Accumulator Examples

AccumulatorListener.scala

import scala.collection.mutable.Map

import org.apache.spark.{Accumulator, AccumulatorParam, SparkContext}
import org.apache.spark.scheduler.{SparkListenerStageCompleted, SparkListener}
import org.apache.spark.SparkContext._


/**
 * just print out the values for all accumulators from the stage.
 * you will only get updates from *named* accumulators, though
 */
object PrintingAccumulatorListener extends SparkListener {
  override def onStageCompleted(event: SparkListenerStageCompleted): Unit = {
    println("Stage accumulator values:")
    event.stageInfo.accumulables.foreach{ case(id, accInfo) =>
      println(s"$id:${accInfo.name}:${accInfo.value}")
    }
  }
}


/**
 * can run aribtrary callback for accumulators when a stage completes.
 */
class FancyAccumulatorListener(sc: SparkContext) extends SparkListener {
  val accumulatorsToPrevValue: Map[Accumulator[_], Any] = Map()
  val accumulatorsToCallback: Map[Accumulator[_], _ => Unit] = Map()

  //really you would want to add in more variants of this function -- accumulable,
  // accumulableCollection, and all allowing a name as well
  def accumulator[T](
      initialValue: T,
      callback: T => Unit
      )(implicit param: AccumulatorParam[T]): Accumulator[T] = {
    val acc = sc.accumulator(initialValue)(param)
    accumulatorsToPrevValue(acc) = initialValue
    accumulatorsToCallback(acc) = callback
    acc
  }

  override def onStageCompleted(event: SparkListenerStageCompleted): Unit = {
    // ignore the stageInfo, b/c
    // (a) accumulableInfo only gives us the toString of the accumulator and
    // (b) we also want info on unnamed accumulators

    // loop through all the accumulators we know about, and see if they were updated.
    // if so, run their callback.
    // Note that this is assuming there aren't *too* many accumulators, and equals is cheap
    accumulatorsToPrevValue.foreach{case (acc, prevValue) =>
      if(acc.value != prevValue) {
        accumulatorsToCallback(acc).asInstanceOf[Any => Unit](acc.value)
        accumulatorsToPrevValue(acc) = acc.value
      }
    }
  }


}


object ExampleProgram {
  def runSimplePrint(sc: SparkContext): Unit = {
    sc.addSparkListener(PrintingAccumulatorListener)
    val data = sc.parallelize(1 to 100)
    val namedAcc = sc.accumulator(0l, "my accumulator")
    val unnamedAcc = sc.accumulator(0l)
    data.foreach{x =>
      namedAcc += x
      unnamedAcc += x
    }
    //after this is run, you'll see an update for the named accumulator printed out, but the unnamed one is ignored by the listener
  }

  def runFancy(sc: SparkContext): Unit = {
    val accRegister = new FancyAccumulatorListener(sc)
    sc.addSparkListener(accRegister)
    val data = sc.parallelize(1 to 100)
    val acc1 = accRegister.accumulator(0l, {x: Long =>
      println("accumulator #1 has been updated to value " + x)
    })
    val acc2 = accRegister.accumulator(0l, {x: Long =>
      println("Numero dos updated!  now its " + x)
    })
    
    println("running stage 1")
    data.foreach{x =>
      acc1 += x
    }
    println("done w/ stage 1, sleeping")
    //we should see the update for acc1
    
    Thread.sleep(5000)  //just to make it clear what is going on w/ the listener
    
    println("running stage 2")
    data.foreach { x =>
      acc2 += x
    }
    //now we'll see the update for acc2

  }

}

ErrorTracker.scala

// counts errors, and keep one error record for debugging
// WARNING: b/c this uses accumulators, the semantics around counting are *extremely* confusing
// if the RDD ever gets recomputed, do to shared lineage, cache eviction, or stage retries.
// use with caution.

import org.apache.spark.{Accumulator, AccumulatorParam, SparkContext}

class ErrorTracker[T] private(val name: String) extends Serializable {

  var errorCounts = 0L
  var successCounts = 0L
  var errorSample: Option[T] = None

  def ok(): Unit = successCounts += 1

  def error(t: T): Unit = {
    errorCounts += 1
    errorSample = Some(t)
  }

  override def toString(): String = {
    val total = errorCounts + successCounts
    val frac = errorCounts.toDouble / total
    f"$errorCounts%d errors / $total%d total ($frac%2.2f). " +
      s"${errorSample.map{e => s"One random error: $e"}.getOrElse("")}"
  }
}

object ErrorTracker {
  def apply[T](name: String, sc: SparkContext): Accumulator[ErrorTracker[T]] = {
    sc.accumulator(new ErrorTracker[T](name), name)(new ErrorTrackerAccumulator[T])
  }
}

private class ErrorTrackerAccumulator[T] extends AccumulatorParam[ErrorTracker[T]] {
  override def addInPlace(r1: ErrorTracker[T], r2: ErrorTracker[T]): ErrorTracker[T] = {
    r1.errorCounts += r2.errorCounts
    r1.successCounts += r2.successCounts
    if (r1.errorSample.isEmpty) {
      r1.errorSample = r2.errorSample
    }
    r1
  }

  override def zero(initialValue: ErrorTracker[T]): ErrorTracker[T] = initialValue
}

MonoidAccumulators.scala

// uses algebird to help define accumulators over complex types.
// saves you the headache of defining lots of accumulator params
// https://github.com/twitter/algebird

// WARNING -- this makes it really tempting to do things that are NOT scalable.
// accumulators get merged on the *driver*, so if you make big accumulators or have
// a lot of tasks, merging the accumulators on the driver will become a bottleneck

import org.apache.spark.{Accumulator, AccumulatorParam, SparkContext}
import com.twitter.algebird._
import com.twitter.algebird.Operators._

class MonoidAccumulator[T: Monoid](t: T) extends AccumulatorParam[T] {
  val monoid: Monoid[T] = implicitly[Monoid[T]]
  def addInPlace(r1: T, r2: T): T = monoid.plus(r1, r2)
  def zero(initialValue: T): T = monoid.zero
}

val data = sc.parallelize(1 to 100)
// algebird lets you easily define different ways of merging things.  We'll have one
// map that sums the values ... 
// (NB: algebird requires these to be immutable Maps)
val summingMapZero = Map[Int,Int]()
val summingMapAcc = sc.accumulator(summingMapZero)(new MonoidAccumulator(summingMapZero))
// ... and another one that take the max of the values
val maxMapZero = Map[Int, Max[Int]]()
val maxMapAcc = sc.accumulator(maxMapZero)(new MonoidAccumulator(maxMapZero))
data.foreach{x =>
  summingMapAcc += Map((x%10) -> x)
  maxMapAcc += Map((x%10) -> Max(x))
}
println("sums:")
summingMapAcc.value.foreach{println}
println("maxs:")
maxMapAcc.value.foreach{println}

how to use ErrorTracker? can you give a sample, thank you!

Unfortunately PrintingAccumulatorListener will work in spark-shell only. Seems StageInfo does not contain user defined accumulators in other modes