在漫衍式系统中，往往需要一些漫衍式同步原语来做一些协同事情，上一篇文章先容了Zookeeper的根基道理，本文先容下基于Zookeeper的Lock和Queue的实现，主要代码都来自Zookeeper的官方recipe。

锁（Lock）

完全漫衍式锁是全局同步的，这意味着在任何时刻没有两个客户端会同时认为它们都拥有沟通的锁，利用 Zookeeper 可以实现漫衍式锁，需要首先界说一个锁节点（lock root node）。

需要得到锁的客户端凭据以下步调来获取锁：

1. 担保锁节点（lock root node）这个父根节点的存在，这个节点是每个要获取lock客户端共用的，这个节点是PERSISTENT的。
2. 第一次需要建设本客户端要获取lock的节点，挪用 create( )，并配置 节点为EPHEMERAL_SEQUENTIAL范例，暗示该节点为姑且的温顺序的。假如获取锁的节点挂掉，则该节点自动失效，可以让其他节点获取锁。
3. 在父锁节点（lock root node）上挪用 getChildren( ) ，不需要配置监督符号。 (为了制止“羊群效应”).
4. 凭据Fair竞争的原则，将步调3中的子节点（要获取锁的节点）凭据节点顺序的巨细做排序，取出编号最小的一个节点做为lock的owner，判定本身的节点id
5. 是否就为owner id，劳务派遣管理系统，假如是则返回，lock乐成。假如不是则挪用 exists( )监听比本身小的前一位的id，存眷它锁释放的操纵（也就是exist watch）。
6. 假如第4步监听exist的watch被触发，则继承按4中的原则判定本身是否能获取到lock。

释放锁：需要释放锁的客户端只需要删除在第2步中建设的节点即可。

留意事项：

一个节点的删除只会导致一个客户端被叫醒，因为每个节点只被一个客户端watch，这制止了“羊群效应”。

一个漫衍式lock的实现：

package org.apache.zookeeper.recipes.lock;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import static org.apache.zookeeper.CreateMode.EPHEMERAL_SEQUENTIAL;
import org.apache.zookeeper.ZooKeeper;
import org.apache.zookeeper.data.ACL;
import org.apache.zookeeper.data.Stat;

import java.util.List;
import java.util.SortedSet;
import java.util.TreeSet;

/**
 * A <a href="package.html">protocol to implement an exclusive
 *  write lock or to elect a leader</a>. <p/> You invoke {@link #lock()} to 
 *  start the process of grabbing the lock; you may get the lock then or it may be 
 *  some time later. <p/> You can register a listener so that you are invoked 
 *  when you get the lock; otherwise you can ask if you have the lock
 *  by calling {@link #isOwner()}
 *
 */
public class WriteLock extends ProtocolSupport {
    private static final Logger LOG = LoggerFactory.getLogger(WriteLock.class);

    private final String dir;
    private String id;
    private ZNodeName idName;
    private String ownerId;
    private String lastChildId;
    private byte[] data = {0x12, 0x34};
    private LockListener callback;
    private LockZooKeeperOperation zop;
    
    /**
     * zookeeper contructor for writelock
     * @param zookeeper zookeeper client instance
     * @param dir the parent path you want to use for locking
     * @param acls the acls that you want to use for all the paths, 
     * if null world read/write is used.
     */
    public WriteLock(ZooKeeper zookeeper, String dir, List<ACL> acl) {
        super(zookeeper);
        this.dir = dir;
        if (acl != null) {
            setAcl(acl);
        }
        this.zop = new LockZooKeeperOperation();
    }
    
    /**
     * zookeeper contructor for writelock with callback
     * @param zookeeper the zookeeper client instance
     * @param dir the parent path you want to use for locking
     * @param acl the acls that you want to use for all the paths
     * @param callback the call back instance
     */
    public WriteLock(ZooKeeper zookeeper, String dir, List<ACL> acl, 
            LockListener callback) {
        this(zookeeper, dir, acl);
        this.callback = callback;
    }

    /**
     * return the current locklistener
     * @return the locklistener
     */
    public LockListener getLockListener() {
        return this.callback;
    }
    
    /**
     * register a different call back listener
     * @param callback the call back instance
     */
    public void setLockListener(LockListener callback) {
        this.callback = callback;
    }

    /**
     * Removes the lock or associated znode if 
     * you no longer require the lock. this also 
     * removes your request in the queue for locking
     * in case you do not already hold the lock.
     * @throws RuntimeException throws a runtime exception
     * if it cannot connect to zookeeper.
     */
    public synchronized void unlock() throws RuntimeException {
        
        if (!isClosed() && id != null) {
            // we don't need to retry this operation in the case of failure
            // as ZK will remove ephemeral files and we don't wanna hang
            // this process when closing if we cannot reconnect to ZK
            try {
                ZooKeeperOperation zopdel = new ZooKeeperOperation() {
                    public boolean execute() throws KeeperException,
                        InterruptedException {
                        zookeeper.delete(id, -1);   
                        return Boolean.TRUE;
                    }
                };
                zopdel.execute();
            } catch (InterruptedException e) {
                LOG.warn("Caught: " + e, e);
                //set that we have been interrupted.
               Thread.currentThread().interrupt();
            } catch (KeeperException.NoNodeException e) {
                // do nothing
            } catch (KeeperException e) {
                LOG.warn("Caught: " + e, e);
                throw (RuntimeException) new RuntimeException(e.getMessage()).
                    initCause(e);
            }
            finally {
                if (callback != null) {
                    callback.lockReleased();
                }
                id = null;
            }
        }
    }
    
    /** 
     * the watcher called on  
     * getting watch while watching 
     * my predecessor
     */
    private class LockWatcher implements Watcher {
        public void process(WatchedEvent event) {
            // lets either become the leader or watch the new/updated node
            LOG.debug("Watcher fired on path: " + event.getPath() + " state: " + 
                    event.getState() + " type " + event.getType());
            try {
                lock();
            } catch (Exception e) {
                LOG.warn("Failed to acquire lock: " + e, e);
            }
        }
    }
    
    /**
     * a zoookeeper operation that is mainly responsible
     * for all the magic required for locking.
     */
    private  class LockZooKeeperOperation implements ZooKeeperOperation {
        
        /** find if we have been created earler if not create our node
         * 
         * @param prefix the prefix node
         * @param zookeeper teh zookeeper client
         * @param dir the dir paretn
         * @throws KeeperException
         * @throws InterruptedException
         */
        private void findPrefixInChildren(String prefix, ZooKeeper zookeeper, String dir) 
            throws KeeperException, InterruptedException {
            List<String> names = zookeeper.getChildren(dir, false);
            for (String name : names) {
                if (name.startsWith(prefix)) {
                    id = dir + "/" + name;
                    if (LOG.isDebugEnabled()) {
                        LOG.debug("Found id created last time: " + id);
                    }
                    break;
                }
            }
            if (id == null) {
                id = zookeeper.create(dir + "/" + prefix, data, 
                        getAcl(), EPHEMERAL_SEQUENTIAL);

                if (LOG.isDebugEnabled()) {
                    LOG.debug("Created id: " + id);
                }
            }

        }
        
        /**
         * the command that is run and retried for actually 
         * obtaining the lock
         * @return if the command was successful or not
         */
        public boolean execute() throws KeeperException, InterruptedException {
            do {
                if (id == null) {
                    long sessionId = zookeeper.getSessionId();
                    String prefix = "x-" + sessionId + "-";
                    // lets try look up the current ID if we failed 
                    // in the middle of creating the znode
                    findPrefixInChildren(prefix, zookeeper, dir);
                    idName = new ZNodeName(id);
                }
                if (id != null) {
                    List<String> names = zookeeper.getChildren(dir, false);
                    if (names.isEmpty()) {
                        LOG.warn("No children in: " + dir + " when we've just " +
                        "created one! Lets recreate it...");
                        // lets force the recreation of the id
                        id = null;
                    } else {
                        // lets sort them explicitly (though they do seem to come back in order ususally :)
                        SortedSet<ZNodeName> sortedNames = new TreeSet<ZNodeName>();
                        for (String name : names) {
                            sortedNames.add(new ZNodeName(dir + "/" + name));
                        }
                        ownerId = sortedNames.first().getName();
                        SortedSet<ZNodeName> lessThanMe = sortedNames.headSet(idName);
                        if (!lessThanMe.isEmpty()) {
                            ZNodeName lastChildName = lessThanMe.last();
                            lastChildId = lastChildName.getName();
                            if (LOG.isDebugEnabled()) {
                                LOG.debug("watching less than me node: " + lastChildId);
                            }
                            Stat stat = zookeeper.exists(lastChildId, new LockWatcher());
                            if (stat != null) {
                                return Boolean.FALSE;
                            } else {
                                LOG.warn("Could not find the" +
                                        " stats for less than me: " + lastChildName.getName());
                            }
                        } else {
                            if (isOwner()) {
                                if (callback != null) {
                                    callback.lockAcquired();
                                }
                                return Boolean.TRUE;
                            }
                        }
                    }
                }
            }
            while (id == null);
            return Boolean.FALSE;
        }
    };

    /**
     * Attempts to acquire the exclusive write lock returning whether or not it was
     * acquired. Note that the exclusive lock may be acquired some time later after
     * this method has been invoked due to the current lock owner going away.
     */
    public synchronized boolean lock() throws KeeperException, InterruptedException {
        if (isClosed()) {
            return false;
        }
        ensurePathExists(dir);

        return (Boolean) retryOperation(zop);
    }

    /**
     * return the parent dir for lock
     * @return the parent dir used for locks.
     */
    public String getDir() {
        return dir;
    }

    /**
     * Returns true if this node is the owner of the
     *  lock (or the leader)
     */
    public boolean isOwner() {
        return id != null && ownerId != null && id.equals(ownerId);
    }

    /**
     * return the id for this lock
     * @return the id for this lock
     */
    public String getId() {
       return this.id;
    }
}

留意这里的lock，大概会失败，会实验多次，每次失败后会Sleep一段时间。

PS：官方的代码有个小bug，id和ownerId应该都是全路径，即id = dir + “/” + name;原代码在findPrefixInChildren里有问题。