Spark Streaming updateStateByKey和mapWithState源码解密
本篇从二个方面进行源码分析：
一、updateStateByKey解密
二、mapWithState解密
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进行黑名单动态生成和过滤例子中会用到updateStateByKey方法，此方法在DStream类中没有定义，需要在
DStream的object区域通过隐式转换来找，如下面的代码：
object DStream {
// `toPairDStreamFunctions` was in SparkContext before
1.3 and users had to
// `import StreamingContext._` to
enable it. Now we move it here to make the compiler find
// it automatically. However, we still
keep the old function in StreamingContext for backward
// compatibility and forward to the
following function directly.
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)
}
继续跟踪PairDStreamFunctions类中有次方法定义：
/**
* Return a new "state" DStream where the state for each key is updated by applying
* the given function on the previous state of the key and the new values of each key.
* Hash partitioning is used to generate the RDDs with `numPartitions` partitions.
* @param updateFunc State update function. If `this` function returns None, then
* corresponding state key-value pair will be eliminated.
* @param numPartitions Number of partitions of each RDD in the new DStream.
* @tparam S State type
*/
def updateStateByKey[S: ClassTag](
updateFunc: (Seq[V], Option[S]) => Option[S],
numPartitions: Int
): DStream[(K, S)] = ssc.withScope {
updateStateByKey(updateFunc, defaultPartitioner(numPartitions))
}
继续返回DStream类：
HashPartitioner的特点是效率高，spark1.2之前采用的主要目的是效率高，不需要排序之类的，设置并行度：
private[streaming] def defaultPartitioner(numPartitions: Int = self.ssc.sc.defaultParallelism) = {
new HashPartitioner(numPartitions)
}
/**
* Return a new "state" DStream where the state for each key is updated by applying
* the given function on the previous state of the key and the new values of each key.
* org.apache.spark.Partitioner is used to control the partitioning of each RDD.
* @param updateFunc State update function. Note, that this function may generate a different
* tuple with a different key than the input key. Therefore keys may be removed
* or added in this way. It is up to the developer to decide whether to
* remember the partitioner despite the key being changed.
* @param partitioner Partitioner for controlling the partitioning of each RDD in the new
* DStream
* @param rememberPartitioner Whether to remember the paritioner object in the generated RDDs.
* @tparam S State type
*/
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)
}
继续跟踪StateDStream，继承了DStream，如果对状态不断的操作就会产生很多的StateDStream状态对象：
private[streaming]
class StateDStream[K: ClassTag, V: ClassTag, S: ClassTag](
parent: DStream[(K, V)],
updateFunc: (Iterator[(K, Seq[V], Option[S])]) => Iterator[(K, S)],
partitioner: Partitioner,
preservePartitioning: Boolean,
initialRDD : Option[RDD[(K, S)]]
) extends DStream[(K, S)](parent.ssc) {
super.persist(StorageLevel.MEMORY_ONLY_SER)
看一段关键的代码：
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)
}//效率角度
val stateRDD = prevStateRDD.mapPartitions(finalFunc, preservePartitioning)
Some(stateRDD)
}
}
}
根据代码分析，把函数传进来，看cogroup，按照key对value进行聚合，按照key对所有数据进行扫描然后聚合，这样做好处是对rdd的计算；
不好的地方就是性能，cogroup对所有数据进行扫描，随着时间流逝数据规模越来越大性能越低，cogroup rdd和另一个
cogroup rdd数据进行扫描合并。如下关键代码:
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)
}
/**
* For each key k in `this` or `other`, return a resulting RDD that contains a tuple with the
* list of values for that key in `this` as well as `other`.
*/
def cogroup[W](other: RDD[(K, W)], partitioner: Partitioner)
: RDD[(K, (Iterable[V], Iterable[W]))] = self.withScope {
if (partitioner.isInstanceOf[HashPartitioner] && keyClass.isArray) {
throw new SparkException("Default partitioner cannot partition array keys.")
}
val cg = new CoGroupedRDD[K](Seq(self, other), partitioner)
cg.mapValues { case Array(vs, w1s) =>
(vs.asInstanceOf[Iterable[V]], w1s.asInstanceOf[Iterable[W]])
}
}
继续剖析mapWithState解密
再看mapWithState，返回的是一个DStream，维护历史状态、更新历史状态都是基于key来维护，state相当于内存数据表，其实是在删除一张表，这张表中
记录了历史状态，一张key、value、state的表，所有历史状态都放在这张表中，根据key 在satate的基础上更新value，如单词计数，不断累积计数：
/**
* :: Experimental ::
* Return a [[MapWithStateDStream]] by applying a function to every key-value element of
* `this` stream, while maintaining some state data for each unique key. The mapping function
* and other specification (e.g. partitioners, timeouts, initial state data, etc.) of this
* transformation can be specified using [[StateSpec]] class. The state data is accessible in
* as a parameter of type [[State]] in the mapping function.
*
* Example of using `mapWithState`:
* {{{
* // A mapping function that maintains an integer state and return a String
* def mappingFunction(key: String, value: Option[Int], state: State[Int]): Option[String] = {
* // Use state.exists(), state.get(), state.update() and state.remove()
* // to manage state, and return the necessary string
* }
*
* val spec = StateSpec.function(mappingFunction).numPartitions(10)
*
* val mapWithStateDStream = keyValueDStream.mapWithState[StateType, MappedType](spec)
* }}}
*
* @param spec Specification of this transformation
* @tparam StateType Class type of the state data
* @tparam MappedType Class type of the mapped data
*/
@Experimental
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]]
)
}
内存数据表都会有：defined、timingOut、updated、removed：
/**
* :: Experimental ::
* Abstract class for getting and updating the state in mapping function used in the `mapWithState`
* operation of a [[org.apache.spark.streaming.dstream.PairDStreamFunctions pair DStream]] (Scala)
* or a [[org.apache.spark.streaming.api.java.JavaPairDStream JavaPairDStream]] (Java).
*
* Scala example of using `State`:
* {{{
* // A mapping function that maintains an integer state and returns a String
* def mappingFunction(key: String, value: Option[Int], state: State[Int]): Option[String] = {
* // Check if state exists
* if (state.exists) {
* val existingState = state.get // Get the existing state
* val shouldRemove = ... // Decide whether to remove the state
* if (shouldRemove) {
* state.remove() // Remove the state
* } else {
* val newState = ...
* state.update(newState) // Set the new state
* }
* } else {
* val initialState = ...
* state.update(initialState) // Set the initial state
* }
* ... // return something
* }
*
* }}}
/** Internal implementation of the [[State]] interface */
private[streaming] class StateImpl[S] extends State[S] {
private var state: S = null.asInstanceOf[S]
private var defined: Boolean = false
private var timingOut: Boolean = false
private var updated: Boolean = false
private var removed: Boolean = false
// ========= Public API =========
override def exists(): Boolean = {
defined
}
override def get(): S = {
if (defined) {
state
} else {
throw new NoSuchElementException("State is not set")
}
}
override def update(newState: S): Unit = {
require(!removed, "Cannot update the state after it has been removed")
require(!timingOut, "Cannot update the state that is timing out")
state = newState
defined = true
updated = true
}
下面的代码V就是外面传入的函数：
/** Internal implementation of [[org.apache.spark.streaming.StateSpec]] interface. */
private[streaming]
case class StateSpecImpl[K, V, S, T](
function: (Time, K, Option[V], State[S]) => Option[T]) extends StateSpec[K, V, S, T] {
/** 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 } }
}
基于历史数据的更新，有内存数据结构，更新已有数据结构，而不是在已有的基础上创建内存数据结构：
/**
* A DStream that allows per-key state to be maintains, and arbitrary records to be generated
* based on updates to the state. This is the main DStream that implements the `mapWithState`
* operation on DStreams.
*
* @param parent Parent (key, value) stream that is the source
* @param spec Specifications of the mapWithState operation
* @tparam K Key type
* @tparam V Value type
* @tparam S Type of the state maintained
* @tparam E Type of the mapped data
*/
private[streaming]
class InternalMapWithStateDStream[K: ClassTag, V: ClassTag, S: ClassTag, E: ClassTag](
parent: DStream[(K, V)], spec: StateSpecImpl[K, V, S, E])
extends DStream[MapWithStateRDDRecord[K, S, E]](parent.context) {
persist(StorageLevel.MEMORY_ONLY)
基于时间窗口创建一个新rdd，是所有故事下面开始：
Some(new MapWithStateRDD(
prevStateRDD, partitionedDataRDD, mappingFunction, validTime, timeoutThresholdTime))
** 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))
}
}
看下MapWithStateRDD：
/**
* RDD storing the keyed states of `mapWithState` operation and corresponding mapped data.
* Each partition of this RDD has a single record of type [[MapWithStateRDDRecord]]. This contains a
* [[StateMap]] (containing the keyed-states) and the sequence of records returned by the mapping
* function of `mapWithState`.
* @param prevStateRDD The previous MapWithStateRDD on whose StateMap data `this` RDD
* will be created
* @param partitionedDataRDD The partitioned data RDD which is used update the previous StateMaps
* in the `prevStateRDD` to create `this` RDD
* @param mappingFunction The function that will be used to update state and return new data
* @param batchTime The time of the batch to which this RDD belongs to. Use to update
* @param timeoutThresholdTime The time to indicate which keys are timeout
*/
private[streaming] class MapWithStateRDD[K: ClassTag, V: ClassTag, S: ClassTag, E: ClassTag](
private var prevStateRDD: RDD[MapWithStateRDDRecord[K, S, E]],
private var partitionedDataRDD: RDD[(K, V)],
mappingFunction: (Time, K, Option[V], State[S]) => Option[E],
batchTime: Time,
timeoutThresholdTime: Option[Long]
) extends RDD[MapWithStateRDDRecord[K, S, E]](
partitionedDataRDD.sparkContext,
List(
new OneToOneDependency[MapWithStateRDDRecord[K, S, E]](prevStateRDD),
new OneToOneDependency(partitionedDataRDD))
) {
每个partition被一个MapWithStateRDDRecord代表的，里面有一个数据结构stateMap，再看此类的重点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)
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)
}
private[streaming] object MapWithStateRDDRecord {
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] = {
// Create a new state map by cloning the previous one (if it exists) or by creating an empty one
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
dataIterator.foreach { case (key, value) =>
wrappedState.wrap(newStateMap.get(key))
val returned = mappingFunction(batchTime, key, Some(value), wrappedState)
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)
}
wrappedState是可以不断被赋值的，mappedData代表最后返回的值。根据当前batch的数据进行计算，更新了newStateMap的数据结构，保存了历史数据，
没有对历史数据进行计算或遍历，只会进行更新、插入操作。Record代表一个partition，MapWithStateRDDRecord中record记录并没改变。
DStream操作RDD，RDD内部变了。所以不可变的rdd可以处理变化的rdd。
Spark Streaming发行版笔记14
