[SPARK-14654][CORE] New accumulator API

## What changes were proposed in this pull request?

This PR introduces a new accumulator API  which is much simpler than before:

1. the type hierarchy is simplified, now we only have an `Accumulator` class
2. Combine `initialValue` and `zeroValue` concepts into just one concept: `zeroValue`
3. there in only one `register` method, the accumulator registration and cleanup registration are combined.
4. the `id`,`name` and `countFailedValues` are combined into an `AccumulatorMetadata`, and is provided during registration.

`SQLMetric` is a good example to show the simplicity of this new API.

What we break:

1. no `setValue` anymore. In the new API, the intermedia type can be different from the result type, it's very hard to implement a general `setValue`
2. accumulator can't be serialized before registered.

Problems need to be addressed in follow-ups:

1. with this new API, `AccumulatorInfo` doesn't make a lot of sense, the partial output is not partial updates, we need to expose the intermediate value.
2. `ExceptionFailure` should not carry the accumulator updates. Why do users care about accumulator updates for failed cases? It looks like we only use this feature to update the internal metrics, how about we sending a heartbeat to update internal metrics after the failure event?
3. the public event `SparkListenerTaskEnd` carries a `TaskMetrics`. Ideally this `TaskMetrics` don't need to carry external accumulators, as the only method of `TaskMetrics` that can access external accumulators is `private[spark]`. However, `SQLListener` use it to retrieve sql metrics.

## How was this patch tested?

existing tests

Author: Wenchen Fan <[email protected]>

Closes #12612 from cloud-fan/acc.

Author: cloud-fan

core/src/main/scala/org/apache/spark/Accumulable.scala

@@ -63,7 +63,7 @@ class Accumulable[R, T] private (
       param: AccumulableParam[R, T],
       name: Option[String],
       countFailedValues: Boolean) = {
-    this(Accumulators.newId(), initialValue, param, name, countFailedValues)
+    this(AccumulatorContext.newId(), initialValue, param, name, countFailedValues)
   }
 
   private[spark] def this(initialValue: R, param: AccumulableParam[R, T], name: Option[String]) = {
@@ -72,61 +72,44 @@ class Accumulable[R, T] private (
 
   def this(initialValue: R, param: AccumulableParam[R, T]) = this(initialValue, param, None)
 
-  @volatile @transient private var value_ : R = initialValue // Current value on driver
-  val zero = param.zero(initialValue) // Zero value to be passed to executors
-  private var deserialized = false
-
-  Accumulators.register(this)
-
-  /**
-   * Return a copy of this [[Accumulable]].
-   *
-   * The copy will have the same ID as the original and will not be registered with
-   * [[Accumulators]] again. This method exists so that the caller can avoid passing the
-   * same mutable instance around.
-   */
-  private[spark] def copy(): Accumulable[R, T] = {
-    new Accumulable[R, T](id, initialValue, param, name, countFailedValues)
-  }
+  val zero = param.zero(initialValue)
+  private[spark] val newAcc = new LegacyAccumulatorWrapper(initialValue, param)
+  newAcc.metadata = AccumulatorMetadata(id, name, countFailedValues)
+  // Register the new accumulator in ctor, to follow the previous behaviour.
+  AccumulatorContext.register(newAcc)
 
   /**
    * Add more data to this accumulator / accumulable
    * @param term the data to add
    */
-  def += (term: T) { value_ = param.addAccumulator(value_, term) }
+  def += (term: T) { newAcc.add(term) }
 
   /**
    * Add more data to this accumulator / accumulable
    * @param term the data to add
    */
-  def add(term: T) { value_ = param.addAccumulator(value_, term) }
+  def add(term: T) { newAcc.add(term) }
 
   /**
    * Merge two accumulable objects together
    *
    * Normally, a user will not want to use this version, but will instead call `+=`.
    * @param term the other `R` that will get merged with this
    */
-  def ++= (term: R) { value_ = param.addInPlace(value_, term)}
+  def ++= (term: R) { newAcc._value = param.addInPlace(newAcc._value, term) }
 
   /**
    * Merge two accumulable objects together
    *
    * Normally, a user will not want to use this version, but will instead call `add`.
    * @param term the other `R` that will get merged with this
    */
-  def merge(term: R) { value_ = param.addInPlace(value_, term)}
+  def merge(term: R) { newAcc._value = param.addInPlace(newAcc._value, term) }
 
   /**
    * Access the accumulator's current value; only allowed on driver.
    */
-  def value: R = {
-    if (!deserialized) {
-      value_
-    } else {
-      throw new UnsupportedOperationException("Can't read accumulator value in task")
-    }
-  }
+  def value: R = newAcc.value
 
   /**
    * Get the current value of this accumulator from within a task.
@@ -137,14 +120,14 @@ class Accumulable[R, T] private (
    * The typical use of this method is to directly mutate the local value, eg., to add
    * an element to a Set.
    */
-  def localValue: R = value_
+  def localValue: R = newAcc.localValue
 
   /**
    * Set the accumulator's value; only allowed on driver.
    */
   def value_= (newValue: R) {
-    if (!deserialized) {
-      value_ = newValue
+    if (newAcc.isAtDriverSide) {
+      newAcc._value = newValue
     } else {
       throw new UnsupportedOperationException("Can't assign accumulator value in task")
     }
@@ -153,7 +136,7 @@ class Accumulable[R, T] private (
   /**
    * Set the accumulator's value. For internal use only.
    */
-  def setValue(newValue: R): Unit = { value_ = newValue }
+  def setValue(newValue: R): Unit = { newAcc._value = newValue }
 
   /**
    * Set the accumulator's value. For internal use only.
@@ -168,22 +151,7 @@ class Accumulable[R, T] private (
     new AccumulableInfo(id, name, update, value, isInternal, countFailedValues)
   }
 
-  // Called by Java when deserializing an object
-  private def readObject(in: ObjectInputStream): Unit = Utils.tryOrIOException {
-    in.defaultReadObject()
-    value_ = zero
-    deserialized = true
-
-    // Automatically register the accumulator when it is deserialized with the task closure.
-    // This is for external accumulators and internal ones that do not represent task level
-    // metrics, e.g. internal SQL metrics, which are per-operator.
-    val taskContext = TaskContext.get()
-    if (taskContext != null) {
-      taskContext.registerAccumulator(this)
-    }
-  }
-
-  override def toString: String = if (value_ == null) "null" else value_.toString
+  override def toString: String = if (newAcc._value == null) "null" else newAcc._value.toString
 }
 
 

core/src/main/scala/org/apache/spark/Accumulator.scala

@@ -68,73 +68,6 @@ class Accumulator[T] private[spark] (
   extends Accumulable[T, T](initialValue, param, name, countFailedValues)
 
 
-// TODO: The multi-thread support in accumulators is kind of lame; check
-// if there's a more intuitive way of doing it right
-private[spark] object Accumulators extends Logging {
-  /**
-   * This global map holds the original accumulator objects that are created on the driver.
-   * It keeps weak references to these objects so that accumulators can be garbage-collected
-   * once the RDDs and user-code that reference them are cleaned up.
-   * TODO: Don't use a global map; these should be tied to a SparkContext (SPARK-13051).
-   */
-  @GuardedBy("Accumulators")
-  val originals = mutable.Map[Long, WeakReference[Accumulable[_, _]]]()
-
-  private val nextId = new AtomicLong(0L)
-
-  /**
-   * Return a globally unique ID for a new [[Accumulable]].
-   * Note: Once you copy the [[Accumulable]] the ID is no longer unique.
-   */
-  def newId(): Long = nextId.getAndIncrement
-
-  /**
-   * Register an [[Accumulable]] created on the driver such that it can be used on the executors.
-   *
-   * All accumulators registered here can later be used as a container for accumulating partial
-   * values across multiple tasks. This is what [[org.apache.spark.scheduler.DAGScheduler]] does.
-   * Note: if an accumulator is registered here, it should also be registered with the active
-   * context cleaner for cleanup so as to avoid memory leaks.
-   *
-   * If an [[Accumulable]] with the same ID was already registered, this does nothing instead
-   * of overwriting it. This happens when we copy accumulators, e.g. when we reconstruct
-   * [[org.apache.spark.executor.TaskMetrics]] from accumulator updates.
-   */
-  def register(a: Accumulable[_, _]): Unit = synchronized {
-    if (!originals.contains(a.id)) {
-      originals(a.id) = new WeakReference[Accumulable[_, _]](a)
-    }
-  }
-
-  /**
-   * Unregister the [[Accumulable]] with the given ID, if any.
-   */
-  def remove(accId: Long): Unit = synchronized {
-    originals.remove(accId)
-  }
-
-  /**
-   * Return the [[Accumulable]] registered with the given ID, if any.
-   */
-  def get(id: Long): Option[Accumulable[_, _]] = synchronized {
-    originals.get(id).map { weakRef =>
-      // Since we are storing weak references, we must check whether the underlying data is valid.
-      weakRef.get.getOrElse {
-        throw new IllegalAccessError(s"Attempted to access garbage collected accumulator $id")
-      }
-    }
-  }
-
-  /**
-   * Clear all registered [[Accumulable]]s. For testing only.
-   */
-  def clear(): Unit = synchronized {
-    originals.clear()
-  }
-
-}
-
-
 /**
  * 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

core/src/main/scala/org/apache/spark/ContextCleaner.scala

@@ -144,7 +144,7 @@ private[spark] class ContextCleaner(sc: SparkContext) extends Logging {
     registerForCleanup(rdd, CleanRDD(rdd.id))
   }
 
-  def registerAccumulatorForCleanup(a: Accumulable[_, _]): Unit = {
+  def registerAccumulatorForCleanup(a: NewAccumulator[_, _]): Unit = {
     registerForCleanup(a, CleanAccum(a.id))
   }
 
@@ -241,7 +241,7 @@ private[spark] class ContextCleaner(sc: SparkContext) extends Logging {
   def doCleanupAccum(accId: Long, blocking: Boolean): Unit = {
     try {
       logDebug("Cleaning accumulator " + accId)
-      Accumulators.remove(accId)
+      AccumulatorContext.remove(accId)
       listeners.asScala.foreach(_.accumCleaned(accId))
       logInfo("Cleaned accumulator " + accId)
     } catch {

core/src/main/scala/org/apache/spark/HeartbeatReceiver.scala

@@ -35,7 +35,7 @@ import org.apache.spark.util.{Clock, SystemClock, ThreadUtils, Utils}
  */
 private[spark] case class Heartbeat(
     executorId: String,
-    accumUpdates: Array[(Long, Seq[AccumulableInfo])], // taskId -> accum updates
+    accumUpdates: Array[(Long, Seq[NewAccumulator[_, _]])], // taskId -> accumulator updates
     blockManagerId: BlockManagerId)
 
 /**