001/*
002 * Licensed to the Apache Software Foundation (ASF) under one
003 * or more contributor license agreements.  See the NOTICE file
004 * distributed with this work for additional information
005 * regarding copyright ownership.  The ASF licenses this file
006 * to you under the Apache License, Version 2.0 (the
007 * "License"); you may not use this file except in compliance
008 * with the License.  You may obtain a copy of the License at
009 *
010 *     http://www.apache.org/licenses/LICENSE-2.0
011 *
012 * Unless required by applicable law or agreed to in writing, software
013 * distributed under the License is distributed on an "AS IS" BASIS,
014 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
015 * See the License for the specific language governing permissions and
016 * limitations under the License.
017 */
018
019package org.apache.hadoop.hbase.procedure2;
020
021import org.apache.yetus.audience.InterfaceAudience;
022import org.slf4j.Logger;
023import org.slf4j.LoggerFactory;
024import org.apache.hbase.thirdparty.com.google.common.annotations.VisibleForTesting;
025
026/**
027 * Basic ProcedureEvent that contains an "object", which can be a description or a reference to the
028 * resource to wait on, and a queue for suspended procedures.
029 */
030@InterfaceAudience.Private
031public class ProcedureEvent<T> {
032  private static final Logger LOG = LoggerFactory.getLogger(ProcedureEvent.class);
033
034  private final T object;
035  private boolean ready = false;
036  private ProcedureDeque suspendedProcedures = new ProcedureDeque();
037
038  public ProcedureEvent(final T object) {
039    this.object = object;
040  }
041
042  public synchronized boolean isReady() {
043    return ready;
044  }
045
046  /**
047   * @return true if event is not ready and adds procedure to suspended queue, else returns false.
048   */
049  public synchronized boolean suspendIfNotReady(Procedure proc) {
050    if (!ready) {
051      suspendedProcedures.addLast(proc);
052    }
053    return !ready;
054  }
055
056  /** Mark the event as not ready. */
057  public synchronized void suspend() {
058    ready = false;
059    if (LOG.isTraceEnabled()) {
060      LOG.trace("Suspend " + toString());
061    }
062  }
063
064  /**
065   * Wakes up the suspended procedures by pushing them back into scheduler queues and sets the
066   * event as ready.
067   * See {@link #wakeInternal(AbstractProcedureScheduler)} for why this is not synchronized.
068   */
069  public void wake(AbstractProcedureScheduler procedureScheduler) {
070    procedureScheduler.wakeEvents(new ProcedureEvent[]{this});
071  }
072
073  /**
074   * Wakes up all the given events and puts the procedures waiting on them back into
075   * ProcedureScheduler queues.
076   */
077  public static void wakeEvents(AbstractProcedureScheduler scheduler, ProcedureEvent ... events) {
078    scheduler.wakeEvents(events);
079  }
080
081  /**
082   * Only to be used by ProcedureScheduler implementations.
083   * Reason: To wake up multiple events, locking sequence is
084   * schedLock --> synchronized (event)
085   * To wake up an event, both schedLock() and synchronized(event) are required.
086   * The order is schedLock() --> synchronized(event) because when waking up multiple events
087   * simultaneously, we keep the scheduler locked until all procedures suspended on these events
088   * have been added back to the queue (Maybe it's not required? Evaluate!)
089   * To avoid deadlocks, we want to keep the locking order same even when waking up single event.
090   * That's why, {@link #wake(AbstractProcedureScheduler)} above uses the same code path as used
091   * when waking up multiple events.
092   * Access should remain package-private.
093   */
094  synchronized void wakeInternal(AbstractProcedureScheduler procedureScheduler) {
095    if (ready && !suspendedProcedures.isEmpty()) {
096      LOG.warn("Found procedures suspended in a ready event! Size=" + suspendedProcedures.size());
097    }
098    ready = true;
099    if (LOG.isTraceEnabled()) {
100      LOG.trace("Unsuspend " + toString());
101    }
102    // wakeProcedure adds to the front of queue, so we start from last in the
103    // waitQueue' queue, so that the procedure which was added first goes in the front for
104    // the scheduler queue.
105    procedureScheduler.addFront(suspendedProcedures.descendingIterator());
106    suspendedProcedures.clear();
107  }
108
109  /**
110   * Access to suspendedProcedures is 'synchronized' on this object, but it's fine to return it
111   * here for tests.
112   */
113  @VisibleForTesting
114  public ProcedureDeque getSuspendedProcedures() {
115    return suspendedProcedures;
116  }
117
118  @Override
119  public String toString() {
120    return getClass().getSimpleName() + " for " + object + ", ready=" + isReady() +
121        ", " + suspendedProcedures;
122  }
123}