基于zookeeper实现的分布式锁

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			<h1 class="entry-title" itemprop="name headline">基于zookeeper实现的分布式锁</h1>
	
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		<time class="entry-date" datetime="2011-01-27T01:37:13+00:00" pubdate="" itemprop="datePublished">2011-01-27</time> <span class="dot">•</span> <span class="categories-links" itemprop="keywords"><a href="http://www.jiacheo.org/cat/tech" rel="category tag">技术</a></span> <span class="dot">•</span> <a href="http://www.jiacheo.org/blog/122#comments" class="comments-link">7 条评论</a> <span class="dot">•</span> <a class="author vcard" itemprop="author" itemscope="" itemtype="http://schema.org/Person" rel="author" href="http://www.jiacheo.org/blog/author/jiacheo"><span itemprop="name" class="fn">jiacheo</span></a> <span class="dot">•</span>14,941 阅读		</div>
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			<p>A distributed lock base on <a title="zookeeper" href="http://hadoop.apache.org/zookeeper/" target="_blank">zookeeper</a>.</p>

zookeeper是hadoop下面的一个子项目, 用来协调跟hadoop相关的一些分布式的框架, 如hadoop, hive, pig等, 其实他们都是动物, 所以叫zookeeper(本人歪歪).

zookeeper其实是集群中每个节点都维护着一棵相同的树, 树的结构跟linux的目录结构的概念差不多, 以/为跟节点, 下边可以扩展任意的节点和叶子节点, 每个节点都可以写入数据. 基于zookeeper的分布式锁的实现, 其实是得益于zookeeper同步文件的强大性, 我们相信每时每刻我们访问zookeeper的树时, 相同节点返回的数据都是一致的. 这要靠zookeeper内部的一些算法来实现. 特别是leader的选举算法, 这里就不说了, 感兴趣的话可以去搜索一下看看.

我们知道了zookeeper集群的每个节点的数据都是一致的, 那么我们可以通过这些节点来作为锁的标志.

首先给锁设置一下API, 至少要包含, lock(锁住), unlock(解锁), isLocked(是否锁住)三个方法

然后我们可以创建一个工厂(LockFactory), 用来专门生产锁.

锁的创建过程如下描述:

前提:每个锁都需要一个路径来指定(如:/jiacheo/lock)

1.根据指定的路径, 查找zookeeper集群下的这个节点是否存在.(说明已经有锁了)

2. 如果存在, 根据查询者的一些特征数据(如ip地址/hostname), 当前的锁是不是查询者的

3. 如果不是查询者的锁, 则返回null, 说明创建锁失败

4. 如果是查询者的锁, 则把这个锁返回给查询者

5. 如果这个节点不存在, 说明当前没有锁, 那么创建一个临时节点, 并将查询者的特征信息写入这个节点的数据中, 然后返回这个锁.

根据以上5部, 一个分布式的锁就可以创建了.

创建的锁有三种状态:

1. 创建失败(null), 说明该锁被其他查询者使用了.'

2. 创建成功, 但当前没有锁住(unlocked), 可以使用

3. 创建成功, 但当前已经锁住(locked)了, 不能继续加锁.

如图, 如果我们getLock("/jiacheo/lock1","192.168.0.100"), 想要获取/jiacheo/lock1这个锁的话, 我们先判断这个节点是否存在, 存在的话获取他的数据(data), 然后通过解析data, 我们可以知道这个节点是不是我们查询者创建的(通过ip地址写入节点数据中), 然后就可以返回一个锁了.

具体的java实现(implementation)代码如下:
1. Lock.java

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package org.jiacheo.zkdl.lock;   import java.net.InetAddress; import java.net.UnknownHostException;   import org.apache.zookeeper.KeeperException; import org.apache.zookeeper.ZooKeeper; import org.apache.zookeeper.data.Stat;   /**  * 类名：<b>Lock</b> <br/>  * <p>  * 类描述：  * </p>  * 创建人：jiacheo <br/>  * 创建时间：2011-1-27 上午01:30:25  <br/>   * @version 2011-1-27   *  */ public class Lock {     private String path;     private ZooKeeper zooKeeper;     public Lock(String path){         this.path = path;     }           /**      * <p>      * 方法描述: 上锁 lock it      * </p>      * 创建人：jiacheo <br/>      * 创建时间：2011-1-27 上午01:30:50  <br/>      * @throws Exception      */     public synchronized void lock() throws Exception{         Stat stat = zooKeeper.exists(path, true);         String data = InetAddress.getLocalHost().getHostAddress()+":lock";         zooKeeper.setData(path, data.getBytes(), stat.getVersion());     }           /**      * <p>      * 方法描述：开锁 unlock it      * </p>      * 创建人：jiacheo <br/>      * 创建时间：2011-1-27 上午01:31:20  <br/>      * @throws Exception      */     public synchronized void unLock() throws Exception{         Stat stat = zooKeeper.exists(path, true);         String data = InetAddress.getLocalHost().getHostAddress()+":unlock";         zooKeeper.setData(path, data.getBytes(), stat.getVersion());     }           /**      * <p>      * 方法描述：是否锁住了, isLocked?      * </p>      * 创建人：jiacheo <br/>      * 创建时间：2011-1-27 上午01:31:43  <br/>      * @return      */     public synchronized boolean isLock(){         try {             Stat stat = zooKeeper.exists(path, true);             String data = InetAddress.getLocalHost().getHostAddress()+":lock";             String nodeData = new String(zooKeeper.getData(path, true, stat));             if(data.equals(nodeData)){ //              lock = true;                 return true;             }         } catch (UnknownHostException e) {             // ignore it         } catch (KeeperException e) {             //TODO use log system and throw a new exception         } catch (InterruptedException e) {             // TODO use log system and throw a new exception         }         return false;     }       public String getPath() {         return path;     }       public void setPath(String path) {         this.path = path;     }       public void setZooKeeper(ZooKeeper zooKeeper) {         this.zooKeeper = zooKeeper;     }             }

2.LockFactory.java

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package org.jiacheo.zkdl.lock;   import java.io.IOException; import java.net.InetAddress; import java.util.Collections;   import org.apache.zookeeper.CreateMode; import org.apache.zookeeper.WatchedEvent; import org.apache.zookeeper.Watcher; import org.apache.zookeeper.ZooKeeper; import org.apache.zookeeper.ZooDefs.Ids; import org.apache.zookeeper.ZooDefs.Perms; import org.apache.zookeeper.data.ACL; import org.apache.zookeeper.data.Stat;   public class LockFactory {           public static final ZooKeeper DEFAULT_ZOOKEEPER = getDefaultZookeeper();     //data格式:  ip:stat  如: 10.232.35.70:lock 10.232.35.70:unlock     public static synchronized Lock getLock(String path,String ip) throws Exception{         if(DEFAULT_ZOOKEEPER != null){             Stat stat = null;             try{                 stat = DEFAULT_ZOOKEEPER.exists(path, true);             }catch (Exception e) {                 // TODO: use log system and throw new exception             }             if(stat!=null){                 byte[] data = DEFAULT_ZOOKEEPER.getData(path, null, stat);                 String dataStr = new String(data);                 String[] ipv = dataStr.split(":");                 if(ip.equals(ipv[0])){                     Lock lock = new Lock(path);                     lock.setZooKeeper(DEFAULT_ZOOKEEPER);                     return lock;                 }                 //is not your lock, return null                 else{                     return null;                 }             }             //no lock created yet, you can get it             else{                 createZnode(path);                 Lock lock = new Lock(path);                 lock.setZooKeeper(DEFAULT_ZOOKEEPER);                 return lock;             }         }         return null;     }           private static ZooKeeper getDefaultZookeeper() {         try {             ZooKeeper zooKeeper = new ZooKeeper("10.232.35.72", 10*1000, new Watcher(){                 public void process(WatchedEvent event) {                     //节点的事件处理. you can do something when the node's data change //                  System.out.println("event " + event.getType() + " has happened!");                 }             });             return zooKeeper;         } catch (IOException e) {             e.printStackTrace();         }         return null;     }       private static void createZnode(String path) throws Exception{                   if(DEFAULT_ZOOKEEPER!=null){             InetAddress address = InetAddress.getLocalHost();             String data = address.getHostAddress()+":unlock";             DEFAULT_ZOOKEEPER.create(path, data.getBytes(),Collections.singletonList(new ACL(Perms.ALL,Ids.ANYONE_ID_UNSAFE)) , CreateMode.EPHEMERAL);         }     } }