runners/google-cloud-dataflow-java/worker/src/main/java/org/apache/beam/runners/dataflow/worker/DataflowElementExecutionTracker.java - beam - Git at Google

 /*
  * Licensed to the Apache Software Foundation (ASF) under one
  * or more contributor license agreements.  See the NOTICE file
  * distributed with this work for additional information
  * regarding copyright ownership.  The ASF licenses this file
  * to you under the Apache License, Version 2.0 (the
  * "License"); you may not use this file except in compliance
  * with the License.  You may obtain a copy of the License at
  *
  *     http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
 package org.apache.beam.runners.dataflow.worker;

 import static org.apache.beam.vendor.guava.v20_0.com.google.common.base.Preconditions.checkState;

 import java.time.Duration;
 import java.util.ArrayDeque;
 import java.util.Deque;
 import java.util.HashMap;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
 import java.util.NoSuchElementException;
 import java.util.Optional;
 import java.util.concurrent.ConcurrentLinkedQueue;
 import org.apache.beam.runners.core.metrics.ExecutionStateSampler;
 import org.apache.beam.runners.dataflow.options.DataflowPipelineDebugOptions;
 import org.apache.beam.runners.dataflow.worker.counters.Counter;
 import org.apache.beam.runners.dataflow.worker.counters.CounterFactory;
 import org.apache.beam.runners.dataflow.worker.counters.NameContext;
 import org.apache.beam.runners.dataflow.worker.util.common.worker.ElementExecutionTracker;
 import org.apache.beam.sdk.options.PipelineOptions;
 import org.apache.beam.vendor.guava.v20_0.com.google.common.annotations.VisibleForTesting;
 import org.apache.beam.vendor.guava.v20_0.com.google.common.base.MoreObjects;
 import org.apache.beam.vendor.guava.v20_0.com.google.common.collect.Iterators;
 import org.apache.beam.vendor.guava.v20_0.com.google.common.collect.PeekingIterator;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;

 /**
  * Dataflow-specific implementation of {@link ElementExecutionTracker}.
  *
  * <p>Each processed element through each step in the fused stage gets tracked as a {@code
  * ElementExecution}. Processing time is sampled using {@link ExecutionStateSampler} and distributed
  * amongst all elements which are executed since the last sample period.
  *
  * <p>Elements are processed in potentially many fragments of execution as we move up and down the
  * stage graph via outputting. Each fragment of execution is counted equally for attributing sampled
  * processing time.
  *
  * <p>When an element is finished processing it is held in the {@code doneJournal} collection until
  * the next sampling round in order to calculate the final processing time. Eventually the total
  * element processing time is reported to the counter and the state is cleaned up.
  *
  * <p>This is not implemented in {@link ElementExecutionTracker} itself because it uses
  * Beam/Dataflow-specific classes, such as {@link PipelineOptions} which cannot be shared with
  * Flume.
  */
 public class DataflowElementExecutionTracker extends ElementExecutionTracker {
   @VisibleForTesting
   public static final String TIME_PER_ELEMENT_EXPERIMENT = "time_per_element_counter";

   private static final Logger LOG = LoggerFactory.getLogger(DataflowElementExecutionTracker.class);

   public static ElementExecutionTracker create(
       CounterFactory counterFactory, PipelineOptions options) {
     // TODO: Remove once feature has launched.
     if (!hasExperiment(options, TIME_PER_ELEMENT_EXPERIMENT)) {
       return ElementExecutionTracker.NO_OP_INSTANCE;
     }

     // TODO: Remove log statement when functionality is enabled by default.
     LOG.info("{} counter enabled.", ElementExecutionTracker.COUNTER_NAME);

     return new DataflowElementExecutionTracker(counterFactory);
   }

   private static boolean hasExperiment(PipelineOptions options, String experiment) {
     List<String> experiments = options.as(DataflowPipelineDebugOptions.class).getExperiments();
     return experiments != null && experiments.contains(experiment);
   }

   /**
    * Tracking object for the execution of a single input element in a step.
    *
    * <p>Each {@link ElementExecution} instance represents a distinct element, implicitly represented
    * by the instance identity. As such, {@link ElementExecution} instances compare using reference
    * equality rather than value equality.
    */
   private static class ElementExecution {
     /** Marker execution to represent when there is no element currently being processed. */
     static final ElementExecution IDLE = new ElementExecution();

     /** Only empty for {@see IDLE}. */
     final Optional<NameContext> step;

     ElementExecution(NameContext step) {
       this.step = Optional.of(step);
     }

     /** Only used for {@see IDLE}. */
     private ElementExecution() {
       step = Optional.empty();
     }

     @Override
     public String toString() {
       if (this == ElementExecution.IDLE) {
         return "IDLE_EXECUTION";
       }

       return MoreObjects.toStringHelper(this)
           .add("step", step)
           .add("executionId", System.identityHashCode(this))
           .toString();
     }
   }

   private final ExecutionJournalWriter executionWriter;
   private final ExecutionJournalReader executionReader;

   private DataflowElementExecutionTracker(CounterFactory counterFactory) {
     ReaderWriterState readerWriterState = new ReaderWriterState();
     executionWriter = new ExecutionJournalWriter(readerWriterState);
     executionReader = new ExecutionJournalReader(counterFactory, readerWriterState);
   }

   @Override
   public void enter(NameContext step) {
     executionWriter.startProcessing(step);
   }

   @Override
   public void exit() {
     executionWriter.doneProcessing();
   }

   @Override
   public void takeSample(long millisSinceLastSample) {
     executionReader.takeSample(Duration.ofMillis(millisSinceLastSample));
   }

   /** State shared between the {@link ExecutionJournalWriter} and {@link ExecutionJournalReader}. */
   private static class ReaderWriterState {
     /**
      * Journal of fragments of execution per element to count for attributing processing time. Each
      * time we transition up or down the stage fusion graph we add an execution fragment for the
      * currently processing element with an incremented snapshot version. Each snapshot version must
      * have a representative value in the {@code executionJournal}, or {@see IDLE_EXECUTION} to
      * represent completion of processing.
      */
     private final Journal<ElementExecution> executionJournal;

     /**
      * Elements which have completed processing but are still pending final timing calculation. The
      * elements are moved here when they exit the {@code executionStack} and are held here until the
      * counter value can be reported in the next sampling round.
      */
     private final Journal<ElementExecution> doneJournal;

     /**
      * Monotonically increasing snapshot version number which tracks the latest execution state
      * ready to be reported. Snapshot versions are associated with values in the {@code
      * executionJournal} and {@code doneJournal}.
      *
      * <p>This value written by the single {@link ExecutionJournalWriter} thread, and read by the
      * {@link ExecutionJournalReader} thread.
      */
     private volatile long latestSnapshot;

     private ReaderWriterState() {
       executionJournal = new Journal<>();
       doneJournal = new Journal<>();
       latestSnapshot = 0L;
     }
   }

   /**
    * Writes journal entries on element processing state changes.
    *
    * <p>Write operations are executed by a single thread inside the stage operation hot loop and
    * must be highly performant. {@link ExecutionJournalWriter} methods are thread-safe only for
    * state shared with the {@link ExecutionJournalReader} thread.
    */
   private static class ExecutionJournalWriter {
     /**
      * Execution stack of processing elements. Elements are pushed onto the stack when we enter the
      * process() function and popped on process() return. This stack mirrors the actual Java runtime
      * stack and contains the step + element context in order to attribute sampled execution time.
      */
     private final Deque<ElementExecution> executionStack;

     private final ReaderWriterState sharedState;

     public ExecutionJournalWriter(ReaderWriterState sharedState) {
       this.sharedState = sharedState;
       this.executionStack = new ArrayDeque<>();

       addExecution(ElementExecution.IDLE);
     }

     /** Create and journal a new {@link ElementExecution} to track a processing element. */
     public void startProcessing(NameContext step) {
       ElementExecution execution = new ElementExecution(step);
       addExecution(execution);
     }

     private void addExecution(ElementExecution execution) {
       long nextSnapshot = sharedState.latestSnapshot + 1;
       executionStack.addLast(execution);
       sharedState.executionJournal.add(execution, nextSnapshot);
       sharedState.latestSnapshot = nextSnapshot;
     }

     /**
      * Indicates that the execution thread has exited the process method for an element.
      *
      * <p>When an element is finished processing, it is popped from the execution stack, but will be
      * tracked in the {@code doneJournal} collection until the next sampling round in order to
      * account timing for the final fragment of execution.
      */
     public void doneProcessing() {
       // The execution stack in initialized with the IDLE execution which should never be removed.
       checkState(executionStack.size() > 1, "No processing elements currently tracked.");

       ElementExecution execution = executionStack.removeLast();
       long nextSnapshot = sharedState.latestSnapshot + 1;
       sharedState.doneJournal.add(execution, nextSnapshot);
       ElementExecution nextElement = executionStack.getLast();
       sharedState.executionJournal.add(nextElement, nextSnapshot);
       sharedState.latestSnapshot = nextSnapshot;
     }
   }

   /**
    * Accounts sampled time to processed elements based on execution journal entries.
    *
    * <p>Read operations are executed by a single thread in charge of state sampling, and are
    * generally called more infrequently than {@link ExecutionJournalWriter} operations. {@link
    * ExecutionJournalReader} methods are thread-safe only for state shared with the {@link
    * ExecutionJournalWriter} thread.
    */
   private static class ExecutionJournalReader {
     /**
      * Accumulated execution time per element. Once an element has finished processing and execution
      * time has been attributed, the total execution time is reported via the counter and removed
      * from the collection.
      */
     private final Map<ElementExecution, Duration> executionTimes;

     private final CounterFactory counterFactory;
     private final ReaderWriterState sharedState;

     /** Cache of per-step distribution counters. */
     private final Map<NameContext, Counter<Long, ?>> counterCache;

     public ExecutionJournalReader(CounterFactory counterFactory, ReaderWriterState sharedState) {
       this.sharedState = sharedState;
       executionTimes = new HashMap<>();
       this.counterFactory = counterFactory;
       counterCache = new HashMap<>();
     }

     /**
      * Account the specified processing time duration to elements which have processed since the
      * last sampling round, and report counters for completed elements.
      */
     public void takeSample(Duration sampleTime) {
       long snapshot = sharedState.latestSnapshot;
       attributeProcessingTime(sampleTime, snapshot);

       // TODO: If possible, this should report tentative counter values before they are
       // finalized. Currently, root steps in a stage fusion will not have counters reported until an
       // element is completely processed through the stage subgraph.
       reportCounters(snapshot);
       pruneJournals(snapshot);
     }

     /**
      * Attribute processing time to elements from {@code executionJournal} up to the specified
      * snapshot.
      */
     private void attributeProcessingTime(Duration duration, long snapshot) {
       // TODO: This algorithm is used to compute "per-element-processing-time" counter
       // values, but a slightly different algorithm is used for msec counters. Values for both
       // counters should be derived from the same algorithm to avoid unexpected discrepancies.

       // Calculate total execution counts
       int totalExecutions = 0;
       Map<ElementExecution, Integer> executionsPerElement = new HashMap<>();
       for (ElementExecution execution : sharedState.executionJournal.readUntil(snapshot)) {
         totalExecutions++;
         if (execution != ElementExecution.IDLE) {
           executionsPerElement.compute(execution, (unused, count) -> count == null ? 1 : count + 1);
         }
       }

       // Attribute processing time
       final int totalExecutionsFinal = totalExecutions;
       for (Map.Entry<ElementExecution, Integer> executionCount : executionsPerElement.entrySet()) {
         executionTimes.compute(
             executionCount.getKey(),
             (unused, total) -> {
               int numExecutions = executionCount.getValue();
               Duration attributedSampleTime =
                   duration.dividedBy(totalExecutionsFinal).multipliedBy(numExecutions);
               return total == null ? attributedSampleTime : total.plus(attributedSampleTime);
             });
       }
     }

     /** Report counter values for done elements up to the given snapshot. */
     private void reportCounters(long snapshot) {
       // Report counter values for completed elements
       for (ElementExecution execution : sharedState.doneJournal.readUntil(snapshot)) {
         checkState(
             execution.step.isPresent(),
             "Unexpected execution in doneJournal with empty step: %s",
             execution);
         Counter<Long, ?> counter = getCounter(execution.step.get());
         counter.addValue(executionTimes.get(execution).toMillis());
       }
     }

     /** Retrieve the per-element processing time counter for the specified step. */
     private Counter<Long, ?> getCounter(NameContext step) {
       return counterCache.computeIfAbsent(
           step,
           s ->
               counterFactory.distribution(
                   ElementExecutionTracker.COUNTER_NAME.withOriginalName(s)));
     }

     /**
      * Prune journal entries which have been accounted.
      *
      * @param snapshot Snapshot value which has been accounted.
      */
     private void pruneJournals(long snapshot) {
       sharedState.doneJournal.pruneUntil(snapshot);

       // Keep the currently executing element in the journal so its remaining execution time
       // is counted in the next sampling round.
       sharedState.executionJournal.pruneUntil(snapshot - 1);
     }
   }

   /**
    * Concurrent queue-based data structure for passing journaled events between a single journal
    * reader and single journal writer.
    *
    * <p>Each event is journaled with an externally-managed snapshot version. Snapshot versions are
    * unique and monotonically increasing.
    */
   private static class Journal<T> {
     private final ConcurrentLinkedQueue<SnapshottedItem<T>> queue;
     private long maxSnapshot;

     public Journal() {
       queue = new ConcurrentLinkedQueue<>();
       maxSnapshot = Long.MIN_VALUE;
     }

     /** Add a new event to the journal with the given snapshot. */
     public void add(T item, long snapshot) {
       if (snapshot <= maxSnapshot) {
         throw new IllegalArgumentException(
             String.format(
                 "Timestamps must be monotonically increasing. "
                     + "Specified snapshot '%d' is not greater than max snapshot '%d'",
                 snapshot, maxSnapshot));
       }
       queue.add(new SnapshottedItem<>(item, snapshot));
       maxSnapshot = snapshot;
     }

     /**
      * Retrieve an iterable of events from the journal up to and including the specified snapshot
      * version.
      *
      * <p>Returns an empty iterable if the journal is empty or there are no events less than the
      * specified snapshot version.
      *
      * <p>The returned iterable returns a view of the queue at the time of construction, consistent
      * with {@link ConcurrentLinkedQueue#iterator()}.
      */
     public Iterable<T> readUntil(long snapshot) {
       // Implement our own iterator in order to short-circuit early once we've seen a snapshot
       // version greater than the value specified.
       // Streams will have takeWhile() in JDK9, and Guava may some day have
       // Iterables.limit(Predicate<T>): https://github.com/google/guava/issues/477
       PeekingIterator<SnapshottedItem<T>> iter = Iterators.peekingIterator(queue.iterator());
       return () ->
           new Iterator<T>() {
             @Override
             public boolean hasNext() {
               return iter.hasNext() && iter.peek().snapshot <= snapshot;
             }

             @Override
             public T next() {
               SnapshottedItem<T> next = iter.next();
               checkValidSnapshot(next);
               return next.item;
             }

             @Override
             public void remove() {
               iter.remove();
             }

             private void checkValidSnapshot(SnapshottedItem<T> next) {
               if (next.snapshot > snapshot) {
                 throw new NoSuchElementException();
               }
             }
           };
     }

     /** Prune events from the journal up to and including the specified snapshot. */
     public void pruneUntil(long snapshot) {
       Iterator<SnapshottedItem<T>> iterator = queue.iterator();
       while (iterator.hasNext()) {
         if (iterator.next().snapshot <= snapshot) {
           iterator.remove();
         } else {
           break;
         }
       }
     }

     private static class SnapshottedItem<E> {
       private final E item;
       private final long snapshot;

       public SnapshottedItem(E item, long snapshot) {
         this.item = item;
         this.snapshot = snapshot;
       }
     }
   }
 }
	/*
	* Licensed to the Apache Software Foundation (ASF) under one
	* or more contributor license agreements. See the NOTICE file
	* distributed with this work for additional information
	* regarding copyright ownership. The ASF licenses this file
	* to you under the Apache License, Version 2.0 (the
	* "License"); you may not use this file except in compliance
	* with the License. You may obtain a copy of the License at
	*
	* http://www.apache.org/licenses/LICENSE-2.0
	*
	* Unless required by applicable law or agreed to in writing, software
	* distributed under the License is distributed on an "AS IS" BASIS,
	* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	* See the License for the specific language governing permissions and
	* limitations under the License.
	*/
	package org.apache.beam.runners.dataflow.worker;

	import static org.apache.beam.vendor.guava.v20_0.com.google.common.base.Preconditions.checkState;

	import java.time.Duration;
	import java.util.ArrayDeque;
	import java.util.Deque;
	import java.util.HashMap;
	import java.util.Iterator;
	import java.util.List;
	import java.util.Map;
	import java.util.NoSuchElementException;
	import java.util.Optional;
	import java.util.concurrent.ConcurrentLinkedQueue;
	import org.apache.beam.runners.core.metrics.ExecutionStateSampler;
	import org.apache.beam.runners.dataflow.options.DataflowPipelineDebugOptions;
	import org.apache.beam.runners.dataflow.worker.counters.Counter;
	import org.apache.beam.runners.dataflow.worker.counters.CounterFactory;
	import org.apache.beam.runners.dataflow.worker.counters.NameContext;
	import org.apache.beam.runners.dataflow.worker.util.common.worker.ElementExecutionTracker;
	import org.apache.beam.sdk.options.PipelineOptions;
	import org.apache.beam.vendor.guava.v20_0.com.google.common.annotations.VisibleForTesting;
	import org.apache.beam.vendor.guava.v20_0.com.google.common.base.MoreObjects;
	import org.apache.beam.vendor.guava.v20_0.com.google.common.collect.Iterators;
	import org.apache.beam.vendor.guava.v20_0.com.google.common.collect.PeekingIterator;
	import org.slf4j.Logger;
	import org.slf4j.LoggerFactory;

	/**
	* Dataflow-specific implementation of {@link ElementExecutionTracker}.
	*
	* <p>Each processed element through each step in the fused stage gets tracked as a {@code
	* ElementExecution}. Processing time is sampled using {@link ExecutionStateSampler} and distributed
	* amongst all elements which are executed since the last sample period.
	*
	* <p>Elements are processed in potentially many fragments of execution as we move up and down the
	* stage graph via outputting. Each fragment of execution is counted equally for attributing sampled
	* processing time.
	*
	* <p>When an element is finished processing it is held in the {@code doneJournal} collection until
	* the next sampling round in order to calculate the final processing time. Eventually the total
	* element processing time is reported to the counter and the state is cleaned up.
	*
	* <p>This is not implemented in {@link ElementExecutionTracker} itself because it uses
	* Beam/Dataflow-specific classes, such as {@link PipelineOptions} which cannot be shared with
	* Flume.
	*/
	public class DataflowElementExecutionTracker extends ElementExecutionTracker {
	@VisibleForTesting
	public static final String TIME_PER_ELEMENT_EXPERIMENT = "time_per_element_counter";

	private static final Logger LOG = LoggerFactory.getLogger(DataflowElementExecutionTracker.class);

	public static ElementExecutionTracker create(
	CounterFactory counterFactory, PipelineOptions options) {
	// TODO: Remove once feature has launched.
	if (!hasExperiment(options, TIME_PER_ELEMENT_EXPERIMENT)) {
	return ElementExecutionTracker.NO_OP_INSTANCE;
	}

	// TODO: Remove log statement when functionality is enabled by default.
	LOG.info("{} counter enabled.", ElementExecutionTracker.COUNTER_NAME);

	return new DataflowElementExecutionTracker(counterFactory);
	}

	private static boolean hasExperiment(PipelineOptions options, String experiment) {
	List<String> experiments = options.as(DataflowPipelineDebugOptions.class).getExperiments();
	return experiments != null && experiments.contains(experiment);
	}

	/**
	* Tracking object for the execution of a single input element in a step.
	*
	* <p>Each {@link ElementExecution} instance represents a distinct element, implicitly represented
	* by the instance identity. As such, {@link ElementExecution} instances compare using reference
	* equality rather than value equality.
	*/
	private static class ElementExecution {
	/** Marker execution to represent when there is no element currently being processed. */
	static final ElementExecution IDLE = new ElementExecution();

	/** Only empty for {@see IDLE}. */
	final Optional<NameContext> step;

	ElementExecution(NameContext step) {
	this.step = Optional.of(step);
	}

	/** Only used for {@see IDLE}. */
	private ElementExecution() {
	step = Optional.empty();
	}

	@Override
	public String toString() {
	if (this == ElementExecution.IDLE) {
	return "IDLE_EXECUTION";
	}

	return MoreObjects.toStringHelper(this)
	.add("step", step)
	.add("executionId", System.identityHashCode(this))
	.toString();
	}
	}

	private final ExecutionJournalWriter executionWriter;
	private final ExecutionJournalReader executionReader;

	private DataflowElementExecutionTracker(CounterFactory counterFactory) {
	ReaderWriterState readerWriterState = new ReaderWriterState();
	executionWriter = new ExecutionJournalWriter(readerWriterState);
	executionReader = new ExecutionJournalReader(counterFactory, readerWriterState);
	}

	@Override
	public void enter(NameContext step) {
	executionWriter.startProcessing(step);
	}

	@Override
	public void exit() {
	executionWriter.doneProcessing();
	}

	@Override
	public void takeSample(long millisSinceLastSample) {
	executionReader.takeSample(Duration.ofMillis(millisSinceLastSample));
	}

	/** State shared between the {@link ExecutionJournalWriter} and {@link ExecutionJournalReader}. */
	private static class ReaderWriterState {
	/**
	* Journal of fragments of execution per element to count for attributing processing time. Each
	* time we transition up or down the stage fusion graph we add an execution fragment for the
	* currently processing element with an incremented snapshot version. Each snapshot version must
	* have a representative value in the {@code executionJournal}, or {@see IDLE_EXECUTION} to
	* represent completion of processing.
	*/
	private final Journal<ElementExecution> executionJournal;

	/**
	* Elements which have completed processing but are still pending final timing calculation. The
	* elements are moved here when they exit the {@code executionStack} and are held here until the
	* counter value can be reported in the next sampling round.
	*/
	private final Journal<ElementExecution> doneJournal;

	/**
	* Monotonically increasing snapshot version number which tracks the latest execution state
	* ready to be reported. Snapshot versions are associated with values in the {@code
	* executionJournal} and {@code doneJournal}.
	*
	* <p>This value written by the single {@link ExecutionJournalWriter} thread, and read by the
	* {@link ExecutionJournalReader} thread.
	*/
	private volatile long latestSnapshot;

	private ReaderWriterState() {
	executionJournal = new Journal<>();
	doneJournal = new Journal<>();
	latestSnapshot = 0L;
	}
	}

	/**
	* Writes journal entries on element processing state changes.
	*
	* <p>Write operations are executed by a single thread inside the stage operation hot loop and
	* must be highly performant. {@link ExecutionJournalWriter} methods are thread-safe only for
	* state shared with the {@link ExecutionJournalReader} thread.
	*/
	private static class ExecutionJournalWriter {
	/**
	* Execution stack of processing elements. Elements are pushed onto the stack when we enter the
	* process() function and popped on process() return. This stack mirrors the actual Java runtime
	* stack and contains the step + element context in order to attribute sampled execution time.
	*/
	private final Deque<ElementExecution> executionStack;

	private final ReaderWriterState sharedState;

	public ExecutionJournalWriter(ReaderWriterState sharedState) {
	this.sharedState = sharedState;
	this.executionStack = new ArrayDeque<>();

	addExecution(ElementExecution.IDLE);
	}

	/** Create and journal a new {@link ElementExecution} to track a processing element. */
	public void startProcessing(NameContext step) {
	ElementExecution execution = new ElementExecution(step);
	addExecution(execution);
	}

	private void addExecution(ElementExecution execution) {
	long nextSnapshot = sharedState.latestSnapshot + 1;
	executionStack.addLast(execution);
	sharedState.executionJournal.add(execution, nextSnapshot);
	sharedState.latestSnapshot = nextSnapshot;
	}

	/**
	* Indicates that the execution thread has exited the process method for an element.
	*
	* <p>When an element is finished processing, it is popped from the execution stack, but will be
	* tracked in the {@code doneJournal} collection until the next sampling round in order to
	* account timing for the final fragment of execution.
	*/
	public void doneProcessing() {
	// The execution stack in initialized with the IDLE execution which should never be removed.
	checkState(executionStack.size() > 1, "No processing elements currently tracked.");

	ElementExecution execution = executionStack.removeLast();
	long nextSnapshot = sharedState.latestSnapshot + 1;
	sharedState.doneJournal.add(execution, nextSnapshot);
	ElementExecution nextElement = executionStack.getLast();
	sharedState.executionJournal.add(nextElement, nextSnapshot);
	sharedState.latestSnapshot = nextSnapshot;
	}
	}

	/**
	* Accounts sampled time to processed elements based on execution journal entries.
	*
	* <p>Read operations are executed by a single thread in charge of state sampling, and are
	* generally called more infrequently than {@link ExecutionJournalWriter} operations. {@link
	* ExecutionJournalReader} methods are thread-safe only for state shared with the {@link
	* ExecutionJournalWriter} thread.
	*/
	private static class ExecutionJournalReader {
	/**
	* Accumulated execution time per element. Once an element has finished processing and execution
	* time has been attributed, the total execution time is reported via the counter and removed
	* from the collection.
	*/
	private final Map<ElementExecution, Duration> executionTimes;

	private final CounterFactory counterFactory;
	private final ReaderWriterState sharedState;

	/** Cache of per-step distribution counters. */
	private final Map<NameContext, Counter<Long, ?>> counterCache;

	public ExecutionJournalReader(CounterFactory counterFactory, ReaderWriterState sharedState) {
	this.sharedState = sharedState;
	executionTimes = new HashMap<>();
	this.counterFactory = counterFactory;
	counterCache = new HashMap<>();
	}

	/**
	* Account the specified processing time duration to elements which have processed since the
	* last sampling round, and report counters for completed elements.
	*/
	public void takeSample(Duration sampleTime) {
	long snapshot = sharedState.latestSnapshot;
	attributeProcessingTime(sampleTime, snapshot);

	// TODO: If possible, this should report tentative counter values before they are
	// finalized. Currently, root steps in a stage fusion will not have counters reported until an
	// element is completely processed through the stage subgraph.
	reportCounters(snapshot);
	pruneJournals(snapshot);
	}

	/**
	* Attribute processing time to elements from {@code executionJournal} up to the specified
	* snapshot.
	*/
	private void attributeProcessingTime(Duration duration, long snapshot) {
	// TODO: This algorithm is used to compute "per-element-processing-time" counter
	// values, but a slightly different algorithm is used for msec counters. Values for both
	// counters should be derived from the same algorithm to avoid unexpected discrepancies.

	// Calculate total execution counts
	int totalExecutions = 0;
	Map<ElementExecution, Integer> executionsPerElement = new HashMap<>();
	for (ElementExecution execution : sharedState.executionJournal.readUntil(snapshot)) {
	totalExecutions++;
	if (execution != ElementExecution.IDLE) {
	executionsPerElement.compute(execution, (unused, count) -> count == null ? 1 : count + 1);
	}
	}

	// Attribute processing time
	final int totalExecutionsFinal = totalExecutions;
	for (Map.Entry<ElementExecution, Integer> executionCount : executionsPerElement.entrySet()) {
	executionTimes.compute(
	executionCount.getKey(),
	(unused, total) -> {
	int numExecutions = executionCount.getValue();
	Duration attributedSampleTime =
	duration.dividedBy(totalExecutionsFinal).multipliedBy(numExecutions);
	return total == null ? attributedSampleTime : total.plus(attributedSampleTime);
	});
	}
	}

	/** Report counter values for done elements up to the given snapshot. */
	private void reportCounters(long snapshot) {
	// Report counter values for completed elements
	for (ElementExecution execution : sharedState.doneJournal.readUntil(snapshot)) {
	checkState(
	execution.step.isPresent(),
	"Unexpected execution in doneJournal with empty step: %s",
	execution);
	Counter<Long, ?> counter = getCounter(execution.step.get());
	counter.addValue(executionTimes.get(execution).toMillis());
	}
	}

	/** Retrieve the per-element processing time counter for the specified step. */
	private Counter<Long, ?> getCounter(NameContext step) {
	return counterCache.computeIfAbsent(
	step,
	s ->
	counterFactory.distribution(
	ElementExecutionTracker.COUNTER_NAME.withOriginalName(s)));
	}

	/**
	* Prune journal entries which have been accounted.
	*
	* @param snapshot Snapshot value which has been accounted.
	*/
	private void pruneJournals(long snapshot) {
	sharedState.doneJournal.pruneUntil(snapshot);

	// Keep the currently executing element in the journal so its remaining execution time
	// is counted in the next sampling round.
	sharedState.executionJournal.pruneUntil(snapshot - 1);
	}
	}

	/**
	* Concurrent queue-based data structure for passing journaled events between a single journal
	* reader and single journal writer.
	*
	* <p>Each event is journaled with an externally-managed snapshot version. Snapshot versions are
	* unique and monotonically increasing.
	*/
	private static class Journal<T> {
	private final ConcurrentLinkedQueue<SnapshottedItem<T>> queue;
	private long maxSnapshot;

	public Journal() {
	queue = new ConcurrentLinkedQueue<>();
	maxSnapshot = Long.MIN_VALUE;
	}

	/** Add a new event to the journal with the given snapshot. */
	public void add(T item, long snapshot) {
	if (snapshot <= maxSnapshot) {
	throw new IllegalArgumentException(
	String.format(
	"Timestamps must be monotonically increasing. "
	+ "Specified snapshot '%d' is not greater than max snapshot '%d'",
	snapshot, maxSnapshot));
	}
	queue.add(new SnapshottedItem<>(item, snapshot));
	maxSnapshot = snapshot;
	}

	/**
	* Retrieve an iterable of events from the journal up to and including the specified snapshot
	* version.
	*
	* <p>Returns an empty iterable if the journal is empty or there are no events less than the
	* specified snapshot version.
	*
	* <p>The returned iterable returns a view of the queue at the time of construction, consistent
	* with {@link ConcurrentLinkedQueue#iterator()}.
	*/
	public Iterable<T> readUntil(long snapshot) {
	// Implement our own iterator in order to short-circuit early once we've seen a snapshot
	// version greater than the value specified.
	// Streams will have takeWhile() in JDK9, and Guava may some day have
	// Iterables.limit(Predicate<T>): https://github.com/google/guava/issues/477
	PeekingIterator<SnapshottedItem<T>> iter = Iterators.peekingIterator(queue.iterator());
	return () ->
	new Iterator<T>() {
	@Override
	public boolean hasNext() {
	return iter.hasNext() && iter.peek().snapshot <= snapshot;
	}

	@Override
	public T next() {
	SnapshottedItem<T> next = iter.next();
	checkValidSnapshot(next);
	return next.item;
	}

	@Override
	public void remove() {
	iter.remove();
	}

	private void checkValidSnapshot(SnapshottedItem<T> next) {
	if (next.snapshot > snapshot) {
	throw new NoSuchElementException();
	}
	}
	};
	}

	/** Prune events from the journal up to and including the specified snapshot. */
	public void pruneUntil(long snapshot) {
	Iterator<SnapshottedItem<T>> iterator = queue.iterator();
	while (iterator.hasNext()) {
	if (iterator.next().snapshot <= snapshot) {
	iterator.remove();
	} else {
	break;
	}
	}
	}

	private static class SnapshottedItem<E> {
	private final E item;
	private final long snapshot;

	public SnapshottedItem(E item, long snapshot) {
	this.item = item;
	this.snapshot = snapshot;
	}
	}
	}
	}