org.apache.hadoop.contrib.bkjournal.WriteLock.java Source code

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Here is the source code for org.apache.hadoop.contrib.bkjournal.WriteLock.java

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/**
 * 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.hadoop.contrib.bkjournal;

import org.apache.zookeeper.ZooKeeper;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.Watcher.Event.KeeperState;
import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.ZooDefs.Ids;

import java.util.concurrent.atomic.AtomicInteger;
import java.util.List;
import java.util.Collections;
import java.util.Comparator;

import java.net.InetAddress;
import java.io.IOException;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

/**
 * Distributed lock, using ZooKeeper.
 *
 * The lock is vulnerable to timing issues. For example, the process could
 * encounter a really long GC cycle between acquiring the lock, and writing to
 * a ledger. This could have timed out the lock, and another process could have
 * acquired the lock and started writing to bookkeeper. Therefore other
 * mechanisms are required to ensure correctness (i.e. Fencing).
 */
class WriteLock implements Watcher {
    static final Log LOG = LogFactory.getLog(WriteLock.class);

    private final ZooKeeper zkc;
    private final String lockpath;

    private AtomicInteger lockCount = new AtomicInteger(0);
    private String myznode = null;

    WriteLock(ZooKeeper zkc, String lockpath) throws IOException {
        this.lockpath = lockpath;

        this.zkc = zkc;
        try {
            if (zkc.exists(lockpath, false) == null) {
                String localString = InetAddress.getLocalHost().toString();
                zkc.create(lockpath, localString.getBytes(), Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
            }
        } catch (Exception e) {
            throw new IOException("Exception accessing Zookeeper", e);
        }
    }

    void acquire() throws IOException {
        while (true) {
            if (lockCount.get() == 0) {
                try {
                    synchronized (this) {
                        if (lockCount.get() > 0) {
                            lockCount.incrementAndGet();
                            return;
                        }
                        myznode = zkc.create(lockpath + "/lock-", new byte[] { '0' }, Ids.OPEN_ACL_UNSAFE,
                                CreateMode.EPHEMERAL_SEQUENTIAL);
                        if (LOG.isTraceEnabled()) {
                            LOG.trace("Acquiring lock, trying " + myznode);
                        }

                        List<String> nodes = zkc.getChildren(lockpath, false);
                        Collections.sort(nodes, new Comparator<String>() {
                            public int compare(String o1, String o2) {
                                Integer l1 = Integer.valueOf(o1.replace("lock-", ""));
                                Integer l2 = Integer.valueOf(o2.replace("lock-", ""));
                                return l1 - l2;
                            }
                        });
                        if ((lockpath + "/" + nodes.get(0)).equals(myznode)) {
                            if (LOG.isTraceEnabled()) {
                                LOG.trace("Lock acquired - " + myznode);
                            }
                            lockCount.set(1);
                            zkc.exists(myznode, this);
                            return;
                        } else {
                            LOG.error("Failed to acquire lock with " + myznode + ", " + nodes.get(0)
                                    + " already has it");
                            throw new IOException("Could not acquire lock");
                        }
                    }
                } catch (KeeperException e) {
                    throw new IOException("Exception accessing Zookeeper", e);
                } catch (InterruptedException ie) {
                    throw new IOException("Exception accessing Zookeeper", ie);
                }
            } else {
                int ret = lockCount.getAndIncrement();
                if (ret == 0) {
                    lockCount.decrementAndGet();
                    continue; // try again;
                } else {
                    return;
                }
            }
        }
    }

    void release() throws IOException {
        try {
            if (lockCount.decrementAndGet() <= 0) {
                if (lockCount.get() < 0) {
                    LOG.warn("Unbalanced lock handling somewhere, lockCount down to " + lockCount.get());
                }
                synchronized (this) {
                    if (lockCount.get() <= 0) {
                        if (LOG.isTraceEnabled()) {
                            LOG.trace("releasing lock " + myznode);
                        }
                        if (myznode != null) {
                            zkc.delete(myznode, -1);
                            myznode = null;
                        }
                    }
                }
            }
        } catch (Exception e) {
            throw new IOException("Exception accessing Zookeeper", e);
        }
    }

    public void checkWriteLock() throws IOException {
        if (!haveLock()) {
            throw new IOException("Lost writer lock");
        }
    }

    boolean haveLock() throws IOException {
        return lockCount.get() > 0;
    }

    public void process(WatchedEvent event) {
        if (event.getState() == KeeperState.Disconnected || event.getState() == KeeperState.Expired) {
            LOG.warn("Lost zookeeper session, lost lock ");
            lockCount.set(0);
        } else {
            // reapply the watch
            synchronized (this) {
                LOG.info("Zookeeper event " + event + " received, reapplying watch to " + myznode);
                if (myznode != null) {
                    try {
                        zkc.exists(myznode, this);
                    } catch (Exception e) {
                        LOG.warn("Could not set watch on lock, releasing", e);
                        try {
                            release();
                        } catch (IOException ioe) {
                            LOG.error("Could not release Zk lock", ioe);
                        }
                    }
                }
            }
        }
    }
}