001/*
002 * Copyright (C) 2011 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
005 * in compliance with the License. You may obtain a copy of the License at
006 *
007 * http://www.apache.org/licenses/LICENSE-2.0
008 *
009 * Unless required by applicable law or agreed to in writing, software distributed under the License
010 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
011 * or implied. See the License for the specific language governing permissions and limitations under
012 * the License.
013 */
014
015package com.google.common.util.concurrent;
016
017import static com.google.common.base.Preconditions.checkArgument;
018import static com.google.common.base.Preconditions.checkNotNull;
019import static com.google.common.util.concurrent.MoreExecutors.directExecutor;
020
021import com.google.common.annotations.Beta;
022import com.google.common.annotations.GwtIncompatible;
023import com.google.common.base.Supplier;
024import com.google.errorprone.annotations.CanIgnoreReturnValue;
025import com.google.errorprone.annotations.concurrent.GuardedBy;
026import com.google.j2objc.annotations.WeakOuter;
027import java.util.concurrent.Callable;
028import java.util.concurrent.Executor;
029import java.util.concurrent.Executors;
030import java.util.concurrent.Future;
031import java.util.concurrent.ScheduledExecutorService;
032import java.util.concurrent.ThreadFactory;
033import java.util.concurrent.TimeUnit;
034import java.util.concurrent.TimeoutException;
035import java.util.concurrent.locks.ReentrantLock;
036import java.util.logging.Level;
037import java.util.logging.Logger;
038import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
039import org.checkerframework.checker.nullness.qual.Nullable;
040
041/**
042 * Base class for services that can implement {@link #startUp} and {@link #shutDown} but while in
043 * the "running" state need to perform a periodic task. Subclasses can implement {@link #startUp},
044 * {@link #shutDown} and also a {@link #runOneIteration} method that will be executed periodically.
045 *
046 * <p>This class uses the {@link ScheduledExecutorService} returned from {@link #executor} to run
047 * the {@link #startUp} and {@link #shutDown} methods and also uses that service to schedule the
048 * {@link #runOneIteration} that will be executed periodically as specified by its {@link
049 * Scheduler}. When this service is asked to stop via {@link #stopAsync} it will cancel the periodic
050 * task (but not interrupt it) and wait for it to stop before running the {@link #shutDown} method.
051 *
052 * <p>Subclasses are guaranteed that the life cycle methods ({@link #runOneIteration}, {@link
053 * #startUp} and {@link #shutDown}) will never run concurrently. Notably, if any execution of {@link
054 * #runOneIteration} takes longer than its schedule defines, then subsequent executions may start
055 * late. Also, all life cycle methods are executed with a lock held, so subclasses can safely modify
056 * shared state without additional synchronization necessary for visibility to later executions of
057 * the life cycle methods.
058 *
059 * <h3>Usage Example</h3>
060 *
061 * <p>Here is a sketch of a service which crawls a website and uses the scheduling capabilities to
062 * rate limit itself.
063 *
064 * <pre>{@code
065 * class CrawlingService extends AbstractScheduledService {
066 *   private Set<Uri> visited;
067 *   private Queue<Uri> toCrawl;
068 *   protected void startUp() throws Exception {
069 *     toCrawl = readStartingUris();
070 *   }
071 *
072 *   protected void runOneIteration() throws Exception {
073 *     Uri uri = toCrawl.remove();
074 *     Collection<Uri> newUris = crawl(uri);
075 *     visited.add(uri);
076 *     for (Uri newUri : newUris) {
077 *       if (!visited.contains(newUri)) { toCrawl.add(newUri); }
078 *     }
079 *   }
080 *
081 *   protected void shutDown() throws Exception {
082 *     saveUris(toCrawl);
083 *   }
084 *
085 *   protected Scheduler scheduler() {
086 *     return Scheduler.newFixedRateSchedule(0, 1, TimeUnit.SECONDS);
087 *   }
088 * }
089 * }</pre>
090 *
091 * <p>This class uses the life cycle methods to read in a list of starting URIs and save the set of
092 * outstanding URIs when shutting down. Also, it takes advantage of the scheduling functionality to
093 * rate limit the number of queries we perform.
094 *
095 * @author Luke Sandberg
096 * @since 11.0
097 */
098@Beta
099@GwtIncompatible
100public abstract class AbstractScheduledService implements Service {
101  private static final Logger logger = Logger.getLogger(AbstractScheduledService.class.getName());
102
103  /**
104   * A scheduler defines the policy for how the {@link AbstractScheduledService} should run its
105   * task.
106   *
107   * <p>Consider using the {@link #newFixedDelaySchedule} and {@link #newFixedRateSchedule} factory
108   * methods, these provide {@link Scheduler} instances for the common use case of running the
109   * service with a fixed schedule. If more flexibility is needed then consider subclassing {@link
110   * CustomScheduler}.
111   *
112   * @author Luke Sandberg
113   * @since 11.0
114   */
115  public abstract static class Scheduler {
116    /**
117     * Returns a {@link Scheduler} that schedules the task using the {@link
118     * ScheduledExecutorService#scheduleWithFixedDelay} method.
119     *
120     * @param initialDelay the time to delay first execution
121     * @param delay the delay between the termination of one execution and the commencement of the
122     *     next
123     * @param unit the time unit of the initialDelay and delay parameters
124     */
125    public static Scheduler newFixedDelaySchedule(
126        final long initialDelay, final long delay, final TimeUnit unit) {
127      checkNotNull(unit);
128      checkArgument(delay > 0, "delay must be > 0, found %s", delay);
129      return new Scheduler() {
130        @Override
131        public Future<?> schedule(
132            AbstractService service, ScheduledExecutorService executor, Runnable task) {
133          return executor.scheduleWithFixedDelay(task, initialDelay, delay, unit);
134        }
135      };
136    }
137
138    /**
139     * Returns a {@link Scheduler} that schedules the task using the {@link
140     * ScheduledExecutorService#scheduleAtFixedRate} method.
141     *
142     * @param initialDelay the time to delay first execution
143     * @param period the period between successive executions of the task
144     * @param unit the time unit of the initialDelay and period parameters
145     */
146    public static Scheduler newFixedRateSchedule(
147        final long initialDelay, final long period, final TimeUnit unit) {
148      checkNotNull(unit);
149      checkArgument(period > 0, "period must be > 0, found %s", period);
150      return new Scheduler() {
151        @Override
152        public Future<?> schedule(
153            AbstractService service, ScheduledExecutorService executor, Runnable task) {
154          return executor.scheduleAtFixedRate(task, initialDelay, period, unit);
155        }
156      };
157    }
158
159    /** Schedules the task to run on the provided executor on behalf of the service. */
160    abstract Future<?> schedule(
161        AbstractService service, ScheduledExecutorService executor, Runnable runnable);
162
163    private Scheduler() {}
164  }
165
166  /* use AbstractService for state management */
167  private final AbstractService delegate = new ServiceDelegate();
168
169  @WeakOuter
170  private final class ServiceDelegate extends AbstractService {
171
172    // A handle to the running task so that we can stop it when a shutdown has been requested.
173    // These two fields are volatile because their values will be accessed from multiple threads.
174    @MonotonicNonNull private volatile Future<?> runningTask;
175    @MonotonicNonNull private volatile ScheduledExecutorService executorService;
176
177    // This lock protects the task so we can ensure that none of the template methods (startUp,
178    // shutDown or runOneIteration) run concurrently with one another.
179    // TODO(lukes): why don't we use ListenableFuture to sequence things? Then we could drop the
180    // lock.
181    private final ReentrantLock lock = new ReentrantLock();
182
183    @WeakOuter
184    class Task implements Runnable {
185      @Override
186      public void run() {
187        lock.lock();
188        try {
189          if (runningTask.isCancelled()) {
190            // task may have been cancelled while blocked on the lock.
191            return;
192          }
193          AbstractScheduledService.this.runOneIteration();
194        } catch (Throwable t) {
195          try {
196            shutDown();
197          } catch (Exception ignored) {
198            logger.log(
199                Level.WARNING,
200                "Error while attempting to shut down the service after failure.",
201                ignored);
202          }
203          notifyFailed(t);
204          runningTask.cancel(false); // prevent future invocations.
205        } finally {
206          lock.unlock();
207        }
208      }
209    }
210
211    private final Runnable task = new Task();
212
213    @Override
214    protected final void doStart() {
215      executorService =
216          MoreExecutors.renamingDecorator(
217              executor(),
218              new Supplier<String>() {
219                @Override
220                public String get() {
221                  return serviceName() + " " + state();
222                }
223              });
224      executorService.execute(
225          new Runnable() {
226            @Override
227            public void run() {
228              lock.lock();
229              try {
230                startUp();
231                runningTask = scheduler().schedule(delegate, executorService, task);
232                notifyStarted();
233              } catch (Throwable t) {
234                notifyFailed(t);
235                if (runningTask != null) {
236                  // prevent the task from running if possible
237                  runningTask.cancel(false);
238                }
239              } finally {
240                lock.unlock();
241              }
242            }
243          });
244    }
245
246    @Override
247    protected final void doStop() {
248      runningTask.cancel(false);
249      executorService.execute(
250          new Runnable() {
251            @Override
252            public void run() {
253              try {
254                lock.lock();
255                try {
256                  if (state() != State.STOPPING) {
257                    // This means that the state has changed since we were scheduled. This implies
258                    // that an execution of runOneIteration has thrown an exception and we have
259                    // transitioned to a failed state, also this means that shutDown has already
260                    // been called, so we do not want to call it again.
261                    return;
262                  }
263                  shutDown();
264                } finally {
265                  lock.unlock();
266                }
267                notifyStopped();
268              } catch (Throwable t) {
269                notifyFailed(t);
270              }
271            }
272          });
273    }
274
275    @Override
276    public String toString() {
277      return AbstractScheduledService.this.toString();
278    }
279  }
280
281  /** Constructor for use by subclasses. */
282  protected AbstractScheduledService() {}
283
284  /**
285   * Run one iteration of the scheduled task. If any invocation of this method throws an exception,
286   * the service will transition to the {@link Service.State#FAILED} state and this method will no
287   * longer be called.
288   */
289  protected abstract void runOneIteration() throws Exception;
290
291  /**
292   * Start the service.
293   *
294   * <p>By default this method does nothing.
295   */
296  protected void startUp() throws Exception {}
297
298  /**
299   * Stop the service. This is guaranteed not to run concurrently with {@link #runOneIteration}.
300   *
301   * <p>By default this method does nothing.
302   */
303  protected void shutDown() throws Exception {}
304
305  /**
306   * Returns the {@link Scheduler} object used to configure this service. This method will only be
307   * called once.
308   */
309  // TODO(cpovirk): @ForOverride
310  protected abstract Scheduler scheduler();
311
312  /**
313   * Returns the {@link ScheduledExecutorService} that will be used to execute the {@link #startUp},
314   * {@link #runOneIteration} and {@link #shutDown} methods. If this method is overridden the
315   * executor will not be {@linkplain ScheduledExecutorService#shutdown shutdown} when this service
316   * {@linkplain Service.State#TERMINATED terminates} or {@linkplain Service.State#TERMINATED
317   * fails}. Subclasses may override this method to supply a custom {@link ScheduledExecutorService}
318   * instance. This method is guaranteed to only be called once.
319   *
320   * <p>By default this returns a new {@link ScheduledExecutorService} with a single thread thread
321   * pool that sets the name of the thread to the {@linkplain #serviceName() service name}. Also,
322   * the pool will be {@linkplain ScheduledExecutorService#shutdown() shut down} when the service
323   * {@linkplain Service.State#TERMINATED terminates} or {@linkplain Service.State#TERMINATED
324   * fails}.
325   */
326  protected ScheduledExecutorService executor() {
327    @WeakOuter
328    class ThreadFactoryImpl implements ThreadFactory {
329      @Override
330      public Thread newThread(Runnable runnable) {
331        return MoreExecutors.newThread(serviceName(), runnable);
332      }
333    }
334    final ScheduledExecutorService executor =
335        Executors.newSingleThreadScheduledExecutor(new ThreadFactoryImpl());
336    // Add a listener to shutdown the executor after the service is stopped. This ensures that the
337    // JVM shutdown will not be prevented from exiting after this service has stopped or failed.
338    // Technically this listener is added after start() was called so it is a little gross, but it
339    // is called within doStart() so we know that the service cannot terminate or fail concurrently
340    // with adding this listener so it is impossible to miss an event that we are interested in.
341    addListener(
342        new Listener() {
343          @Override
344          public void terminated(State from) {
345            executor.shutdown();
346          }
347
348          @Override
349          public void failed(State from, Throwable failure) {
350            executor.shutdown();
351          }
352        },
353        directExecutor());
354    return executor;
355  }
356
357  /**
358   * Returns the name of this service. {@link AbstractScheduledService} may include the name in
359   * debugging output.
360   *
361   * @since 14.0
362   */
363  protected String serviceName() {
364    return getClass().getSimpleName();
365  }
366
367  @Override
368  public String toString() {
369    return serviceName() + " [" + state() + "]";
370  }
371
372  @Override
373  public final boolean isRunning() {
374    return delegate.isRunning();
375  }
376
377  @Override
378  public final State state() {
379    return delegate.state();
380  }
381
382  /** @since 13.0 */
383  @Override
384  public final void addListener(Listener listener, Executor executor) {
385    delegate.addListener(listener, executor);
386  }
387
388  /** @since 14.0 */
389  @Override
390  public final Throwable failureCause() {
391    return delegate.failureCause();
392  }
393
394  /** @since 15.0 */
395  @CanIgnoreReturnValue
396  @Override
397  public final Service startAsync() {
398    delegate.startAsync();
399    return this;
400  }
401
402  /** @since 15.0 */
403  @CanIgnoreReturnValue
404  @Override
405  public final Service stopAsync() {
406    delegate.stopAsync();
407    return this;
408  }
409
410  /** @since 15.0 */
411  @Override
412  public final void awaitRunning() {
413    delegate.awaitRunning();
414  }
415
416  /** @since 15.0 */
417  @Override
418  public final void awaitRunning(long timeout, TimeUnit unit) throws TimeoutException {
419    delegate.awaitRunning(timeout, unit);
420  }
421
422  /** @since 15.0 */
423  @Override
424  public final void awaitTerminated() {
425    delegate.awaitTerminated();
426  }
427
428  /** @since 15.0 */
429  @Override
430  public final void awaitTerminated(long timeout, TimeUnit unit) throws TimeoutException {
431    delegate.awaitTerminated(timeout, unit);
432  }
433
434  /**
435   * A {@link Scheduler} that provides a convenient way for the {@link AbstractScheduledService} to
436   * use a dynamically changing schedule. After every execution of the task, assuming it hasn't been
437   * cancelled, the {@link #getNextSchedule} method will be called.
438   *
439   * @author Luke Sandberg
440   * @since 11.0
441   */
442  @Beta
443  public abstract static class CustomScheduler extends Scheduler {
444
445    /** A callable class that can reschedule itself using a {@link CustomScheduler}. */
446    private class ReschedulableCallable extends ForwardingFuture<Void> implements Callable<Void> {
447
448      /** The underlying task. */
449      private final Runnable wrappedRunnable;
450
451      /** The executor on which this Callable will be scheduled. */
452      private final ScheduledExecutorService executor;
453
454      /**
455       * The service that is managing this callable. This is used so that failure can be reported
456       * properly.
457       */
458      private final AbstractService service;
459
460      /**
461       * This lock is used to ensure safe and correct cancellation, it ensures that a new task is
462       * not scheduled while a cancel is ongoing. Also it protects the currentFuture variable to
463       * ensure that it is assigned atomically with being scheduled.
464       */
465      private final ReentrantLock lock = new ReentrantLock();
466
467      /** The future that represents the next execution of this task. */
468      @GuardedBy("lock")
469      private @Nullable Future<Void> currentFuture;
470
471      ReschedulableCallable(
472          AbstractService service, ScheduledExecutorService executor, Runnable runnable) {
473        this.wrappedRunnable = runnable;
474        this.executor = executor;
475        this.service = service;
476      }
477
478      @Override
479      public Void call() throws Exception {
480        wrappedRunnable.run();
481        reschedule();
482        return null;
483      }
484
485      /** Atomically reschedules this task and assigns the new future to {@link #currentFuture}. */
486      public void reschedule() {
487        // invoke the callback outside the lock, prevents some shenanigans.
488        Schedule schedule;
489        try {
490          schedule = CustomScheduler.this.getNextSchedule();
491        } catch (Throwable t) {
492          service.notifyFailed(t);
493          return;
494        }
495        // We reschedule ourselves with a lock held for two reasons. 1. we want to make sure that
496        // cancel calls cancel on the correct future. 2. we want to make sure that the assignment
497        // to currentFuture doesn't race with itself so that currentFuture is assigned in the
498        // correct order.
499        Throwable scheduleFailure = null;
500        lock.lock();
501        try {
502          if (currentFuture == null || !currentFuture.isCancelled()) {
503            currentFuture = executor.schedule(this, schedule.delay, schedule.unit);
504          }
505        } catch (Throwable e) {
506          // If an exception is thrown by the subclass then we need to make sure that the service
507          // notices and transitions to the FAILED state. We do it by calling notifyFailed directly
508          // because the service does not monitor the state of the future so if the exception is not
509          // caught and forwarded to the service the task would stop executing but the service would
510          // have no idea.
511          // TODO(lukes): consider building everything in terms of ListenableScheduledFuture then
512          // the AbstractService could monitor the future directly. Rescheduling is still hard...
513          // but it would help with some of these lock ordering issues.
514          scheduleFailure = e;
515        } finally {
516          lock.unlock();
517        }
518        // Call notifyFailed outside the lock to avoid lock ordering issues.
519        if (scheduleFailure != null) {
520          service.notifyFailed(scheduleFailure);
521        }
522      }
523
524      // N.B. Only protect cancel and isCancelled because those are the only methods that are
525      // invoked by the AbstractScheduledService.
526      @Override
527      public boolean cancel(boolean mayInterruptIfRunning) {
528        // Ensure that a task cannot be rescheduled while a cancel is ongoing.
529        lock.lock();
530        try {
531          return currentFuture.cancel(mayInterruptIfRunning);
532        } finally {
533          lock.unlock();
534        }
535      }
536
537      @Override
538      public boolean isCancelled() {
539        lock.lock();
540        try {
541          return currentFuture.isCancelled();
542        } finally {
543          lock.unlock();
544        }
545      }
546
547      @Override
548      protected Future<Void> delegate() {
549        throw new UnsupportedOperationException(
550            "Only cancel and isCancelled is supported by this future");
551      }
552    }
553
554    @Override
555    final Future<?> schedule(
556        AbstractService service, ScheduledExecutorService executor, Runnable runnable) {
557      ReschedulableCallable task = new ReschedulableCallable(service, executor, runnable);
558      task.reschedule();
559      return task;
560    }
561
562    /**
563     * A value object that represents an absolute delay until a task should be invoked.
564     *
565     * @author Luke Sandberg
566     * @since 11.0
567     */
568    @Beta
569    protected static final class Schedule {
570
571      private final long delay;
572      private final TimeUnit unit;
573
574      /**
575       * @param delay the time from now to delay execution
576       * @param unit the time unit of the delay parameter
577       */
578      public Schedule(long delay, TimeUnit unit) {
579        this.delay = delay;
580        this.unit = checkNotNull(unit);
581      }
582    }
583
584    /**
585     * Calculates the time at which to next invoke the task.
586     *
587     * <p>This is guaranteed to be called immediately after the task has completed an iteration and
588     * on the same thread as the previous execution of {@link
589     * AbstractScheduledService#runOneIteration}.
590     *
591     * @return a schedule that defines the delay before the next execution.
592     */
593    // TODO(cpovirk): @ForOverride
594    protected abstract Schedule getNextSchedule() throws Exception;
595  }
596}