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From lvfangmin <...@git.apache.org>
Subject [GitHub] zookeeper pull request #590: [ZOOKEEPER-1177] Add the memory optimized watch...
Date Tue, 04 Sep 2018 16:43:51 GMT
Github user lvfangmin commented on a diff in the pull request:

    https://github.com/apache/zookeeper/pull/590#discussion_r214987142
  
    --- Diff: src/java/main/org/apache/zookeeper/server/watch/WatchManagerOptimized.java ---
    @@ -0,0 +1,355 @@
    +/**
    + * 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.zookeeper.server.watch;
    +
    +import java.io.PrintWriter;
    +import java.util.HashMap;
    +import java.util.BitSet;
    +import java.util.HashSet;
    +import java.util.ArrayList;
    +import java.util.LinkedHashMap;
    +import java.util.Map;
    +import java.util.Map.Entry;
    +import java.util.Random;
    +import java.util.Set;
    +import java.util.Iterator;
    +import java.lang.Iterable;
    +import java.util.concurrent.ConcurrentHashMap;
    +import java.util.concurrent.locks.ReadWriteLock;
    +import java.util.concurrent.locks.ReentrantReadWriteLock;
    +
    +import org.apache.zookeeper.WatchedEvent;
    +import org.apache.zookeeper.Watcher;
    +import org.apache.zookeeper.Watcher.Event.EventType;
    +import org.apache.zookeeper.Watcher.Event.KeeperState;
    +import org.apache.zookeeper.server.ServerCnxn;
    +import org.apache.zookeeper.server.util.BitMap;
    +import org.slf4j.Logger;
    +import org.slf4j.LoggerFactory;
    +
    +/**
    + * Optimized in memory and time complexity, compared to WatchManager, both the
    + * memory consumption and time complexity improved a lot, but it cannot
    + * efficiently remove the watcher when the session or socket is closed, for
    + * majority usecase this is not a problem.
    + *
    + * Changed made compared to WatchManager:
    + *
    + * - Use HashSet and BitSet to store the watchers to find a balance between
    + *   memory usage and time complexity
    + * - Use ReadWriteLock instead of synchronized to reduce lock retention
    + * - Lazily clean up the closed watchers
    + */
    +public class WatchManagerOptimized
    +        implements IWatchManager, DeadWatcherListener {
    +
    +    private static final Logger LOG =
    +            LoggerFactory.getLogger(WatchManagerOptimized.class);
    +
    +    private final ConcurrentHashMap<String, BitHashSet> pathWatches =
    +            new ConcurrentHashMap<String, BitHashSet>();
    +
    +    // watcher to bit id mapping
    +    private final BitMap<Watcher> watcherBitIdMap = new BitMap<Watcher>();
    +
    +    // used to lazily remove the dead watchers
    +    private final WatcherCleaner watcherCleaner;
    +
    +    private final ReentrantReadWriteLock addRemovePathRWLock = new ReentrantReadWriteLock();
    +
    +    public WatchManagerOptimized() {
    +        watcherCleaner = new WatcherCleaner(this);
    +        watcherCleaner.start();
    +    }
    +
    +    @Override
    +    public boolean addWatch(String path, Watcher watcher) {
    +        boolean result = false;
    +        addRemovePathRWLock.readLock().lock();
    +        try {
    +            // avoid race condition of adding a on flying dead watcher
    +            if (isDeadWatcher(watcher)) {
    +                LOG.debug("Ignoring addWatch with closed cnxn");
    +            } else {
    +                Integer bit = watcherBitIdMap.add(watcher);
    +                BitHashSet watchers = pathWatches.get(path);
    +                if (watchers == null) {
    +                    watchers = new BitHashSet();
    +                    BitHashSet existingWatchers = pathWatches.putIfAbsent(path, watchers);
    +                    if (existingWatchers != null) {
    +                        watchers = existingWatchers;
    +                    }
    +                }
    +                result = watchers.add(bit);
    +            }
    +        } finally {
    +            addRemovePathRWLock.readLock().unlock();
    +        }
    +        return result;
    +    }
    +
    +    @Override
    +    public boolean containsWatcher(String path, Watcher watcher) {
    +        BitHashSet watchers = pathWatches.get(path);
    +        if (watchers == null || !watchers.contains(watcherBitIdMap.getBit(watcher)))
{
    +            return false;
    +        }
    +        return true;
    +    }
    +
    +    @Override
    +    public boolean removeWatcher(String path, Watcher watcher) {
    +        addRemovePathRWLock.writeLock().lock();
    --- End diff --
    
    Actually, this is based on the scenario optimization, like cache hit, when user send removeWatcher
request, it's more likely the watcher is actually exist and haven't fired yet, so instead
of multiple read lock and API call to check exist then switch to write lock, it's actually
cheaper to only call write lock once.


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