由于最近在项目中需要用到Spark的累加器，同时需要自己去自定义实现Spark的累加器，从而满足生产上的需求。对此，对Spark的累加器实现机制进行了追踪学习。
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本系列文章，将从以下几个方面入手，对Spark累加器进行剖析：
Spark累加器的基本概念
累加器的重点类构成
累加器的源码解析
累加器的执行过程
累加器使用中的坑
自定义累加器的实现
Spark提供的Accumulator，主要用于多个节点对一个变量进行共享性的操作。Accumulator只提供了累加的功能，只能累加，不能减少累加器只能在Driver端构建，并只能从Driver端读取结果，在Task端只能进行累加。
至于这里为什么只能在Task累加呢？下面的内容将会进行详细的介绍，先简单介绍下：
在Task节点，准确的就是说在executor上；
每个Task都会有一个累加器的变量，被序列化传输到executor端运行之后再返回过来都是独立运行的；
如果在Task端去获取值的话，只能获取到当前Task的，Task与Task之间不会有影响累加器不会改变Spark lazy计算的特点，只会在Job触发的时候进行相关的累加操作
现有累加器类型:
class Accumulator extends Accumulable
源码（源码中已经对这个类的作用做了十分详细的解释）：
/**
* A simpler value of [[Accumulable]] where the result type being accumulated is the same
* as the types of elements being merged, i.e. variables that are only "added" to through an
* associative operation and can therefore be efficiently supported in parallel. They can be used
* to implement counters (as in MapReduce) or sums. Spark natively supports accumulators of numeric
* value types, and programmers can add support for new types.
*
* An accumulator is created from an initial value `v` by calling [[SparkContext#accumulator]].
* Tasks running on the cluster can then add to it using the [[Accumulable#+=]] operator.
* However, they cannot read its value. Only the driver program can read the accumulator's value,
* using its value method.
*
* @param initialValue initial value of accumulator
* @param param helper object defining how to add elements of type `T`
* @tparam T result type
*/
class Accumulator[T] private[spark] (
@transient private[spark] val initialValue: T,
param: AccumulatorParam[T],
name: Option[String],
internal: Boolean)
extends Accumulable[T, T](initialValue, param, name, internal) {
def this(initialValue: T, param: AccumulatorParam[T], name: Option[String]) = {
this(initialValue, param, name, false)
}
def this(initialValue: T, param: AccumulatorParam[T]) = {
this(initialValue, param, None, false)
}
}
主要实现了累加器的初始化及封装了相关的累加器操作方法
同时在类对象构建的时候向Accumulators注册累加器
累加器的add操作的返回值类型和传入进去的值类型可以不一样
所以一定要定义好两步操作（即add方法）：累加操作/合并操作object Accumulators
该方法在Driver端管理着累加器，也包含了累加器的聚合操作trait AccumulatorParam[T] extends AccumulableParam[T, T]
源码：
/**
* A simpler version of [[org.apache.spark.AccumulableParam]] where the only data type you can add
* in is the same type as the accumulated value. An implicit AccumulatorParam object needs to be
* available when you create Accumulators of a specific type.
*
* @tparam T type of value to accumulate
*/
trait AccumulatorParam[T] extends AccumulableParam[T, T] {
def addAccumulator(t1: T, t2: T): T = {
addInPlace(t1, t2)
}
}
AccumulatorParam的addAccumulator操作的泛型封装
具体的实现还是需要在具体实现类里面实现addInPlace方法
自定义实现累加器的关键object AccumulatorParam
源码：
object AccumulatorParam {
// The following implicit objects were in SparkContext before 1.2 and users had to
// `import SparkContext._` to enable them. Now we move them here to make the compiler find
// them automatically. However, as there are duplicate codes in SparkContext for backward
// compatibility, please update them accordingly if you modify the following implicit objects.
implicit object DoubleAccumulatorParam extends AccumulatorParam[Double] {
def addInPlace(t1: Double, t2: Double): Double = t1 + t2
def zero(initialValue: Double): Double = 0.0
}
implicit object IntAccumulatorParam extends AccumulatorParam[Int] {
def addInPlace(t1: Int, t2: Int): Int = t1 + t2
def zero(initialValue: Int): Int = 0
}
implicit object LongAccumulatorParam extends AccumulatorParam[Long] {
def addInPlace(t1: Long, t2: Long): Long = t1 + t2
def zero(initialValue: Long): Long = 0L
}
implicit object FloatAccumulatorParam extends AccumulatorParam[Float] {
def addInPlace(t1: Float, t2: Float): Float = t1 + t2
def zero(initialValue: Float): Float = 0f
}
// TODO: Add AccumulatorParams for other types, e.g. lists and strings
}
从源码中大量的implicit关键词，可以发现该类主要进行隐式类型转换的操作TaskContextImpl
在Executor端管理着我们的累加器，累加器是通过该类进行返回的Driver端
accumulator方法
以下列这段代码中的accumulator方法为入口点，进入到相应的源码中去
val acc = new Accumulator(initialValue, param, Some(name))源码：
class Accumulator[T] private[spark] (
@transient private[spark] val initialValue: T,
param: AccumulatorParam[T],
name: Option[String],
internal: Boolean)
extends Accumulable[T, T](initialValue, param, name, internal) {
def this(initialValue: T, param: AccumulatorParam[T], name: Option[String]) = {
this(initialValue, param, name, false)
}
def this(initialValue: T, param: AccumulatorParam[T]) = {
this(initialValue, param, None, false)
}
}继承的Accumulable[T, T]
源码：
class Accumulable[R, T] private[spark] (
initialValue: R,
param: AccumulableParam[R, T],
val name: Option[String],
internal: Boolean)
extends Serializable {
…
// 这里的_value并不支持序列化
// 注：有@transient的都不会被序列化
@volatile @transient private var value_ : R = initialValue // Current value on master
…
// 注册了当前的累加器
Accumulators.register(this)
…,
}Accumulators.register()
源码：
// 传入参数，注册累加器
def register(a: Accumulable[_, _]): Unit = synchronized {
// 构造成WeakReference
originals(a.id) = new WeakReference[Accumulable[_, _]](a)
}至此，Driver端的初始化已经完成
Executor端
Executor端的反序列化是一个得到我们的对象的过程
初始化是在反序列化的时候就完成的，同时反序列化的时候还完成了Accumulator向TaskContextImpl的注册TaskRunner中的run方法
// 在计算的过程中，会将RDD和function经过序列化之后传给Executor端
private[spark] class Executor(
executorId: String,
executorHostname: String,
env: SparkEnv,
userClassPath: Seq[URL] = Nil,
isLocal: Boolean = false)
extends Logging {
...
class TaskRunner(
execBackend: ExecutorBackend,
val taskId: Long,
val attemptNumber: Int,
taskName: String,
serializedTask: ByteBuffer)
extends Runnable {
…
override def run(): Unit = {
…
val (value, accumUpdates) = try {
// 调用TaskRunner中的task.run方法，触发task的运行
val res = task.run(
taskAttemptId = taskId,
attemptNumber = attemptNumber,
metricsSystem = env.metricsSystem)
threwException = false
res
} finally {
…
}
…
}Task中的collectAccumulators()方法
private[spark] abstract class Task[T](
final def run(
taskAttemptId: Long,
attemptNumber: Int,
metricsSystem: MetricsSystem)
: (T, AccumulatorUpdates) = {
…
try {
// 返回累加器，并运行task
// 调用TaskContextImpl的collectAccumulators，返回值的类型为一个Map
(runTask(context), context.collectAccumulators())
} finally {
…
}
…
}
)ResultTask中的runTask方法
override def runTask(context: TaskContext): U = {
// Deserialize the RDD and the func using the broadcast variables.
val deserializeStartTime = System.currentTimeMillis()
val ser = SparkEnv.get.closureSerializer.newInstance()
// 反序列化是在调用ResultTask的runTask方法的时候做的
// 会反序列化出来RDD和自己定义的function
val (rdd, func) = ser.deserialize[(RDD[T], (TaskContext, Iterator[T]) => U)](
ByteBuffer.wrap(taskBinary.value), Thread.currentThread.getContextClassLoader)
_executorDeserializeTime = System.currentTimeMillis() - deserializeStartTime
metrics = Some(context.taskMetrics)
func(context, rdd.iterator(partition, context))
}Accumulable中的readObject方法
// 在反序列化的过程中会调用Accumulable.readObject方法
// Called by Java when deserializing an object
private def readObject(in: ObjectInputStream): Unit = Utils.tryOrIOException {
in.defaultReadObject()
// value的初始值为zero；该值是会被序列化的
value_ = zero
deserialized = true
// Automatically register the accumulator when it is deserialized with the task closure.
//
// Note internal accumulators sent with task are deserialized before the TaskContext is created
// and are registered in the TaskContext constructor. Other internal accumulators, such SQL
// metrics, still need to register here.
val taskContext = TaskContext.get()
if (taskContext != null) {
// 当前反序列化所得到的对象会被注册到TaskContext中
// 这样TaskContext就可以获取到累加器
// 任务运行结束之后，就可以通过context.collectAccumulators()返回给executor
taskContext.registerAccumulator(this)
}
}Executor.scala
// 在executor端拿到accumuUpdates值之后，会去构造一个DirectTaskResult
val directResult = new DirectTaskResult(valueBytes, accumUpdates, task.metrics.orNull)
val serializedDirectResult = ser.serialize(directResult)
val resultSize = serializedDirectResult.limit
…
// 最终由ExecutorBackend的statusUpdate方法发送至Driver端
// ExecutorBackend为一个Trait，有多种实现
execBackend.statusUpdate(taskId, TaskState.FINISHED, serializedResult)CoarseGrainedExecutorBackend中的statusUpdate方法
// 通过ExecutorBackend的一个实现类：CoarseGrainedExecutorBackend 中的statusUpdate方法
// 将数据发送至Driver端
override def statusUpdate(taskId: Long, state: TaskState, data: ByteBuffer) {
val msg = StatusUpdate(executorId, taskId, state, data)
driver match {
case Some(driverRef) => driverRef.send(msg)
case None => logWarning(s"Drop $msg because has not yet connected to driver")
}
}CoarseGrainedSchedulerBackend中的receive方法
// Driver端在接收到消息之后，会调用CoarseGrainedSchedulerBackend中的receive方法
override def receive: PartialFunction[Any, Unit] = {
case StatusUpdate(executorId, taskId, state, data) =>
// 会在DAGScheduler的handleTaskCompletion方法中将结果返回
scheduler.statusUpdate(taskId, state, data.value)
…
}TaskSchedulerImpl的statusUpdate方法
def statusUpdate(tid: Long, state: TaskState, serializedData: ByteBuffer) {
…
if (state == TaskState.FINISHED) {
taskSet.removeRunningTask(tid)
// 将成功的Task入队
taskResultGetter.enqueueSuccessfulTask(taskSet, tid, serializedData)
} else if (Set(TaskState.FAILED, TaskState.KILLED, TaskState.LOST).contains(state)) {
taskSet.removeRunningTask(tid)
taskResultGetter.enqueueFailedTask(taskSet, tid, state, serializedData)
}
…
}TaskResultGetter的enqueueSuccessfulTask方法
def enqueueSuccessfulTask(taskSetManager: TaskSetManager, tid: Long, serializedData: ByteBuffer) {
…
result.metrics.setResultSize(size)
scheduler.handleSuccessfulTask(taskSetManager, tid, result)
…TaskSchedulerImpl的handleSuccessfulTask方法
def handleSuccessfulTask(
taskSetManager: TaskSetManager,
tid: Long,
taskResult: DirectTaskResult[_]): Unit = synchronized {
taskSetManager.handleSuccessfulTask(tid, taskResult)
}DAGScheduler的taskEnded方法
def taskEnded(
task: Task[_],
reason: TaskEndReason,
result: Any,
accumUpdates: Map[Long, Any],
taskInfo: TaskInfo,
taskMetrics: TaskMetrics): Unit = {
eventProcessLoop.post(
// 给自身的消息循环体发了个CompletionEvent
// 这个CompletionEvent会被handleTaskCompletion方法所接收到
CompletionEvent(task, reason, result, accumUpdates, taskInfo, taskMetrics))
}DAGScheduler的handleTaskCompletion方法
// 与上述CoarseGrainedSchedulerBackend中的receive方法章节对应
// 在handleTaskCompletion方法中，接收CompletionEvent
// 不论是ResultTask还是ShuffleMapTask都会去调用updateAccumulators方法，更新累加器的值
private[scheduler] def handleTaskCompletion(event: CompletionEvent) {
…
event.reason match {
case Success =>
listenerBus.post(SparkListenerTaskEnd(stageId, stage.latestInfo.attemptId, taskType,
event.reason, event.taskInfo, event.taskMetrics))
stage.pendingPartitions -= task.partitionId
task match {
case rt: ResultTask[_, _] =>
// Cast to ResultStage here because it's part of the ResultTask
// TODO Refactor this out to a function that accepts a ResultStage
val resultStage = stage.asInstanceOf[ResultStage]
resultStage.activeJob match {
case Some(job) =>
if (!job.finished(rt.outputId)) {
updateAccumulators(event)
case smt: ShuffleMapTask =>
val shuffleStage = stage.asInstanceOf[ShuffleMapStage]
updateAccumulators(event)
}
…
}DAGScheduler的updateAccumulators方法
private def updateAccumulators(event: CompletionEvent): Unit = {
val task = event.task
val stage = stageIdToStage(task.stageId)
if (event.accumUpdates != null) {
try {
// 调用了累加器的add方法
Accumulators.add(event.accumUpdates)Accumulators的add方法
def add(values: Map[Long, Any]): Unit = synchronized {
// 遍历传进来的值
for ((id, value) <- values) {
if (originals.contains(id)) {
// Since we are now storing weak references, we must check whether the underlying data
// is valid.
// 根据id从注册的Map中取出对应的累加器
originals(id).get match {
// 将值给累加起来，最终将结果加到value里面
// ++=是被重载了
case Some(accum) => accum.asInstanceOf[Accumulable[Any, Any]] ++= value
case None =>
throw new IllegalAccessError("Attempted to access garbage collected Accumulator.")
}
} else {
logWarning(s"Ignoring accumulator update for unknown accumulator id $id")
}
}
}Accumulators的++=方法
def ++= (term: R) { value_ = param.addInPlace(value_, term)}Accumulators的value方法
def value: R = {
if (!deserialized) {
value_
} else {
throw new UnsupportedOperationException("Can't read accumulator value in task")
}
}此时我们的应用程序就可以通过 .value 的方式去获取计数器的值了