本期内容：
1、Spark Streaming元数据清理详解
2、Spark Streaming元数据清理源码解析
一、如何研究Spark Streaming元数据清理
操作DStream的时候会产生元数据，所以要解决RDD的数据清理工作就一定要从DStream入手。因为DStream是RDD的模板，DStream之间有依赖关系。
DStream的操作产生了RDD,接收数据也靠DStream，数据的输入，数据的计算，输出整个生命周期都是由DStream构建的。由此，DStream负责RDD的整个生命周期。因此研究的入口的是DStream。基于Kafka数据来源，通过Direct的方式访问Kafka,DStream随着时间的进行，会不断的在自己的内存数据结构中维护一个HashMap,HashMap维护的就是时间窗口，以及时间窗口下的RDD.按照Batch Duration来存储RDD以及删除RDD.
Spark Streaming本身是一直在运行的，在自己计算的时候会不断的产生RDD，例如每秒Batch Duration都会产生RDD,除此之外可能还有累加器，广播变量。由于不断的产生这些对象，因此Spark Streaming有自己的一套对象，元数据以及数据的清理机制。
Spark Streaming对RDD的管理就相当于JVM的GC
二、源码解析
Spark Streaming是通过我们设定的Batch Durations来不断的产生RDD，Spark Streaming清理元数据跟时钟有关，因为数据是周期性的产生，所以肯定是周期性的释放，这些都跟JobGenerator有关，所以我们先从这开始研究。
1、RecurringTimer: 消息循环器将消息不断的发送给EventLoop
= RecurringTimer(...millisecondslongTime => .post((Time(longTime))))
2、eventLoop：onReceive接收到消息
(): = synchronized {
(!= ) = EventLoop[JobGeneratorEvent]() {
(event: JobGeneratorEvent): = processEvent(event)
(e: ): = {
jobScheduler.reportError(e)
}
}
.start()
(.) {
restart()
} {
startFirstTime()
}
}3、在processEvent中接收清理元数据消息
/** Processes all events */
private def processEvent(event: JobGeneratorEvent) {
logDebug("Got event " + event)
event match {
case GenerateJobs(time) => generateJobs(time)
case ClearMetadata(time) => clearMetadata(time) //清理元数据
case DoCheckpoint(time, clearCheckpointDataLater) =>
doCheckpoint(time, clearCheckpointDataLater)
case ClearCheckpointData(time) => clearCheckpointData(time) //清理checkpoint
}
}
具体的方法实现内容就不再这里说，我们进一步分析下这些清理动作是在什么时候被调用的，在Spark Streaming应用程序中，最终Job是交给JobHandler来执行的，所以我们分析下JobHandler
private class JobHandler(job: Job) extends Runnable with Logging {
import JobScheduler._
def run() {
try {
val formattedTime = UIUtils.formatBatchTime(
job.time.milliseconds, ssc.graph.batchDuration.milliseconds, showYYYYMMSS = false)
val batchUrl = s"/streaming/batch/?id=${job.time.milliseconds}"
val batchLinkText = s"[output operation ${job.outputOpId}, batch time ${formattedTime}]"
ssc.sc.setJobDescription(
s"""Streaming job from <a href="$batchUrl">$batchLinkText</a>""")
ssc.sc.setLocalProperty(BATCH_TIME_PROPERTY_KEY, job.time.milliseconds.toString)
ssc.sc.setLocalProperty(OUTPUT_OP_ID_PROPERTY_KEY, job.outputOpId.toString)
// We need to assign `eventLoop` to a temp variable. Otherwise, because
// `JobScheduler.stop(false)` may set `eventLoop` to null when this method is running, then
// it's possible that when `post` is called, `eventLoop` happens to null.
var _eventLoop = eventLoop
if (_eventLoop != null) {
_eventLoop.post(JobStarted(job, clock.getTimeMillis()))
// Disable checks for existing output directories in jobs launched by the streaming
// scheduler, since we may need to write output to an existing directory during checkpoint
// recovery; see SPARK-4835 for more details.
PairRDDFunctions.disableOutputSpecValidation.withValue(true) {
job.run()
}
_eventLoop = eventLoop
if (_eventLoop != null) {
_eventLoop.post(JobCompleted(job, clock.getTimeMillis()))
}
} else {
// JobScheduler has been stopped.
}
} finally {
ssc.sc.setLocalProperty(JobScheduler.BATCH_TIME_PROPERTY_KEY, null)
ssc.sc.setLocalProperty(JobScheduler.OUTPUT_OP_ID_PROPERTY_KEY, null)
}
}
}
}
当Job完成的时候，会发JobCompleted消息给onReceive，通过processEvent来执行具体的方法
private def processEvent(event: JobSchedulerEvent) {
try {
event match {
case JobStarted(job, startTime) => handleJobStart(job, startTime)
case JobCompleted(job, completedTime) => handleJobCompletion(job, completedTime)
case ErrorReported(m, e) => handleError(m, e)
}
} catch {
case e: Throwable =>
reportError("Error in job scheduler", e)
}
}
private def handleJobCompletion(job: Job, completedTime: Long) {
val jobSet = jobSets.get(job.time)
jobSet.handleJobCompletion(job)
job.setEndTime(completedTime)
listenerBus.post(StreamingListenerOutputOperationCompleted(job.toOutputOperationInfo))
logInfo("Finished job " + job.id + " from job set of time " + jobSet.time)
if (jobSet.hasCompleted) {
jobSets.remove(jobSet.time)
jobGenerator.onBatchCompletion(jobSet.time)
logInfo("Total delay: %.3f s for time %s (execution: %.3f s)".format(
jobSet.totalDelay / 1000.0, jobSet.time.toString,
jobSet.processingDelay / 1000.0
))
listenerBus.post(StreamingListenerBatchCompleted(jobSet.toBatchInfo))
}
job.result match {
case Failure(e) =>
reportError("Error running job " + job, e)
case _ =>
}
}
通过jobGenerator.onBatchCompletion来清理元数据
/**
* Callback called when a batch has been completely processed.
*/
def onBatchCompletion(time: Time) {
eventLoop.post(ClearMetadata(time))
}
到这里Spark Streaming清理元数据的步骤基本上完成了
备注：
资料来源于：DT_大数据梦工厂（Spark发行版本定制）
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