首先简单解释一下什么是state(状态)管理？我们以wordcount为例。每个batchInterval会计算当前batch的单词计数，那如果需要计算从流开始到目前为止的单词出现的次数，该如计算呢？SparkStreaming提供了两种方法：updateStateByKey和mapWithState 。mapWithState 是1.6版本新增功能，目前属于实验阶段。mapWithState具官方说性能较updateStateByKey提升10倍。那么我们来看看他们到底是如何实现的。
一、updateStateByKey 解析
1.1 updateStateByKey 的使用实例
首先看一个updateStateByKey函数使用的例子：
object UpdateStateByKeyDemo {
def main(args: Array[String]) {
val conf = new SparkConf().setAppName("UpdateStateByKeyDemo")
val ssc = new StreamingContext(conf,Seconds(20)) //要使用updateStateByKey方法，必须设置Checkpoint。
ssc.checkpoint("/checkpoint/")
val socketLines = ssc.socketTextStream("localhost",9999)
socketLines.flatMap(_.split(",")).map(word=>(word,1))
.updateStateByKey(
(currValues:Seq[Int],preValue:Option[Int]) =>{
val currValue = currValues.sum//将目前值相加
Some(currValue + preValue.getOrElse(0))//目前值的和加上历史值
}).print()
ssc.start()
ssc.awaitTermination()
ssc.stop()
}
}代码很简单，关键地方写了详细的注释。
1.2 updateStateByKey 方法源码分析
我们知道map返回的是MappedDStream，而MappedDStream并没有updateStateByKey方法，并且它的父类DStream中也没有该方法。但是DStream的伴生对象中有一个隐式转换函数
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)
}
PairDStreamFunction 中updateStateByKey的源码如下： def updateStateByKey[S: ClassTag]( updateFunc: (Seq[V], Option[S]) => Option[S]
): DStream[(K, S)] = ssc.withScope {
updateStateByKey(updateFunc, defaultPartitioner())
}其中updateFunc就要传入的参数，他是一个函数，
Seq[V]表示当前key对应的所有值，Option[S] 是当前key的历史状态，返回的是新的状态。
最终会调用下面的方法：
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)
}在这里面new出了一个StateDStream对象。在其compute方法中，会先获取上一个batch计算出的RDD（包含了至程序开始到上一个batch单词的累计计数），然后在获取本次batch中StateDStream的父类计算出的RDD（本次batch的单词计数）分别是prevStateRDD和parentRDD，然后在调用 computeUsingPreviousRDD 方法：
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)
}两个RDD进行cogroup然后应用updateStateByKey传入的函数。cogroup的性能是比较低下的。
二、mapWithState方法解析
2.1 mapWithState方法使用实例：
object StatefulNetworkWordCount {
def main(args: Array[String]) { if (args.length < 2) {
System.err.println("Usage: StatefulNetworkWordCount <hostname> <port>")
System.exit(1)
}
StreamingExamples.setStreamingLogLevels()
val sparkConf = new SparkConf().setAppName("StatefulNetworkWordCount") // Create the context with a 1 second batch size
val ssc = new StreamingContext(sparkConf, Seconds(1))
ssc.checkpoint(".")
// Initial state RDD for mapWithState operation
val initialRDD = ssc.sparkContext.parallelize(List(("hello", 1), ("world", 1)))
// Create a ReceiverInputDStream on target ip:port and count the
// words in input stream of \n delimited test (eg. generated by 'nc')
val lines = ssc.socketTextStream(args(0), args(1).toInt)
val words = lines.flatMap(_.split(" "))
val wordDstream = words.map(x => (x, 1))
// Update the cumulative count using mapWithState
// This will give a DStream made of state (which is the cumulative count of the words)
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 stateDstream = wordDstream.mapWithState(
StateSpec.function(mappingFunc).initialState(initialRDD))
stateDstream.print()
ssc.start()
ssc.awaitTermination()
}
}mapWithState接收的参数是一个StateSpec对象。在StateSpec中封装了状态管理的函数
mapWithState函数中创建了MapWithStateDStreamImpl对象
def 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]]
)
}MapWithStateDStreamImpl 中创建了一个InternalMapWithStateDStream类型对象internalStream，在MapWithStateDStreamImpl的compute方法中调用了internalStream的getOrCompute方法。
/** Internal implementation of the [[MapWithStateDStream]] */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 { _.flatMap[MappedType] { _.mappedData } }
}InternalMapWithStateDStream中没有getOrCompute方法，这里调用的是其父类 DStream 的getOrCpmpute方法，该方法中最终会调用InternalMapWithStateDStream的Compute方法：
/** Method that generates a RDD for the given time */
override def compute(validTime: Time): Option[RDD[MapWithStateRDDRecord[K, S, E]]] = { // Get the previous state or create a new empty state 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](
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
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))
}根据给定的时间生成一个MapWithStateRDD，首先获取了先前状态的RDD：preStateRDD和当前时间的RDD:dataRDD，然后对dataRDD基于先前状态RDD的分区器进行重新分区获取partitionedDataRDD。最后将preStateRDD，partitionedDataRDD和用户定义的函数mappingFunction传给新生成的MapWithStateRDD对象返回。
下面看一下MapWithStateRDD的compute方法：
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)
//prevRecord 代表一个分区的数据
val prevRecord = if (prevStateRDDIterator.hasNext) Some(prevStateRDDIterator.next()) else None
val newRecord = MapWithStateRDDRecord.updateRecordWithData(
prevRecord,
dataIterator,
mappingFunction,
batchTime,
timeoutThresholdTime,
removeTimedoutData = doFullScan // remove timedout data only when full scan is enabled
)
Iterator(newRecord)
}MapWithStateRDDRecord 对应MapWithStateRDD 的一个分区：
private[streaming] case class MapWithStateRDDRecord[K, S, E]( var stateMap: StateMap[K, S], var mappedData: Seq[E])
其中stateMap存储了key的状态，mappedData存储了mapping function函数的返回值
看一下MapWithStateRDDRecord的
updateRecordWithData方法
def updateRecordWithData[K: ClassTag, V: ClassTag, S: ClassTag, E: ClassTag](
prevRecord: Option[MapWithStateRDDRecord[K, S, E]], dataIterator: Iterator[(K, V)], mappingFunction: (Time, K, Option[V], State[S]) => Option[E], batchTime: Time, timeoutThresholdTime: Option[Long], removeTimedoutData: Boolean
): MapWithStateRDDRecord[K, S, E] = {// 创建一个新的 state map 从过去的Recoord中复制 (如果存在) 否则创建一下空的StateMap对象
val newStateMap = prevRecord.map { _.stateMap.copy() }. getOrElse { new EmptyStateMap[K, S]() }
val mappedData = new ArrayBuffer[E]
//状态
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
dataIterator.foreach { case (key, value) =>
//获取key对应的状态
wrappedState.wrap(newStateMap.get(key))
//调用mappingFunction获取返回值
val returned = mappingFunction(batchTime, key, Some(value), wrappedState)
//维护newStateMap的值 if (wrappedState.isRemoved) {
newStateMap.remove(key)
} else if (wrappedState.isUpdated
|| (wrappedState.exists && timeoutThresholdTime.isDefined)) {
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)
}
}
MapWithStateRDDRecord(newStateMap, mappedData)
}最终返回MapWithStateRDDRecord
对象交个MapWithStateRDD的compute函数，MapWithStateRDD的compute函数将其封装成Iterator返回。
作者：飞帅记忆
链接：https://www.jianshu.com/p/261636f397b8
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