本节的主要内容：
一、ReceivedBlockTracker容错安全性
二、DStreamGraph和JobGenerator容错安全性
从数据层面，ReceivedBlockTracker为整个Spark Streaming应用程序记录元数据信息。
从调度层面，DStreamGraph和JobGenerator是Spark Streaming调度的核心，记录当前调度到哪一进度，和业务有关。
ReceivedBlockTracker在接收到元数据信息后调用addBlock方法，先写入磁盘中，然后在写入内存，
看源码：
private type ReceivedBlockQueue = mutable.Queue[ReceivedBlockInfo]
private val streamIdToUnallocatedBlockQueues = new mutable.HashMap[Int, ReceivedBlockQueue]
private val timeToAllocatedBlocks = new mutable.HashMap[Time, AllocatedBlocks]
private val writeAheadLogOption = createWriteAheadLog()
private var lastAllocatedBatchTime: Time = null
// Recover block information from write ahead logs
if (recoverFromWriteAheadLog) {
recoverPastEvents()
}
/** Add received block. This event will get written to the write ahead log (if enabled). */
def addBlock(receivedBlockInfo: ReceivedBlockInfo): Boolean = {
try {
val writeResult = writeToLog(BlockAdditionEvent(receivedBlockInfo))
if (writeResult) {
synchronized {
getReceivedBlockQueue(receivedBlockInfo.streamId) += receivedBlockInfo
}
logDebug(s"Stream ${receivedBlockInfo.streamId} received " +
s"block ${receivedBlockInfo.blockStoreResult.blockId}")
} else {
logDebug(s"Failed to acknowledge stream ${receivedBlockInfo.streamId} receiving " +
s"block ${receivedBlockInfo.blockStoreResult.blockId} in the Write Ahead Log.")
}
writeResult
} catch {
case NonFatal(e) =>
logError(s"Error adding block $receivedBlockInfo", e)
false
}
}
根据batchTime分配属于当前BatchDuration要处理的数据到timToAllocatedBlocks数据结构，看源码：
/**
* Allocate all unallocated blocks to the given batch.
* This event will get written to the write ahead log (if enabled).
*/
def allocateBlocksToBatch(batchTime: Time): Unit = synchronized {
if (lastAllocatedBatchTime == null || batchTime > lastAllocatedBatchTime) {
val streamIdToBlocks = streamIds.map { streamId =>
(streamId, getReceivedBlockQueue(streamId).dequeueAll(x => true))
}.toMap
val allocatedBlocks = AllocatedBlocks(streamIdToBlocks)
if (writeToLog(BatchAllocationEvent(batchTime, allocatedBlocks))) {
timeToAllocatedBlocks.put(batchTime, allocatedBlocks)
lastAllocatedBatchTime = batchTime //上一个job分配完数据后在接下来分配
} else {
logInfo(s"Possibly processed batch $batchTime need to be processed again in WAL recovery")
}
} else {
// This situation occurs when:
// 1. WAL is ended with BatchAllocationEvent, but without BatchCleanupEvent,
// possibly processed batch job or half-processed batch job need to be processed again,
// so the batchTime will be equal to lastAllocatedBatchTime.
// 2. Slow checkpointing makes recovered batch time older than WAL recovered
// lastAllocatedBatchTime.
// This situation will only occurs in recovery time.
logInfo(s"Possibly processed batch $batchTime need to be processed again in WAL recovery")
}
}
/** Get the blocks allocated to the given batch. */
def getBlocksOfBatch(batchTime: Time): Map[Int, Seq[ReceivedBlockInfo]] = synchronized {
timeToAllocatedBlocks.get(batchTime).map { _.streamIdToAllocatedBlocks }.getOrElse(Map.empty)
}
跟踪Time对象，ReceiverTracker的allocateBlocksToBatch方法中的入参batchTime是被JobGenerator的generateJobs方法调用的，看源码：
/** Allocate all unallocated blocks to the given batch. */
def allocateBlocksToBatch(batchTime: Time): Unit = {
if (receiverInputStreams.nonEmpty) {
receivedBlockTracker.allocateBlocksToBatch(batchTime)
}
}
/** Get the blocks for the given batch and all input streams. */
def getBlocksOfBatch(batchTime: Time): Map[Int, Seq[ReceivedBlockInfo]] = {
receivedBlockTracker.getBlocksOfBatch(batchTime)
}
JobGenerator的generateJobs方法是被定时器发送GenerateJobs消息调用的，看源码：
/** 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)
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))
}
GenerateJobs中的时间参数就是nextTime，而nextTime+=period，这个period就是ssc.graph.batchDuration.milliseconds：
private def triggerActionForNextInterval(): Unit = {
clock.waitTillTime(nextTime)
callback(nextTime)
prevTime = nextTime
nextTime += period
logDebug("Callback for " + name + " called at time " + prevTime)
}
nextTime的初始值是在start方法中传入的startTime赋值的，即RecurringTimer的getStartTime方法的返回值，是当前时间period的(整数倍+1)：
/** Starts the generator for the first time */
private def startFirstTime() {
val startTime = new Time(timer.getStartTime())
graph.start(startTime - graph.batchDuration)
timer.start(startTime.milliseconds)
logInfo("Started JobGenerator at " + startTime)
}
Period这个值是我们调用new StreamingContext来构造StreamingContext时传入的Duration值：
def setBatchDuration(duration: Duration) {
this.synchronized {
require(batchDuration == null,
s"Batch duration already set as $batchDuration. Cannot set it again.")
batchDuration = duration
}
}
private[streaming] val graph: DStreamGraph = {
if (isCheckpointPresent) {
cp_.graph.setContext(this)
cp_.graph.restoreCheckpointData()
cp_.graph
} else {
require(batchDur_ != null, "Batch duration for StreamingContext cannot be null")
val newGraph = new DStreamGraph()
newGraph.setBatchDuration(batchDur_)
newGraph
}
}
ReceivedBlockTracker会清除过期的元数据信息，从HashMap中移除，也是先写入磁盘，然后在写入内存。
元数据的生成，消费和销毁都有WAL，所以失败时就可以从日志中恢复。从源码分析中得出只有设置了checkpoint目录，才进行WAL机制。
总结：
ReceivedBlockTracker是通过WAL方式来进行数据容错的。
DStreamGraph和JobGenerator是通过checkpoint方式来进行数据容错的。
Spark发行版笔记13
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