DStream与RDD关系是什么
本篇内容主要讲解“DStream与RDD关系是什么”,感兴趣的朋友不妨来看看。本文介绍的方法操作简单快捷,实用性强。下面就让小编来带大家学习“DStream与RDD关系是什么”吧!
RDD是怎么生成的?RDD依靠什么生成?RDD生成的依据是什么?Spark Streaming中RDD的执行是否和Spark Core中的RDD执行有所不同?运行之后我们对RDD怎么处理?
RDD本身也是基本的对象,例如说BatchInterval为1秒,那么每一秒都会产生RDD,内存中不能完全容纳该对象。每个BatchInterval的作业执行完后,怎么对已有的RDD进行管理。
ForEachDStream不一定会触发Job的执行,和Job的执行没有关系。
Job的产生是由Spark Streaming框架造成的。
foreachRDD是Spark Streaming的后门,可以直接对RDD进行操作。
DStream就是RDD的模板,后面的DStream与前面的DStream有依赖。
val lines = jsc.socketTextStream("127.0.0.1", 9999)这里产生了SocketInputDStream。
lines.flatMap(_.split(" ")).map(word => (word, 1)).reduceByKey(_ + _).print()这里由SocketInputDStream转换为FlatMappedDStream,再转换为MappedDStream,再转换为ShuffledDStream,再转换为ForEachDStream。
对于DStream类,源码中是这样解释的。
* DStreams internally is characterized by a few basic properties: * - A list of other DStreams that the DStream depends on * - A time interval at which the DStream generates an RDD * - A function that is used to generate an RDD after each time interval |
大致意思是:
1.DStream依赖于其他DStream。
2.每隔BatchDuration,DStream生成一个RDD
3.每隔BatchDuration,DStream内部函数会生成RDD
DStream是从后往前依赖,因为DStream代表Spark Streaming业务逻辑,RDD是从后往前依赖的,DStream是lazy级别的。DStream的依赖关系必须和RDD的依赖关系保持高度一致。
DStream类中generatedRDDs存储着不同时间对应的RDD实例。每一个DStream实例都有自己的generatedRDDs。实际运算的时候,由于是从后往前推,计算只作用于最后一个DStream。
// RDDs generated, marked as private[streaming] so that testsuites can access it @transient private[streaming] var generatedRDDs = new HashMap[Time, RDD[T]] () |
generatedRDDs是如何获取的。DStream的getOrCompute方法,先根据时间判断HashMap中是否已存在该时间对应的RDD,如果没有则调用compute得到RDD,并放入到HashMap中。
/** * Get the RDD corresponding to the given time; either retrieve it from cache * or compute-and-cache it. */ private[streaming] final def getOrCompute(time: Time): Option[RDD[T]] = { // If RDD was already generated, then retrieve it from HashMap, // or else compute the RDD generatedRDDs.get(time).orElse { // Compute the RDD if time is valid (e.g. correct time in a sliding window) // of RDD generation, else generate nothing. if (isTimeValid(time)) { val rddOption = createRDDWithLocalProperties(time, displayInnerRDDOps = false) { // 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. We need to have this call here because // compute() might cause Spark jobs to be launched. PairRDDFunctions.disableOutputSpecValidation.withValue(true) { compute(time) } } rddOption.foreach { case newRDD => // Register the generated RDD for caching and checkpointing if (storageLevel != StorageLevel.NONE) { newRDD.persist(storageLevel) logDebug(s"Persisting RDD ${newRDD.id} for time $time to $storageLevel") } if (checkpointDuration != null && (time - zeroTime).isMultipleOf(checkpointDuration)) { newRDD.checkpoint() logInfo(s"Marking RDD ${newRDD.id} for time $time for checkpointing") } generatedRDDs.put(time, newRDD) } rddOption } else { None } } } |
拿DStream的子类ReceiverInputDStream来说明compute方法,内部调用了createBlockRDD这个方法。
/** * Generates RDDs with blocks received by the receiver of this stream. */ override def compute(validTime: Time): Option[RDD[T]] = { val blockRDD = { if (validTime < graph.startTime) { // If this is called for any time before the start time of the context, // then this returns an empty RDD. This may happen when recovering from a // driver failure without any write ahead log to recover pre-failure data. new BlockRDD[T](ssc.sc, Array.empty) } else { // Otherwise, ask the tracker for all the blocks that have been allocated to this stream // for this batch val receiverTracker = ssc.scheduler.receiverTracker val blockInfos = receiverTracker.getBlocksOfBatch(validTime).getOrElse(id, Seq.empty) // Register the input blocks information into InputInfoTracker val inputInfo = StreamInputInfo(id, blockInfos.flatMap(_.numRecords).sum) ssc.scheduler.inputInfoTracker.reportInfo(validTime, inputInfo) // Create the BlockRDD createBlockRDD(validTime, blockInfos) } } Some(blockRDD) } |
createBlockRDD会返回BlockRDD,由于ReceiverInputDStream没有父依赖,所以自己生成RDD。
private[streaming] def createBlockRDD(time: Time, blockInfos: Seq[ReceivedBlockInfo]): RDD[T] = { if (blockInfos.nonEmpty) { val blockIds = blockInfos.map { _.blockId.asInstanceOf[BlockId] }.toArray // Are WAL record handles present with all the blocks val areWALRecordHandlesPresent = blockInfos.forall { _.walRecordHandleOption.nonEmpty } if (areWALRecordHandlesPresent) { // If all the blocks have WAL record handle, then create a WALBackedBlockRDD val isBlockIdValid = blockInfos.map { _.isBlockIdValid() }.toArray val walRecordHandles = blockInfos.map { _.walRecordHandleOption.get }.toArray new WriteAheadLogBackedBlockRDD[T]( ssc.sparkContext, blockIds, walRecordHandles, isBlockIdValid) } else { // Else, create a BlockRDD. However, if there are some blocks with WAL info but not // others then that is unexpected and log a warning accordingly. if (blockInfos.find(_.walRecordHandleOption.nonEmpty).nonEmpty) { if (WriteAheadLogUtils.enableReceiverLog(ssc.conf)) { logError("Some blocks do not have Write Ahead Log information; " + "this is unexpected and data may not be recoverable after driver failures") } else { logWarning("Some blocks have Write Ahead Log information; this is unexpected") } } val validBlockIds = blockIds.filter { id => ssc.sparkContext.env.blockManager.master.contains(id) } if (validBlockIds.size != blockIds.size) { logWarning("Some blocks could not be recovered as they were not found in memory. " + "To prevent such data loss, enabled Write Ahead Log (see programming guide " + "for more details.") } new BlockRDD[T](ssc.sc, validBlockIds) } } else { // If no block is ready now, creating WriteAheadLogBackedBlockRDD or BlockRDD // according to the configuration if (WriteAheadLogUtils.enableReceiverLog(ssc.conf)) { new WriteAheadLogBackedBlockRDD[T]( ssc.sparkContext, Array.empty, Array.empty, Array.empty) } else { new BlockRDD[T](ssc.sc, Array.empty) } } } |
再拿DStream的子类MappedDStream来说,这里的compute方法,是调用父RDD的getOrCompute方法获得RDD,再使用map操作。
private[streaming] class MappedDStream[T: ClassTag, U: ClassTag] ( parent: DStream[T], mapFunc: T => U ) extends DStream[U](parent.ssc) { override def dependencies: List[DStream[_]] = List(parent) override def slideDuration: Duration = parent.slideDuration override def compute(validTime: Time): Option[RDD[U]] = { parent.getOrCompute(validTime).map(_.map[U](mapFunc)) } } |
从上面两个DStream的子类,可以说明第一个DStream,即InputDStream的comput方法是自己获取数据并计算的,而其他的DStream是依赖父DStream的,调用父DStream的getOrCompute方法,然后进行计算。
以上说明了对DStream的操作最后作用于对RDD的操作。
接着看下DStream的另一个子类ForEachDStream,发现其compute方法没有任何操作,但是重写了generateJob方法。
private[streaming] class ForEachDStream[T: ClassTag] ( parent: DStream[T], foreachFunc: (RDD[T], Time) => Unit, displayInnerRDDOps: Boolean ) extends DStream[Unit](parent.ssc) { override def dependencies: List[DStream[_]] = List(parent) override def slideDuration: Duration = parent.slideDuration override def compute(validTime: Time): Option[RDD[Unit]] = None override def generateJob(time: Time): Option[Job] = { parent.getOrCompute(time) match { case Some(rdd) => val jobFunc = () => createRDDWithLocalProperties(time, displayInnerRDDOps) { foreachFunc(rdd, time) } Some(new Job(time, jobFunc)) case None => None } } } |
从Job生成入手,JobGenerator的generateJobs方法,内部调用的DStreamGraph的generateJobs方法。
/** 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 { //根据特定的时间获取具体的数据 jobScheduler.receiverTracker.allocateBlocksToBatch(time) // allocate received blocks to batch //调用DStreamGraph的generateJobs生成Job 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)) } |
DStreamGraph的generateJobs方法调用了OutputStream的generateJob方法,OutputStream就是ForEachDStream。
def generateJobs(time: Time): Seq[Job] = { logDebug("Generating jobs for time " + time) val jobs = this.synchronized { outputStreams.flatMap { outputStream => val jobOption = outputStream.generateJob(time) jobOption.foreach(_.setCallSite(outputStream.creationSite)) jobOption } } logDebug("Generated " + jobs.length + " jobs for time " + time) jobs } |
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