Spark Streaming中的数据是源源不断流进来的，有时候我们需要计算一些周期性的统计，就不得不维护一下数据的状态。在Spark Streaming中状态管理有两种方式。一种是updateStateByKey,另一种是mapWithState
第一种方式：先获取上一个batch中的状态RDD和当前batch的RDD 做cogroup 得到一个新的状态RDD。这种方式完美的契合了RDD的不变性，但是对性能却会有比较大的影响,因为需要对所有数据做处理，计算量和数据集大小是成线性相关的。
看一下updateStateByKey的代码，在Dstream中并没有找到updateStateByKey()方法，因为updateStateByKey是针对Key-Value的操作，所在可以想到updateStateByKey()方法其实是在PairDStreamFunctions类中，他是通过隐式转换的方式实现的。
代码如下
implicit def toPairDStreamFunctions[K, V](stream: DStream[(K, V)]) (implicit kt: ClassTag[K], vt: ClassTag[V], ord: Ordering[K] = null): PairDStreamFunctions[K, V] = {
new PairDStreamFunctions[K, V](stream)
}接着看updateStateByKey()方法,他有几种重载方式，最终调用以下的updateStateByKey()方法，代码如下
def updateStateByKey[S: ClassTag](
updateFunc: (Iterator[(K, Seq[V], Option[S])]) => Iterator[(K, S)], partitioner: Partitioner, rememberPartitioner: Boolean
): DStream[(K, S)] = ssc.withScope { new StateDStream(self, ssc.sc.clean(updateFunc), partitioner, rememberPartitioner, None)
}这里实例化了一个StateDStream，看一下StateDStream的compute方法，代码如下
override def compute(validTime: Time): Option[RDD[(K, S)]] = { // Try to get the previous state RDD
getOrCompute(validTime - slideDuration) match { case Some(prevStateRDD) => { // If previous state RDD exists
// Try to get the parent RDD
parent.getOrCompute(validTime) match { case Some(parentRDD) => { // If parent RDD exists, then compute as usual
computeUsingPreviousRDD (parentRDD, prevStateRDD)
} case None => { // If parent RDD does not exist
// Re-apply the update function to the old state RDD
val updateFuncLocal = updateFunc
val finalFunc = (iterator: Iterator[(K, S)]) => {
val i = iterator.map(t => (t._1, Seq[V](), Option(t._2)))
updateFuncLocal(i)
}
val stateRDD = prevStateRDD.mapPartitions(finalFunc, preservePartitioning)
Some(stateRDD)
}
}
} case None => { // If previous session RDD does not exist (first input data)
// Try to get the parent RDD
parent.getOrCompute(validTime) match { case Some(parentRDD) => { // If parent RDD exists, then compute as usual
initialRDD match { case None => { // Define the function for the mapPartition operation on grouped RDD;
// first map the grouped tuple to tuples of required type,
// and then apply the update function
val updateFuncLocal = updateFunc
val finalFunc = (iterator : Iterator[(K, Iterable[V])]) => {
updateFuncLocal (iterator.map (tuple => (tuple._1, tuple._2.toSeq, None)))
}
val groupedRDD = parentRDD.groupByKey (partitioner)
val sessionRDD = groupedRDD.mapPartitions (finalFunc, preservePartitioning) // logDebug("Generating state RDD for time " + validTime + " (first)")
Some (sessionRDD)
} case Some (initialStateRDD) => {
computeUsingPreviousRDD(parentRDD, initialStateRDD)
}
}
} case None => { // If parent RDD does not exist, then nothing to do!
// logDebug("Not generating state RDD (no previous state, no parent)")
None
}
}
}
}
}这里代码分几种情况，但最终都调用computeUsingPreviousRDD()方法，关键操作就在computeUsingPreviousRDD()方法中，代码如下
private [this] def computeUsingPreviousRDD (
parentRDD : RDD[(K, V)], prevStateRDD : RDD[(K, S)]) = { // Define the function for the mapPartition operation on cogrouped RDD;
// first map the cogrouped tuple to tuples of required type,
// and then apply the update function
val updateFuncLocal = updateFunc
val finalFunc = (iterator: Iterator[(K, (Iterable[V], Iterable[S]))]) => {
val i = iterator.map(t => {
val itr = t._2._2.iterator
val headOption = if (itr.hasNext) Some(itr.next()) else None
(t._1, t._2._1.toSeq, headOption)
})
updateFuncLocal(i)
}
val cogroupedRDD = parentRDD.cogroup(prevStateRDD, partitioner)
val stateRDD = cogroupedRDD.mapPartitions(finalFunc, preservePartitioning)
Some(stateRDD)
}可以看到当前状态的RDD和前一个状态的RDD进行cogroup操作
val cogroupedRDD = parentRDD.cogroup(prevStateRDD, partitioner)
parentRDD中只要有一条数据就会进行cogroup操作的，并将所有数据都进行更新函数（用户定义的）的操作，所以当数据量不断增加的时候，计算量随着线性增加。
第二种方式，在Spark1.6以后出来一种mapWithState的方式，他是一种变通的实现。因为没法变更RDD/Partition等核心概念，所以Spark Streaming在集合元素上做了文章，定义了MapWithStateRDD，将该RDD的元素做了限定，必须是MapWithStateRDDRecord 这个东西。该MapWithStateRDDRecord 保存分区内的所有key的状态(通过stateMap记录)以及计算结果(mappedData),元素MapWithStateRDDRecord 是可变的，但是RDD 依然是不变的。
mapWithState和updateStateByKey一样都是在PairDtreamFuntions类中,mapWithState代码如下
@Experimentaldef mapWithState[StateType: ClassTag, MappedType: ClassTag]( spec: StateSpec[K, V, StateType, MappedType]
): MapWithStateDStream[K, V, StateType, MappedType] = {
new MapWithStateDStreamImpl[K, V, StateType, MappedType]( self, spec.asInstanceOf[StateSpecImpl[K, V, StateType, MappedType]]
)
}首先看注解，他是一个实验性的方法，官方还没有推荐使用。
再看spec: StateSpec[K, V, StateType, MappedType]，这里并没有接收一个函数，而是一个StateSpec。其实就将函数包装在StateSpec内部而已
这里实例化了一个MapWithStateDStreamImpl，代码如下
private[streaming] class MapWithStateDStreamImpl[ KeyType: ClassTag, ValueType: ClassTag, StateType: ClassTag, MappedType: ClassTag](
dataStream: DStream[(KeyType, ValueType)],
spec: StateSpecImpl[KeyType, ValueType, StateType, MappedType])
extends MapWithStateDStream[KeyType, ValueType, StateType, MappedType](dataStream.context) { private val internalStream = new InternalMapWithStateDStream[KeyType, ValueType, StateType, MappedType](dataStream, spec)
override def slideDuration: Duration = internalStream.slideDuration
override def dependencies: List[DStream[_]] = List(internalStream)
override def compute(validTime: Time): Option[RDD[MappedType]] = {
internalStream.getOrCompute(validTime).map(x=>{
x.flatMap[MappedType](_.mappedData )
})
} /**
* Forward the checkpoint interval to the internal DStream that computes the state maps. This
* to make sure that this DStream does not get checkpointed, only the internal stream.
*/
override def checkpoint(checkpointInterval: Duration): DStream[MappedType] = {
internalStream.checkpoint(checkpointInterval) this
} /** Return a pair DStream where each RDD is the snapshot of the state of all the keys. */
def stateSnapshots(): DStream[(KeyType, StateType)] = {
internalStream.flatMap {
_.stateMap.getAll().map { case (k, s, _) => (k, s) }.toTraversable }
}
def keyClass: Class[_] = implicitly[ClassTag[KeyType]].runtimeClass
def valueClass: Class[_] = implicitly[ClassTag[ValueType]].runtimeClass
def stateClass: Class[_] = implicitly[ClassTag[StateType]].runtimeClass
def mappedClass: Class[_] = implicitly[ClassTag[MappedType]].runtimeClass
}MapWithStateDStreamImpl的compute操作其他没有什么内容，主要是从internalStream中获取计算结果，internalStream是在MapWithStateDStreamImpl实例化的时候创建，代码如下
private val internalStream = new InternalMapWithStateDStream[KeyType, ValueType, StateType, MappedType](dataStream, spec)
看 InternalMapWithStateDStream的compute方法，代码如下
override def compute(validTime: Time): Option[RDD[MapWithStateRDDRecord[K, S, E]]] = { // Get the previous state or create a new empty state RDD
// 得到以前状态的RDD或创建一个空状态的RDD
val prevStateRDD = getOrCompute(validTime - slideDuration) match { case Some(rdd) => if (rdd.partitioner != Some(partitioner)) { // If the RDD is not partitioned the right way, let us repartition it using the
// partition index as the key. This is to ensure that state RDD is always partitioned
// before creating another state RDD using it
MapWithStateRDD.createFromRDD[K, V, S, E](rdd.flatMap(_.stateMap.getAll()), partitioner, validTime)
} else {
rdd
} case None =>
MapWithStateRDD.createFromPairRDD[K, V, S, E]( // 获取用户初始化的状态RDD
spec.getInitialStateRDD().getOrElse(new EmptyRDD[(K, S)](ssc.sparkContext)),
partitioner,
validTime
)
} // Compute the new state RDD with previous state RDD and partitioned data RDD
// Even if there is no data RDD, use an empty one to create a new state RDD
// 获取当前要进行计算的RDD
val dataRDD = parent.getOrCompute(validTime).getOrElse {
context.sparkContext.emptyRDD[(K, V)]
}
val partitionedDataRDD = dataRDD.partitionBy(partitioner)
val timeoutThresholdTime = spec.getTimeoutInterval().map { interval =>
(validTime - interval).milliseconds
}
Some(new MapWithStateRDD(prevStateRDD, partitionedDataRDD, mappingFunction, validTime, timeoutThresholdTime))
}首先获取前一个状态的RDD（prevStateRDD），prevStateRDD在第一次使用的时候调用MapWithStateRDD.createFromPairRDD方法，将自用定义的初始值放在到新生成的stateMap中;如果prevStateRDD分区和当前状态分区不同时会调用MapWithStateRDD.createFromRDD()将状态数据重新分区后放入新生成的stateMap，createFromRDD()方法和createFromPairRDD方法代码如下
def createFromPairRDD[K: ClassTag, V: ClassTag, S: ClassTag, E: ClassTag](
pairRDD: RDD[(K, S)],
partitioner: Partitioner,
updateTime: Time): MapWithStateRDD[K, V, S, E] = {
val stateRDD = pairRDD.partitionBy(partitioner).mapPartitions ({ iterator =>
val stateMap = StateMap.create[K, S](SparkEnv.get.conf) // 把用户定义的初始值放入新创建的stateMap
iterator.foreach { case (key, state) => stateMap.put(key, state, updateTime.milliseconds) } // 把stateMap放在MapWithStateRDDRecord中做为RDD的元素返回
Iterator(MapWithStateRDDRecord(stateMap, Seq.empty[E]))
}, preservesPartitioning = true)
val emptyDataRDD = pairRDD.sparkContext.emptyRDD[(K, V)].partitionBy(partitioner)
val noOpFunc = (time: Time, key: K, value: Option[V], state: State[S]) => None new MapWithStateRDD[K, V, S, E](stateRDD, emptyDataRDD, noOpFunc, updateTime, None)
}
def createFromRDD[K: ClassTag, V: ClassTag, S: ClassTag, E: ClassTag](
rdd: RDD[(K, S, Long)],
partitioner: Partitioner,
updateTime: Time): MapWithStateRDD[K, V, S, E] = {
val pairRDD = rdd.map { x => (x._1, (x._2, x._3)) }
val stateRDD = pairRDD.partitionBy(partitioner).mapPartitions({ iterator =>
val stateMap = StateMap.create[K, S](SparkEnv.get.conf) // 把之前stateMap中的状态数据（key,(state,update)）放入一个stateMap中
iterator.foreach { case (key, (state, updateTime)) =>
stateMap.put(key, state, updateTime)
}
Iterator(MapWithStateRDDRecord(stateMap, Seq.empty[E]))
}, preservesPartitioning = true)
val emptyDataRDD = pairRDD.sparkContext.emptyRDD[(K, V)].partitionBy(partitioner)
val noOpFunc = (time: Time, key: K, value: Option[V], state: State[S]) => None new MapWithStateRDD[K, V, S, E](stateRDD, emptyDataRDD, noOpFunc, updateTime, None)
}prevStateRDD获取之后实例化MapWithStateRDD，将前一个状态RDD和当前要计算的RDD传递进去，看MapWithStateRDD类的代码
private[streaming] class MapWithStateRDD[K: ClassTag, V: ClassTag, S: ClassTag, E: ClassTag]( // 存储State数据的RDD
private var prevStateRDD: RDD[MapWithStateRDDRecord[K, S, E]], // 计算当前数据的RDD
private var partitionedDataRDD: RDD[(K, V)], // 计算函数
mappingFunction: (Time, K, Option[V], State[S]) => Option[E],
batchTime: Time,
timeoutThresholdTime: Option[Long]
) extends RDD[MapWithStateRDDRecord[K, S, E]](
partitionedDataRDD.sparkContext, // MapWithStateRDD依赖两个父RDD，因为有两个数据来源。一个是状态数据，一个是当前数据
List( new OneToOneDependency[MapWithStateRDDRecord[K, S, E]](prevStateRDD), new OneToOneDependency(partitionedDataRDD))
) {
@volatile private var doFullScan = false
require(prevStateRDD.partitioner.nonEmpty)
require(partitionedDataRDD.partitioner == prevStateRDD.partitioner)
override val partitioner = prevStateRDD.partitioner
override def checkpoint(): Unit = {
super.checkpoint()
doFullScan = true
}
override def compute(partition: Partition, context: TaskContext): Iterator[MapWithStateRDDRecord[K, S, E]] = {
val stateRDDPartition = partition.asInstanceOf[MapWithStateRDDPartition]
val prevStateRDDIterator = prevStateRDD.iterator(stateRDDPartition.previousSessionRDDPartition, context)
val dataIterator = partitionedDataRDD.iterator(stateRDDPartition.partitionedDataRDDPartition, context) // 因为prevStateRDD只有一个元素，所有取prevStateRDDIterator.next()
val prevRecord:Option[MapWithStateRDDRecord[K, S, E]] = if (prevStateRDDIterator.hasNext){
Some(prevStateRDDIterator.next())
} else {
None
} // 返回一个新的MapWithStateRDDRecord
val newRecord = MapWithStateRDDRecord.updateRecordWithData(
prevRecord,
dataIterator,
mappingFunction,
batchTime,
timeoutThresholdTime,
removeTimedoutData = doFullScan // remove timedout data only when full scan is enabled
) // 将新生成的MapWithStateRDDRecord放入迭代器，此迭代器还是只有一个元素
Iterator(newRecord)
}
override protected def getPartitions: Array[Partition] = {
Array.tabulate(prevStateRDD.partitions.length) { i => new MapWithStateRDDPartition(i, prevStateRDD, partitionedDataRDD)}
}
override def clearDependencies(): Unit = {
super.clearDependencies()
prevStateRDD = null
partitionedDataRDD = null
}
def setFullScan(): Unit = {
doFullScan = true
}
}主要看newRecord是怎样生成的，因为newRecord里有所有的状态信息和计算结果，看 MapWithStateRDDRecord.updateRecordWithData的代码
def updateRecordWithData[K: ClassTag, V: ClassTag, S: ClassTag, E: ClassTag]( // 前一个MapWithStateRDDRecord
prevRecord: Option[MapWithStateRDDRecord[K, S, E]], // 当前需要计算的数据
dataIterator: Iterator[(K, V)], // 计算函数
mappingFunction: (Time, K, Option[V], State[S]) => Option[E], batchTime: Time, timeoutThresholdTime: Option[Long], removeTimedoutData: Boolean
): MapWithStateRDDRecord[K, S, E] = { // Create a new state map by cloning the previous one (if it exists) or by creating an empty one
// 首先创建一个新的StateMap,这里是从前一个StateMap复制而来的，由于StateMap的复制是采用增量复制，
// 新创建的stateMap会引用旧的stateMap
val newStateMap = prevRecord.map( _.stateMap.copy()). getOrElse { new EmptyStateMap[K, S]() }
val mappedData = new ArrayBuffer[E]
val wrappedState = new StateImpl[S]() // Call the mapping function on each record in the data iterator, and accordingly
// update the states touched, and collect the data returned by the mapping function
// mapWithState操作性能优势就是在这里体现的
dataIterator.foreach { case (key, value) =>
//将newStateMap中的元素包装一下
wrappedState.wrap(newStateMap.get(key)) // 终于看到用户定义的mappingFunction函数了，传入当前key，当前value,和此key的历史数据
val returned = mappingFunction(batchTime, key, Some(value), wrappedState) if (wrappedState.isRemoved) { // 如果更新值被标记删除
newStateMap.remove(key)
} else if (wrappedState.isUpdated || (wrappedState.exists && timeoutThresholdTime.isDefined)) { // 如果当前key的value为标记有更新，就更新newStateMap，重新put操作
newStateMap.put(key, wrappedState.get(), batchTime.milliseconds)
}
mappedData ++= returned
} // Get the timed out state records, call the mapping function on each and collect the
// data returned
if (removeTimedoutData && timeoutThresholdTime.isDefined) {
newStateMap.getByTime(timeoutThresholdTime.get).foreach { case (key, state, _) =>
wrappedState.wrapTimingOutState(state)
val returned = mappingFunction(batchTime, key, None, wrappedState)
mappedData ++= returned
newStateMap.remove(key)
}
} // newStateMap 状态集合
// mappedData 返回计算后的结果，这里要注意：因为上面的迭代操作是基于当前RDD的数据，
// 所以返回计算后的结果只有当前数据的更新值
MapWithStateRDDRecord(newStateMap, mappedData)
}通过上面的注释已经知道MapWithStateRDD[MapWithStateRDDRecord]类型的RDD的数据是怎么计算的了，接着看InternalMapWithStateDStream的computer方法返回后的操作
override def compute(validTime: Time): Option[RDD[MappedType]] = {
internalStream.getOrCompute(validTime).map(x=>{
x.flatMap[MappedType](_.mappedData )
})
}使用flatMap从MapWithStateRDDRecord中获取mappedData(当前RDD进行状态计算后的结果)并返回，到这里mapWithState的操作就完成了
下面看一个例子
看一个计算wordCount状态操作的Demo,代码如下
package cn.lht.spark.streamingimport _root_.kafka.serializer.StringDecoderimport org.apache.spark.SparkConfimport org.apache.spark.streaming.kafka.KafkaUtilsimport org.apache.spark.streaming._
object StateWordCount {
def main(args: Array[String]): Unit = {
val topics = "kafkaforspark"
val brokers = "*.*.*.*:9092,*.*.*.*:9092"
val sparkConf = new SparkConf().setAppName("DirectKafkaWordCount").setMaster("local[2]")
sparkConf.set("spark.testing.memory", "2147480000")
val ssc = new StreamingContext(sparkConf, Seconds(5))
ssc.checkpoint("hdfs://mycluster/ceshi/")
val topicsSet = topics.split(",").toSet
val kafkaParams = Map[String, String]("metadata.broker.list" -> brokers)
val messages = KafkaUtils.createDirectStream[String, String, StringDecoder, StringDecoder](ssc, kafkaParams, topicsSet)
.map(_._2.trim).map((_, 1)) // 1. mapWithState操作
val mappingFunc = (word: String, one: Option[Int], state: State[Int]) => {
val sum = one.getOrElse(0) + state.getOption.getOrElse(0)
val output = (word, sum)
state.update(sum)
output
}
val state = StateSpec.function(mappingFunc)
messages.reduceByKey(_+_).mapWithState(state).print() // 2. updateStateByKey// val addFunc = (currValues: Seq[Int], prevValueState: Option[Int]) => {// //通过Spark内部的reduceByKey按key规约，然后这里传入某key当前批次的Seq/List,再计算当前批次的总和// val currentCount = currValues.sum// // 已累加的值// val previousCount = prevValueState.getOrElse(0)// // 返回累加后的结果，是一个Option[Int]类型// Some(currentCount + previousCount)// }// messages.updateStateByKey(addFunc,2).print()
ssc.start()
ssc.awaitTermination()
}
}输入数据为三组，分别是
1 2 3 4 5 4 5 6 7 8 6 7 8 9
mapWithState操作结果
(4,1) (5,1) (1,1) (2,1) (3,1) ------------------------------------------- Time: 1464516190000 ms ------------------------------------------- (4,2) (8,1) (5,2) (6,1) (7,1) ------------------------------------------- Time: 1464516195000 ms ------------------------------------------- (8,2) (9,1) (6,2) (7,2)
updateStateByKey操作结果
------------------------------------------- Time: 1464516940000 ms ------------------------------------------- (4,1) (2,1) (5,1) (3,1) (1,1) ------------------------------------------- Time: 1464516945000 ms ------------------------------------------- (4,2) (8,1) (6,1) (2,1) (7,1) (5,2) (3,1) (1,1) ------------------------------------------- Time: 1464516950000 ms ------------------------------------------- (8,2) (4,2) (6,2) (2,1) (7,2) (5,2) (9,1) (3,1) (1,1)
看以上两种操作返回的结果是不一样的，mapWithState返回最新数据的状态结果，而updateStateByKey返回了所有状态结果，具体使用要配合业务进行调整
作者：海纳百川_spark
链接：https://www.jianshu.com/p/2edb6d218be9
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