分布式锁 - zk 分布式锁 - 《微服务》

zk分布式锁，其实可以做的比较简单，就是某个节点尝试创建临时znode，此时创建成功了就获取了这个锁；这个时候别的客户端来创建锁会失败，只能注册个监听器监听这个锁。释放锁就是删除这个znode，一旦释放掉就会通知客户端，然后有一个等待着的客户端就可以再次重新枷锁。

/**
* ZooKeeperSession
* @author Administrator
*
*/
public class ZooKeeperSession {
    private static CountDownLatch connectedSemaphore = new CountDownLatch(1);
    private ZooKeeper zookeeper;
    private CountDownLatch latch;
    public ZooKeeperSession() {
        try {
            this.zookeeper = new ZooKeeper(
                "192.168.31.187:2181,192.168.31.19:2181,192.168.31.227:2181", 
                50000, 
                new ZooKeeperWatcher());            
            try {
                connectedSemaphore.await();
            } catch(InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("ZooKeeper session established......");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    /**
    * 获取分布式锁
    * @param productId
    */
    public Boolean acquireDistributedLock(Long productId) {
        String path = "/product-lock-" + productId;
        try {
            zookeeper.create(path, "".getBytes(), 
                             Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL);
            return true;
        } catch (Exception e) {
            while(true) {
                try {
                    Stat stat = zk.exists(path, true); // 相当于是给node注册一个监听器，去看看这个监听器是否存在
                    if(stat != null) {
                        this.latch = new CountDownLatch(1);
                        this.latch.await(waitTime, TimeUnit.MILLISECONDS);
                        this.latch = null;
                    }
                    zookeeper.create(path, "".getBytes(), 
                                     Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL);
                    return true;
                } catch(Exception e) {
                    continue;
                }
            }
            // 很不优雅，我呢就是给大家来演示这么一个思路
            // 比较通用的，我们公司里我们自己封装的基于zookeeper的分布式锁，我们基于zookeeper的临时顺序节点去实现的，比较优雅的
        }
        return true;
    }
    /**
    * 释放掉一个分布式锁
    * @param productId
    */
    public void releaseDistributedLock(Long productId) {
        String path = "/product-lock-" + productId;
        try {
            zookeeper.delete(path, -1); 
            System.out.println("release the lock for product[id=" + productId + "]......");  
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    /**
    * 建立zk session的watcher
    * @author Administrator
    *
    */
    private class ZooKeeperWatcher implements Watcher {
        public void process(WatchedEvent event) {
            System.out.println("Receive watched event: " + event.getState());
            if(KeeperState.SyncConnected == event.getState()) {
                connectedSemaphore.countDown();
            } 
            if(this.latch != null) {  
                this.latch.countDown();  
            }
        }
    }
    /**
    * 封装单例的静态内部类
    * @author Administrator
    *
    */
    private static class Singleton {
        private static ZooKeeperSession instance;
        static {
            instance = new ZooKeeperSession();
        }
        public static ZooKeeperSession getInstance() {
            return instance;
        }
    }
    /**
    * 获取单例
    * @return
    */
    public static ZooKeeperSession getInstance() {
        return Singleton.getInstance();
    }
    /**
    * 初始化单例的便捷方法
    */
    public static void init() {
        getInstance();
    }
}

（3）redis分布式锁和zk分布式锁的对比

redis分布式锁，其实需要自己不断去尝试获取锁，比较消耗性能

zk分布式锁，获取不到锁，注册个监听器即可，不需要不断主动尝试获取锁，性能开销较小

另外一点就是，如果是redis获取锁的那个客户端bug了或者挂了，那么只能等待超时时间之后才能释放锁；而zk的话，因为创建的是临时znode，只要客户端挂了，znode就没了，此时就自动释放锁

redis分布式锁大家每发现好麻烦吗？遍历上锁，计算时间等等。。。zk的分布式锁语义清晰实现简单

所以先不分析太多的东西，就说这两点，我个人实践认为zk的分布式锁比redis的分布式锁牢靠、而且模型简单易用

（4）zookeeper分布式锁实现代码

public class ZooKeeperDistributedLock implements Watcher{

    private ZooKeeper zk;
    private String locksRoot= "/locks";
    private String productId;
    private String waitNode;
    private String lockNode;
    private CountDownLatch latch;
    private CountDownLatch connectedLatch = new CountDownLatch(1);
    private int sessionTimeout = 30000; 

    public ZooKeeperDistributedLock(String productId){
        this.productId = productId;
        try {
            String address = "192.168.31.187:2181,192.168.31.19:2181,192.168.31.227:2181";
            zk = new ZooKeeper(address, sessionTimeout, this);
            connectedLatch.await();
        } catch (IOException e) {
            throw new LockException(e);
        } catch (KeeperException e) {
            throw new LockException(e);
        } catch (InterruptedException e) {
            throw new LockException(e);
        }
    }

    public void process(WatchedEvent event) {
        if(event.getState()==KeeperState.SyncConnected){
            connectedLatch.countDown();
            return;
        }

        if(this.latch != null) {  
            this.latch.countDown(); 
        }
    }

    public void acquireDistributedLock() {   
        try {
            if(this.tryLock()){
                return;
            }
            else{
                waitForLock(waitNode, sessionTimeout);
            }
        } catch (KeeperException e) {
            throw new LockException(e);
        } catch (InterruptedException e) {
            throw new LockException(e);
        } 
    }

    public boolean tryLock() {
        try {
            // 传入进去的locksRoot + “/” + productId
            // 假设productId代表了一个商品id，比如说1
            // locksRoot = locks
            // /locks/10000000000，/locks/10000000001，/locks/10000000002
            lockNode = zk.create(locksRoot + "/" + productId, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);

            // 看看刚创建的节点是不是最小的节点
            // locks：10000000000，10000000001，10000000002
            List<String> locks = zk.getChildren(locksRoot, false);
            Collections.sort(locks);

            if(lockNode.equals(locksRoot+"/"+ locks.get(0))){
                //如果是最小的节点,则表示取得锁
                return true;
            }

            //如果不是最小的节点，找到比自己小1的节点
            int previousLockIndex = -1;
            for(int i = 0; i < locks.size(); i++) {
                if(lockNode.equals(locksRoot + “/” + locks.get(i))) {
                    previousLockIndex = i - 1;
                    break;
                }
            }

            this.waitNode = locks.get(previousLockIndex);
        } catch (KeeperException e) {
            throw new LockException(e);
        } catch (InterruptedException e) {
            throw new LockException(e);
        }
        return false;
    }

    private boolean waitForLock(String waitNode, long waitTime) throws InterruptedException, KeeperException {
        Stat stat = zk.exists(locksRoot + "/" + waitNode, true);
        if(stat != null){
            this.latch = new CountDownLatch(1);
            this.latch.await(waitTime, TimeUnit.MILLISECONDS);                   
            this.latch = null;
        }
        return true;
    }

    public void unlock() {
        try {
            // 删除/locks/10000000000节点
            // 删除/locks/10000000001节点
            System.out.println("unlock " + lockNode);
            zk.delete(lockNode,-1);
            lockNode = null;
            zk.close();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (KeeperException e) {
            e.printStackTrace();
        }
    }

    public class LockException extends RuntimeException {
        private static final long serialVersionUID = 1L;
        public LockException(String e){
            super(e);
        }
        public LockException(Exception e){
            super(e);
        }
    }


}

如果有一把锁，被多个人给竞争，此时多个人会排队，第一个拿到锁的人会执行，然后释放锁，后面的每个人都会去监听排在自己前面的那个人创建的node上，一旦某个人释放了锁，排在自己后面的人就会被zookeeper给通知，一旦被通知了之后，就ok了，自己就获取到了锁，就可以执行代码了