org.apache.pulsar.shade.com.spotify.futures.ConcurrencyReducer Maven / Gradle / Ivy

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/*-
 * -\-\-
 * completable-futures
 * --
 * Copyright (C) 2016 - 2020 Spotify AB
 * --
 * Licensed 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.pulsar.shade.com.spotify.futures;

import static java.util.Objects.requireNonNull;

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.Callable;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.CompletionStage;
import java.util.concurrent.Semaphore;

/**
 * {@link ConcurrencyReducer} is used to queue tasks which will be
 * executed in a manner reducing the number of concurrent tasks.
 *
 * Note: This is a port of ConcurrencyLimiter from futures-extra for use with CompletionStages
 */
public class ConcurrencyReducer {

  private final BlockingQueue> queue;
  private final Semaphore limit;
  private final int maxQueueSize;

  private final int maxConcurrency;

  private ConcurrencyReducer(int maxConcurrency, int maxQueueSize) {
    this.maxConcurrency = maxConcurrency;
    this.maxQueueSize = maxQueueSize;
    if (maxConcurrency <= 0) {
      throw new IllegalArgumentException("maxConcurrency must be at least 0");
    }

    if (maxQueueSize <= 0) {
      throw new IllegalArgumentException("maxQueueSize must be at least 0");
    }
    this.queue = new ArrayBlockingQueue<>(maxQueueSize);
    this.limit = new Semaphore(maxConcurrency);
  }

  /**
   * @param maxConcurrency maximum number of futures in progress,
   * @param maxQueueSize   maximum number of jobs in queue. This is a soft bound and may be
   *                       temporarily exceeded if add() is called concurrently.
   * @return a new concurrency limiter
   */
  public static  ConcurrencyReducer create(int maxConcurrency, int maxQueueSize) {
    return new ConcurrencyReducer<>(maxConcurrency, maxQueueSize);
  }

  /**
   * the callable function will run as soon as the currently active set of futures is less than the
   * maxConcurrency limit.
   *
   * @param callable - a function that creates a future.
   * @return a proxy future that completes with the future created by the input function. This
   * future will be immediately failed with {@link CapacityReachedException} if the soft queue
   * size limit is exceeded.
   */
  public CompletableFuture add(final Callable> callable) {
    requireNonNull(callable);
    final CompletableFuture response = new CompletableFuture<>();
    final Job job = new Job<>(callable, response);
    if (!queue.offer(job)) {
      final String message = "Queue size has reached capacity: " + maxQueueSize;
      return CompletableFutures.exceptionallyCompletedFuture(new CapacityReachedException(message));
    }
    pump();
    return response;
  }

  /**
   * @return the number of callables that are queued up and haven't started yet.
   */
  public int numQueued() {
    return queue.size();
  }

  /**
   * @return the number of currently active futures that have not yet completed.
   */
  public int numActive() {
    return maxConcurrency - limit.availablePermits();
  }

  /**
   * @return the number of additional callables that can be queued before failing.
   */
  public int remainingQueueCapacity() {
    return queue.remainingCapacity();
  }

  /**
   * @return the number of additional callables that can be run without queueing.
   */
  public int remainingActiveCapacity() {
    return limit.availablePermits();
  }

  /**
   * Does one of two things: 1) return a job and acquire a permit from the semaphore 2) return null
   * and does not acquire a permit from the semaphore
   */
  private Job grabJob() {
    if (!limit.tryAcquire()) {
      return null;
    }

    final Job job = queue.poll();
    if (job != null) {
      return job;
    }

    limit.release();
    return null;
  }

  private void pump() {
    Job job;
    while ((job = grabJob()) != null) {
      final CompletableFuture response = job.response;

      if (response.isCancelled()) {
        limit.release();
      } else {
        invoke(response, job.callable);
      }
    }
  }

  private void invoke(final CompletableFuture response,
                      final Callable> callable) {
    final CompletionStage future;
    try {
      future = callable.call();
      if (future == null) {
        limit.release();
        response.completeExceptionally(new NullPointerException());
        return;
      }
    } catch (Throwable e) {
      limit.release();
      response.completeExceptionally(e);
      return;
    }

    future.whenComplete(
        (result, t) -> {
          if (t != null) {
            limit.release();
            response.completeExceptionally(t);
            pump();
          } else {
            limit.release();
            response.complete(result);
            pump();
          }
        });
  }

  private static class Job {

    private final Callable> callable;
    private final CompletableFuture response;

    public Job(Callable> callable, CompletableFuture response) {
      this.callable = callable;
      this.response = response;
    }
  }

  public static class CapacityReachedException extends RuntimeException {

    public CapacityReachedException(String errorMessage) {
      super(errorMessage);
    }
  }
}