基于zookeeper的分布式锁

一。为何使用分布式锁?
当应用服务器数量超过1台，对相同数据的访问可能造成访问冲突（特别是写冲突）。单纯使用关系数据库比如MYSQL的应用可以借助于事务来实现锁，也可以使用版本号等实现乐观锁，最大的缺陷就是可用性降低（性能差）。对于GLEASY这种满足大规模并发访问请求的应用来说，使用数据库事务来实现数据库就有些捉襟见肘了。另外对于一些不依赖数据库的应用，比如分布式文件系统，为了保证同一文件在大量读写操作情况下的正确性，必须引入分布式锁来约束对同一文件的并发操作。

二。对分布式锁的要求
1.高性能（分布式锁不能成为系统的性能瓶颈）
2.避免死锁（拿到锁的结点挂掉不会导致其它结点永远无法继续）
3.支持锁重入

三。方案1，基于zookeeper的分布式锁

 

 

/**
* DistributedLockUtil.java
* 分布式锁工厂类，所有分布式请求都由该工厂类负责
**/
public class DistributedLockUtil {
	private static Object schemeLock = new Object();
	private static Object mutexLock = new Object();
	private static Map<String,Object> mutexLockMap = new ConcurrentHashMap();	
	private String schema;
	private Map<String,DistributedReentrantLock> cache = new ConcurrentHashMap<String,DistributedReentrantLock>();
	
	private static Map<String,DistributedLockUtil> instances = new ConcurrentHashMap();
	public static DistributedLockUtil getInstance(String schema){
		DistributedLockUtil u = instances.get(schema);
		if(u==null){
			synchronized(schemeLock){
				u = instances.get(schema);
				if(u == null){
					u = new DistributedLockUtil(schema);
					instances.put(schema, u);
				}
			}
		}
		return u;
	}
	
	private DistributedLockUtil(String schema){
		this.schema = schema;
	}
	
	private Object getMutex(String key){
		Object mx = mutexLockMap.get(key);
		if(mx == null){
			synchronized(mutexLock){
				mx = mutexLockMap.get(key);
				if(mx==null){
					mx = new Object();
					mutexLockMap.put(key,mx);
				}
			}
		}
		return mx;
	}
	
	private DistributedReentrantLock getLock(String key){
		DistributedReentrantLock lock = cache.get(key);
		if(lock == null){
			synchronized(getMutex(key)){
				lock = cache.get(key);
				if(lock == null){
					lock = new DistributedReentrantLock(key,schema);
					cache.put(key, lock);
				}
			}
		}
		return lock;
	}
	
	public void reset(){
		for(String s : cache.keySet()){
			getLock(s).unlock();
		}
	}
	
	/**
	 * 尝试加锁
	 * 如果当前线程已经拥有该锁的话,直接返回false,表示不用再次加锁,此时不应该再调用unlock进行解锁
	 * 
	 * @param key
	 * @return
	 * @throws InterruptedException
	 * @throws KeeperException
	 */
	public LockStat lock(String key) throws InterruptedException, KeeperException{
		if(getLock(key).isOwner()){
			return LockStat.NONEED;
		}
		getLock(key).lock();
		return LockStat.SUCCESS;
	}
	
	public void clearLock(String key) throws InterruptedException, KeeperException{
		synchronized(getMutex(key)){
			DistributedReentrantLock l = cache.get(key);
			l.clear();
			cache.remove(key);
		}
	}	
	
	public void unlock(String key,LockStat stat) throws InterruptedException, KeeperException{
		unlock(key,stat,false);
	}

	public void unlock(String key,LockStat stat,boolean keepalive) throws InterruptedException, KeeperException{
		if(stat == null) return;
		if(LockStat.SUCCESS.equals(stat)){
			DistributedReentrantLock lock = getLock(key);
			boolean hasWaiter = lock.unlock();
			if(!hasWaiter && !keepalive){
				synchronized(getMutex(key)){
					lock.clear();
					cache.remove(key);
				}
			}
		}
	}
	
	public static enum LockStat{
		NONEED,
		SUCCESS
	}
}

/**
*DistributedReentrantLock.java
*本地线程之间锁争用，先使用虚拟机内部锁机制，减少结点间通信开销
*/
public class DistributedReentrantLock {
	private static final Logger logger = Logger.getLogger(DistributedReentrantLock.class);
 private ReentrantLock reentrantLock = new ReentrantLock();

 private WriteLock writeLock;
 private long timeout = 3*60*1000;
 
 private final Object mutex = new Object();
 private String dir;
 private String schema;
 
 private final ExitListener exitListener = new ExitListener(){
		@Override
		public void execute() {
			initWriteLock();
		}
	};
	
	private synchronized void initWriteLock(){
		logger.debug("初始化writeLock");
		writeLock = new WriteLock(dir,new LockListener(){

			@Override
			public void lockAcquired() {
				synchronized(mutex){
					mutex.notify();
				}
			}
			@Override
			public void lockReleased() {
			}
 		
 	},schema);
		
		if(writeLock != null && writeLock.zk != null){
			writeLock.zk.addExitListener(exitListener);
		}
		
		synchronized(mutex){
			mutex.notify();
		}
	}
	
 public DistributedReentrantLock(String dir,String schema) {	
 	this.dir = dir;
 	this.schema = schema;
 	initWriteLock();
 }

 public void lock(long timeout) throws InterruptedException, KeeperException {
 reentrantLock.lock();//多线程竞争时，先拿到第一层锁
 try{
 	boolean res = writeLock.trylock();
 	if(!res){
	 	synchronized(mutex){
					mutex.wait(timeout);
				}
	 	if(writeLock == null || !writeLock.isOwner()){
	 		throw new InterruptedException("锁超时");
	 	}
 	}
 }catch(InterruptedException e){
 	reentrantLock.unlock();
 	throw e;
 }catch(KeeperException e){
 	reentrantLock.unlock();
 	throw e;
 }
 }
 
 public void lock() throws InterruptedException, KeeperException {
 	lock(timeout);
 }

 public void destroy() throws KeeperException {
 	writeLock.unlock();
 }
 

 public boolean unlock(){
 	if(!isOwner()) return false;
 try{
 	writeLock.unlock();
 	reentrantLock.unlock();//多线程竞争时，释放最外层锁
 }catch(RuntimeException e){
 	reentrantLock.unlock();//多线程竞争时，释放最外层锁
 	throw e;
 }
 
 return reentrantLock.hasQueuedThreads();
 }



 public boolean isOwner() {
 return reentrantLock.isHeldByCurrentThread() && writeLock.isOwner();
 }

	public void clear() {
		writeLock.clear();
	}

}

/**
*WriteLock.java
*基于zk的锁实现
*一个最简单的场景如下：
*1.结点A请求加锁，在特定路径下注册自己（会话自增结点)，得到一个ID号1
*2.结点B请求加锁，在特定路径下注册自己（会话自增结点)，得到一个ID号2
*3.结点A获取所有结点ID，判断出来自己是最小结点号，于是获得锁
*4.结点B获取所有结点ID，判断出来自己不是最小结点，于是监听小于自己的最大结点（结点A）变更事件
*5.结点A拿到锁，处理业务，处理完，释放锁（删除自己）
*6.结点B收到结点A变更事件，判断出来自己已经是最小结点号，于是获得锁。
*/
public class WriteLock extends ZkPrimative {
 private static final Logger LOG = Logger.getLogger(WriteLock.class);

 private final String dir;
 private String id;
 private LockNode idName;
 private String ownerId;
 private String lastChildId;
 private byte[] data = {0x12, 0x34};
 private LockListener callback;
 
 public WriteLock(String dir,String schema) {
 super(schema,true);
 this.dir = dir;
 }
 
 public WriteLock(String dir,LockListener callback,String schema) {
 	this(dir,schema);
 <a href="http://www.nbso.ca/">nbso online casino reviews</a>  this.callback = callback;
 }

 public LockListener getLockListener() {
 return this.callback;
 }
 
 public void setLockListener(LockListener callback) {
 this.callback = callback;
 }

 public synchronized void unlock() throws RuntimeException {
 	if(zk == null || zk.isClosed()){
 		return;
 	}
 if (id != null) {
 try {
 	 zk.delete(id, -1); 
 } 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;
 }
 }
 }
 
 private class LockWatcher implements Watcher {
 public void process(WatchedEvent event) {
 LOG.debug("Watcher fired on path: " event.getPath() " state: " 
 event.getState() " type " event.getType());
 try {
 trylock();
 } catch (Exception e) {
 LOG.warn("Failed to acquire lock: " e, e);
 }
 }
 }
 
 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, 
 acl, EPHEMERAL_SEQUENTIAL);

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

 }

	public void clear() {
		if(zk == null || zk.isClosed()){
 		return;
 	}
		try {
			zk.delete(dir, -1);
		} catch (Exception e) {
			 LOG.error("clear error: " e,e);
		} 
	}
	
 public synchronized boolean trylock() throws KeeperException, InterruptedException {
 	if(zk == null){
 		LOG.info("zk 是空");
 		return false;
 	}
 if (zk.isClosed()) {
 	LOG.info("zk 已经关闭");
 return false;
 }
 ensurePathExists(dir);
 
 LOG.debug("id:" id);
 do {
 if (id == null) {
 long sessionId = zk.getSessionId();
 String prefix = "x-" sessionId "-";
 idName = new LockNode(id);
 LOG.debug("idName:" idName);
 }
 if (id != null) {
 List<String> names = zk.getChildren(dir, false);
 if (names.isEmpty()) {
 LOG.warn("No children in: " dir " when we've just " 
 "created one! Lets recreate it...");
 id = null;
 } else {
 SortedSet<LockNode> sortedNames = new TreeSet<LockNode>();
 for (String name : names) {
 sortedNames.add(new LockNode(dir "/" name));
 }
 ownerId = sortedNames.first().getName();
 LOG.debug("all:" sortedNames);
 SortedSet<LockNode> lessThanMe = sortedNames.headSet(idName);
 LOG.debug("less than me:" lessThanMe);
 if (!lessThanMe.isEmpty()) {
 	LockNode lastChildName = lessThanMe.last();
 lastChildId = lastChildName.getName();
 if (LOG.isDebugEnabled()) {
 LOG.debug("watching less than me node: " lastChildId);
 }
 Stat stat = zk.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;
 }

 public String getDir() {
 return dir;
 }

 public boolean isOwner() {
 return id != null && ownerId != null && id.equals(ownerId);
 }

 public String getId() {
 return this.id;
 }
}


使用本方案实现的分布式锁，可以很好地解决锁重入的问题，而且使用会话结点来避免死锁；性能方面，根据笔者自测结果，加锁解锁各一次算是一个操作，本方案实现的分布式锁，TPS大概为2000-3000，性能比较一般；