
at org.apache.spark.util.EventLoop$$anon$1.run(EventLoop.scala:48)
上面顯示有一個stage同時啟動了兩個TasksetManager，TaskScheduler.submitTasks的代碼如下：
override def submitTasks(taskSet: TaskSet) {
val tasks = taskSet.tasks
logInfo("Adding task set " + taskSet.id + " with " + tasks.length + " tasks")
this.synchronized {
val manager = createTaskSetManager(taskSet, maxTaskFailures)
val stage = taskSet.stageId
val stageTaskSets =
taskSetsByStageIdAndAttempt.getOrElseUpdate(stage, new HashMap[Int, TaskSetManager])
stageTaskSets(taskSet.stageAttemptId) = manager
val conflictingTaskSet = stageTaskSets.exists { case (_, ts) =>
ts.taskSet != taskSet && !ts.isZombie
}
if (conflictingTaskSet) {
throw new IllegalStateException(s"more than one active taskSet for stage $stage:" +
s" ${stageTaskSets.toSeq.map{_._2.taskSet.id}.mkString(",")}")
}
.........
}
看到這震驚了，怎么會出現兩個呢？繼續看之前的日志，發現stage4571114被resubmit了；
[INFO][dag-scheduler-event-loop][2017-03-03+22:16:29.547][org.apache.spark.scheduler.DAGScheduler]Resubmitting ShuffleMapStage 4571114 (map at MainApp.scala:73) because some of its tasks had failed: 0
[INFO][dag-scheduler-event-loop][2017-03-03+22:16:29.547][org.apache.spark.scheduler.DAGScheduler]Submitting ShuffleMapStage 4571114 (MapPartitionsRDD[3719544] at map at MainApp.scala:73), which has no missing parents
查看stage重新提交的代碼，以下代碼截取自DAGScheduler.handleTaskCompletion方法
case smt: ShuffleMapTask =>
val shuffleStage = stage.asInstanceOf[ShuffleMapStage]
updateAccumulators(event)
val status = event.result.asInstanceOf[MapStatus]
val execId = status.location.executorId
logDebug("ShuffleMapTask finished on " + execId)
if (failedEpoch.contains(execId) && smt.epoch <= failedEpoch(execId)) {
logInfo(s"Ignoring possibly bogus $smt completion from executor $execId")
} else {
shuffleStage.addOutputLoc(smt.partitionId, status)
}
if (runningStages.contains(shuffleStage) && shuffleStage.pendingPartitions.isEmpty) {
markStageAsFinished(shuffleStage)
logInfo("looking for newly runnable stages")
logInfo("running: " + runningStages)
logInfo("waiting: " + waitingStages)
logInfo("failed: " + failedStages)
// We supply true to increment the epoch number here in case this is a
// recomputation of the map outputs. In that case, some nodes may have cached
// locations with holes (from when we detected the error) and will need the
// epoch incremented to refetch them.
// TODO: Only increment the epoch number if this is not the first time
// we registered these map outputs.
mapOutputTracker.registerMapOutputs(
shuffleStage.shuffleDep.shuffleId,
shuffleStage.outputLocInMapOutputTrackerFormat(),
changeEpoch = true)
clearCacheLocs()
if (!shuffleStage.isAvailable) {
// Some tasks had failed; let's resubmit this shuffleStage
// TODO: Lower-level scheduler should also deal with this
logInfo("Resubmitting " + shuffleStage + " (" + shuffleStage.name +
") because some of its tasks had failed: " +
shuffleStage.findMissingPartitions().mkString(", "))
submitStage(shuffleStage)
} else {
// Mark any map-stage jobs waiting on this stage as finished
if (shuffleStage.mapStageJobs.nonEmpty) {
val stats = mapOutputTracker.getStatistics(shuffleStage.shuffleDep)
for (job <- shuffleStage.mapStageJobs) {
markMapStageJobAsFinished(job, stats)
}
}
}
可以看出只有shuffleStage.pendingPartitions為空同時shuffleStage.isAvailable為false的時候才會觸發resubmit，我們來看下這兩個變量是什么時候開始，pendingPartitions表示現在正在處理的partition的數量，當task運行結束后會刪除，
val stage = stageIdToStage(task.stageId)
event.reason match {
case Success =>
listenerBus.post(SparkListenerTaskEnd(stageId, stage.latestInfo.attemptId, taskType,
event.reason, event.taskInfo, event.taskMetrics))
//從正在處理的partition中移除
stage.pendingPartitions -= task.partitionId
isAvaible判斷的是已經告知driver的shuffle數據位置的partition數目是否等于總共的partition數目
def isAvailable: Boolean = _numAvailableOutputs == numPartitions
這個變量也是在ShuffleTask運行結束后進行更新的，不過需要注意的是，只有在Shuffle數據所在的executor還是可用的時候才進行更新，如果運行shuffleTask的executor已經掛了，肯定也無法通過該executor獲取磁盤上的shuffle數據
case smt: ShuffleMapTask =>
val shuffleStage = stage.asInstanceOf[ShuffleMapStage]
updateAccumulators(event)
val status = event.result.asInstanceOf[MapStatus]
val execId = status.location.executorId
logDebug("ShuffleMapTask finished on " + execId)
if (failedEpoch.contains(execId) && smt.epoch <= failedEpoch(execId)) {
logInfo(s"Ignoring possibly bogus $smt completion from executor $execId")
} else {
shuffleStage.addOutputLoc(smt.partitionId, status)
}
唯一的可能造成重新調度的就是該處了，根據關鍵信息查詢下日志信息
[INFO][dag-scheduler-event-loop][2017-03-03+22:16:27.427][org.apache.spark.scheduler.DAGScheduler]Ignoring possibly bogus ShuffleMapTask(4571114, 0) completion from executor 4
但就算此時剛運行完shuffleTask的executor掛掉了，造成了stage的重新調度，也不會導致TasksetManager沖突，因為此時taskset.isZombie狀態肯定變了為true，因為TasksetManager.handleSuccessfulTask方法執行在DAGScheduler.handleTaskCompletion之前。
ts.taskSet != taskSet && !ts.isZombie```
TasksetManager.handleSuccessfulTask
def handleSuccessfulTask(tid: Long, result: DirectTaskResult[_]): Unit = {
val info = taskInfos(tid)
val index = info.index
info.markSuccessful()
removeRunningTask(tid)
// This method is called by "TaskSchedulerImpl.handleSuccessfulTask" which holds the
// "TaskSchedulerImpl" lock until exiting. To avoid the SPARK-7655 issue, we should not
// "deserialize" the value when holding a lock to avoid blocking other threads. So we call
// "result.value()" in "TaskResultGetter.enqueueSuccessfulTask" before reaching here.
// Note: "result.value()" only deserializes the value when it's called at the first time, so
// here "result.value()" just returns the value and won't block other threads.
//最終會提交一個CompletionEvent事件到DAGScheduler的事件隊列中等待處理
sched.dagScheduler.taskEnded(
tasks(index), Success, result.value(), result.accumUpdates, info, result.metrics)
if (!successful(index)) {
tasksSuccessful += 1
logInfo("Finished task %s in stage %s (TID %d) in %d ms on %s (%d/%d)".format(
info.id, taskSet.id, info.taskId, info.duration, info.host, tasksSuccessful, numTasks))
// Mark successful and stop if all the tasks have succeeded.
successful(index) = true
if (tasksSuccessful == numTasks) {
isZombie = true
}
} else {
logInfo("Ignoring task-finished event for " + info.id + " in stage " + taskSet.id +
" because task " + index + " has already completed successfully")
}
failedExecutors.remove(index)
maybeFinishTaskSet()
}
可能有的同學已經看出問題來了，為了將問題說的更明白，我畫了一個task執行成功的時序圖
結合時序圖和代碼我們可以看出DAGSchduler.handleCompletion執行發生在了TasksetManager.handleSuccessfulTask方法中isZombie變為true之前，handleSuccessfulTask是在`task-result-getter`線程中執行的,導致isZombie還未變為true，DAGSchduler就觸發了stage的重新提交，最終導致TaskManger沖突。
以下日志分別是resubmit提交的時間和handleSuccessfuleTask的結束時間，從側面(由于isZombie變為true并沒有馬上打印時間)也能夠看出resubmit重新提交的時間早于handleSuccessfuleTask。
handleSuccessfuleTask結束時間
[INFO][task-result-getter-2][2017-03-03+22:16:29.999][org.apache.spark.scheduler.TaskSchedulerImpl]Removed TaskSet 4571114.1, whose tasks have all completed, from pool
resubmit stage任務重新提交時間
[INFO][dag-scheduler-event-loop][2017-03-03+22:16:29.549][org.apache.spark.scheduler.TaskSchedulerImpl]Adding task set 4571114.2 with 1 tasks
事件發生的時間軸
###問題修復
該問題修復其實很簡單，只需要修改TasksetManager.handleSuccessfulTask的方法，在isZombie=true后再發送CompletionEvent事件即可，代碼修復如下
def handleSuccessfulTask(tid: Long, result: DirectTaskResult[_]): Unit = {
val info = taskInfos(tid)
val index = info.index
info.markSuccessful()
removeRunningTask(tid)
// This method is called by "TaskSchedulerImpl.handleSuccessfulTask" which holds the
// "TaskSchedulerImpl" lock until exiting. To avoid the SPARK-7655 issue, we should not
// "deserialize" the value when holding a lock to avoid blocking other threads. So we call
// "result.value()" in "TaskResultGetter.enqueueSuccessfulTask" before reaching here.
// Note: "result.value()" only deserializes the value when it's called at the first time, so
// here "result.value()" just returns the value and won't block other threads.
if (!successful(index)) {
tasksSuccessful += 1
logInfo("Finished task %s in stage %s (TID %d) in %d ms on %s (%d/%d)".format(
info.id, taskSet.id, info.taskId, info.duration, info.host, tasksSuccessful, numTasks))
// Mark successful and stop if all the tasks have succeeded.
successful(index) = true
if (tasksSuccessful == numTasks) {
isZombie = true
}
sched.dagScheduler.taskEnded(
tasks(index), Success, result.value(), result.accumUpdates, info, result.metrics)
} else {
logInfo("Ignoring task-finished event for " + info.id + " in stage " + taskSet.id +
" because task " + index + " has already completed successfully")
}
failedExecutors.remove(index)
maybeFinishTaskSet()
sched.dagScheduler.taskEnded(
tasks(index), Success, result.value(), result.accumUpdates, info, result.metrics)
}
改問題已經提交到spark社區中，詳情見[spark patch](https://github.com/apache/spark/pull/17208)。