1 场景描述

在分布式应用, 往往存在多个进程提供同一服务. 这些进程有可能在相同的机器上, 也有可能分布在不同的机器上. 如果这些进程共享了一些资源, 可能就需要分布式锁来锁定对这些资源的访问。

2 思路

进程需要访问共享数据时, 就在"/locks"节点下创建一个sequence类型的子节点, 称为thisPath. 当thisPath在所有子节点中最小时, 说明该进程获得了锁. 进程获得锁之后, 就可以访问共享资源了. 访问完成后, 需要将thisPath删除. 锁由新的最小的子节点获得.

有了清晰的思路之后, 还需要补充一些细节. 进程如何知道thisPath是所有子节点中最小的呢? 可以在创建的时候, 通过getChildren方法获取子节点列表, 然后在列表中找到排名比thisPath前1位的节点, 称为waitPath, 然后在waitPath上注册监听, 当waitPath被删除后, 进程获得通知, 此时说明该进程获得了锁.

3 算法

1. lock操作过程：

   首先为一个lock场景，在zookeeper中指定对应的一个根节点，用于记录资源竞争的内容；

   每个lock创建后，会lazy在zookeeper中创建一个node节点，表明对应的资源竞争标识。 (小技巧：node节点为EPHEMERAL_SEQUENTIAL，自增长的临时节点)；

   进行lock操作时，获取对应lock根节点下的所有子节点，也即处于竞争中的资源标识；

   按照Fair（公平）竞争的原则，按照对应的自增内容做排序，取出编号最小的一个节点做为lock的owner，判断自己的节点id是否就为owner id，如果是则返回，lock成功。

   如果自己非owner id，按照排序的结果找到序号比自己前一位的id，关注它锁释放的操作(也就是exist watcher)，形成一个链式的触发过程；

2. unlock操作过程：

   将自己id对应的节点删除即可，对应的下一个排队的节点就可以收到Watcher事件，从而被唤醒得到锁后退出；

3. 其中的几个关键点：

   node节点选择为EPHEMERAL_SEQUENTIAL很重要。

   自增长的特性，可以方便构建一个基于Fair特性的锁，前一个节点唤醒后一个节点，形成一个链式的触发过程。可以有效的避免"惊群效应"(一个锁释放，所有等待的线程都被唤醒)，有针对性的唤醒，提升性能。

   选择一个EPHEMERAL临时节点的特性。因为和zookeeper交互是一个网络操作，不可控因素过多，比如网络断了，上一个节点释放锁的操作会失败。临时节点是和对应的session挂接的，session一旦超时或者异常退出其节点就会消失，类似于ReentrantLock中等待队列Thread的被中断处理。

   获取lock操作是一个阻塞的操作，而对应的Watcher是一个异步事件，所以需要使用互斥信号共享锁BooleanMutex进行通知，可以比较方便的解决锁重入的问题。(锁重入可以理解为多次读操作，锁释放为写抢占操作)

4. 注意：

   使用EPHEMERAL会引出一个风险：在非正常情况下，网络延迟比较大会出现session timeout，zookeeper就会认为该client已关闭，从而销毁其id标示，竞争资源的下一个id就可以获取锁。这时可能会有两个process同时拿到锁在跑任务，所以设置好session timeout很重要。

   同样使用PERSISTENT同样会存在一个死锁的风险，进程异常退出后，对应的竞争资源id一直没有删除，下一个id一直无法获取到锁对象。

4 实现

1. DistributedLock.java源码：分布式锁

 package com.king.lock; import java.io.IOException;
 import java.util.List;
 import java.util.SortedSet;
 import java.util.TreeSet; import org.apache.commons.lang3.StringUtils;
 import org.apache.zookeeper.*;
 import org.apache.zookeeper.data.Stat; /**
  * Zookeeper 分布式锁
  */
 public class DistributedLock {     private static final int SESSION_TIMEOUT = 10000;     private static final int DEFAULT_TIMEOUT_PERIOD = 10000;     private static final String CONNECTION_STRING = "127.0.0.1:2180,127.0.0.1:2181,127.0.0.1:2182,127.0.0.1:2183";     private static final byte[] data = {0x12, 0x34};     private ZooKeeper zookeeper;     private String root;     private String id;     private LockNode idName;     private String ownerId;     private String lastChildId;     private Throwable other = null;     private KeeperException exception = null;     private InterruptedException interrupt = null;     public DistributedLock(String root) {
         try {
             this.zookeeper = new ZooKeeper(CONNECTION_STRING, SESSION_TIMEOUT, null);
             this.root = root;
             ensureExists(root);
         } catch (IOException e) {
             e.printStackTrace();
             other = e;
         }
     }     /**
      * 尝试获取锁操作，阻塞式可被中断
      */
     public void lock() throws Exception {
         // 可能初始化的时候就失败了
         if (exception != null) {
             throw exception;
         }         if (interrupt != null) {
             throw interrupt;
         }         if (other != null) {
             throw new Exception("", other);
         }         if (isOwner()) {// 锁重入
             return;
         }         BooleanMutex mutex = new BooleanMutex();
         acquireLock(mutex);
         // 避免zookeeper重启后导致watcher丢失，会出现死锁使用了超时进行重试
         try {
 //            mutex.lockTimeOut(DEFAULT_TIMEOUT_PERIOD, TimeUnit.MICROSECONDS);// 阻塞等待值为true
             mutex.lock();
         } catch (Exception e) {
             e.printStackTrace();
             if (!mutex.state()) {
                 lock();
             }
         }         if (exception != null) {
             throw exception;
         }         if (interrupt != null) {
             throw interrupt;
         }         if (other != null) {
             throw new Exception("", other);
         }
     }     /**
      * 尝试获取锁对象, 不会阻塞
      *
      * @throws InterruptedException
      * @throws KeeperException
      */
     public boolean tryLock() throws Exception {
         // 可能初始化的时候就失败了
         if (exception != null) {
             throw exception;
         }         if (isOwner()) { // 锁重入
             return true;
         }         acquireLock(null);         if (exception != null) {
             throw exception;
         }         if (interrupt != null) {
             Thread.currentThread().interrupt();
         }         if (other != null) {
             throw new Exception("", other);
         }         return isOwner();
     }     /**
      * 释放锁对象
      */
     public void unlock() throws KeeperException {
         if (id != null) {
             try {
                 zookeeper.delete(root + "/" + id, -1);
             } catch (InterruptedException e) {
                 Thread.currentThread().interrupt();
             } catch (KeeperException.NoNodeException e) {
                 // do nothing
             } finally {
                 id = null;
             }
         } else {
             //do nothing
         }
     }     /**
      * 判断某path节点是否存在，不存在就创建
      * @param path
      */
     private void ensureExists(final String path) {
         try {
             Stat stat = zookeeper.exists(path, false);
             if (stat != null) {
                 return;
             }
             zookeeper.create(path, data, ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
         } catch (KeeperException e) {
             exception = e;
         } catch (InterruptedException e) {
             Thread.currentThread().interrupt();
             interrupt = e;
         }
     }     /**
      * 返回锁对象对应的path
      */
     public String getRoot() {
         return root;
     }     /**
      * 判断当前是不是锁的owner
      */
     public boolean isOwner() {
         return id != null && ownerId != null && id.equals(ownerId);
     }     /**
      * 返回当前的节点id
      */
     public String getId() {
         return this.id;
     }     // ===================== helper method =============================     /**
      * 执行lock操作，允许传递watch变量控制是否需要阻塞lock操作
      */
     private Boolean acquireLock(final BooleanMutex mutex) {
         try {
             do {
                 if (id == null) { // 构建当前lock的唯一标识
                     long sessionId = zookeeper.getSessionId();
                     String prefix = "x-" + sessionId + "-";
                     // 如果第一次，则创建一个节点
                     String path = zookeeper.create(root + "/" + prefix, data, ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
                     int index = path.lastIndexOf("/");
                     id = StringUtils.substring(path, index + 1);
                     idName = new LockNode(id);
                 }                 if (id != null) {
                     List<String> names = zookeeper.getChildren(root, false);
                     if (names.isEmpty()) {
                         id = null; // 异常情况，重新创建一个
                     } else {
                         // 对节点进行排序
                         SortedSet<LockNode> sortedNames = new TreeSet<>();
                         for (String name : names) {
                             sortedNames.add(new LockNode(name));
                         }                         if (!sortedNames.contains(idName)) {
                             id = null;// 清空为null，重新创建一个
                             continue;
                         }                         // 将第一个节点做为ownerId
                         ownerId = sortedNames.first().getName();
                         if (mutex != null && isOwner()) {
                             mutex.unlock();// 直接更新状态，返回
                             return true;
                         } else if (mutex == null) {
                             return isOwner();
                         }                         SortedSet<LockNode> lessThanMe = sortedNames.headSet(idName);
                         if (!lessThanMe.isEmpty()) {
                             // 关注一下排队在自己之前的最近的一个节点
                             LockNode lastChildName = lessThanMe.last();
                             lastChildId = lastChildName.getName();
                             // 异步watcher处理
                             Stat stat = zookeeper.exists(root + "/" + lastChildId, new Watcher() {
                                 publicvoidprocess(WatchedEvent event) {
                                     acquireLock(mutex);
                                 }
                             });                             if (stat == null) {
                                 acquireLock(mutex);// 如果节点不存在，需要自己重新触发一下，watcher不会被挂上去
                             }
                         } else {
                             if (isOwner()) {
                                 mutex.unlock();
                             } else {
                                 id = null;// 可能自己的节点已超时挂了，所以id和ownerId不相同
                             }
                         }
                     }
                 }
             } while (id == null);
         } catch (KeeperException e) {
             exception = e;
             if (mutex != null) {
                 mutex.unlock();
             }
         } catch (InterruptedException e) {
             interrupt = e;
             if (mutex != null) {
                 mutex.unlock();
             }
         } catch (Throwable e) {
             other = e;
             if (mutex != null) {
                 mutex.unlock();
             }
         }         if (isOwner() && mutex != null) {
             mutex.unlock();
         }
         return Boolean.FALSE;
     }
 }

2. BooleanMutex.java源码：互斥信号共享锁

 package com.king.lock; import java.util.concurrent.TimeUnit;
 import java.util.concurrent.TimeoutException;
 import java.util.concurrent.locks.AbstractQueuedSynchronizer; /**
  * 互斥信号共享锁
  */
 public class BooleanMutex {     private Sync sync;     public BooleanMutex() {
         sync = new Sync();
         set(false);
     }     /**
      * 阻塞等待Boolean为true
      * @throws InterruptedException
      */
     public void lock() throws InterruptedException {
         sync.innerLock();
     }     /**
      * 阻塞等待Boolean为true,允许设置超时时间
      * @param timeout
      * @param unit
      * @throws InterruptedException
      * @throws TimeoutException
      */
     public void lockTimeOut(long timeout, TimeUnit unit) throws InterruptedException, TimeoutException {
         sync.innerLock(unit.toNanos(timeout));
     }     public void unlock(){
         set(true);
     }     /**
      * 重新设置对应的Boolean mutex
      * @param mutex
      */
     public void set(Boolean mutex) {
         if (mutex) {
             sync.innerSetTrue();
         } else {
             sync.innerSetFalse();
         }
     }     public boolean state() {
         return sync.innerState();
     }     /**
      * 互斥信号共享锁
      */
     private final class Sync extends AbstractQueuedSynchronizer {
         private static final long serialVersionUID = -7828117401763700385L;         /**
          * 状态为1，则唤醒被阻塞在状态为FALSE的所有线程
          */
         private static final int TRUE = 1;
         /**
          * 状态为0，则当前线程阻塞，等待被唤醒
          */
         private static final int FALSE = 0;         /**
          * 返回值大于0，则执行；返回值小于0，则阻塞
          */
         protected int tryAcquireShared(int arg) {
             return getState() == 1 ? 1 : -1;
         }         /**
          * 实现AQS的接口，释放共享锁的判断
          */
         protected boolean tryReleaseShared(int ignore) {
             // 始终返回true，代表可以release
             return true;
         }         privatebooleaninnerState() {
             return getState() == 1;
         }         privatevoidinnerLock()throws InterruptedException {
             acquireSharedInterruptibly(0);
         }         privatevoidinnerLock(long nanosTimeout)throws InterruptedException, TimeoutException {
             if (!tryAcquireSharedNanos(0, nanosTimeout))
                 throw new TimeoutException();
         }         privatevoidinnerSetTrue() {
             for (;;) {
                 int s = getState();
                 if (s == TRUE) {
                     return; // 直接退出
                 }
                 if (compareAndSetState(s, TRUE)) {// cas更新状态，避免并发更新true操作
                     releaseShared(0);// 释放一下锁对象，唤醒一下阻塞的Thread
                 }
             }
         }         privatevoidinnerSetFalse() {
             for (;;) {
                 int s = getState();
                 if (s == FALSE) {
                     return; //直接退出
                 }
                 if (compareAndSetState(s, FALSE)) {//cas更新状态，避免并发更新false操作
                     setState(FALSE);
                 }
             }
         }
     }
 }

3. 相关说明：

4. 测试类：

 package com.king.lock; import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors; import org.apache.zookeeper.KeeperException; /**
  * 分布式锁测试
  * @author taomk
  * @version 1.0
  * @since 15-11-19 上午11:48
  */
 public class DistributedLockTest {     public static void main(String [] args) {
         ExecutorService executor = Executors.newCachedThreadPool();
         final int count = 50;
         final CountDownLatch latch = new CountDownLatch(count);
         for (int i = 0; i < count; i++) {
             final DistributedLock node = new DistributedLock("/locks");
             executor.submit(new Runnable() {
                 public void run() {
                     try {
                         Thread.sleep(1000);
 //                        node.tryLock(); // 无阻塞获取锁
                         node.lock(); // 阻塞获取锁
                         Thread.sleep(100);                         System.out.println("id: " + node.getId() + " is leader: " + node.isOwner());
                     } catch (InterruptedException e) {
                         e.printStackTrace();
                     } catch (KeeperException e) {
                         e.printStackTrace();
                     } catch (Exception e) {
                         e.printStackTrace();
                     } finally {
                         latch.countDown();
                         try {
                             node.unlock();
                         } catch (KeeperException e) {
                             e.printStackTrace();
                         }
                     }                 }
             });
         }         try {
             latch.await();
         } catch (InterruptedException e) {
             e.printStackTrace();
         }         executor.shutdown();
     }
 }
 控制台输出： id: x-239027745716109354-0000000248 is leader: true
 id: x-22854963329433645-0000000249 is leader: true
 id: x-22854963329433646-0000000250 is leader: true
 id: x-166970151413415997-0000000251 is leader: true
 id: x-166970151413415998-0000000252 is leader: true
 id: x-166970151413415999-0000000253 is leader: true
 id: x-166970151413416000-0000000254 is leader: true
 id: x-166970151413416001-0000000255 is leader: true
 id: x-166970151413416002-0000000256 is leader: true
 id: x-22854963329433647-0000000257 is leader: true
 id: x-239027745716109355-0000000258 is leader: true
 id: x-166970151413416003-0000000259 is leader: true
 id: x-94912557367427124-0000000260 is leader: true
 id: x-22854963329433648-0000000261 is leader: true
 id: x-239027745716109356-0000000262 is leader: true
 id: x-239027745716109357-0000000263 is leader: true
 id: x-166970151413416004-0000000264 is leader: true
 id: x-239027745716109358-0000000265 is leader: true
 id: x-239027745716109359-0000000266 is leader: true
 id: x-22854963329433649-0000000267 is leader: true
 id: x-22854963329433650-0000000268 is leader: true
 id: x-94912557367427125-0000000269 is leader: true
 id: x-22854963329433651-0000000270 is leader: true
 id: x-94912557367427126-0000000271 is leader: true
 id: x-239027745716109360-0000000272 is leader: true
 id: x-94912557367427127-0000000273 is leader: true
 id: x-94912557367427128-0000000274 is leader: true
 id: x-166970151413416005-0000000275 is leader: true
 id: x-94912557367427129-0000000276 is leader: true
 id: x-166970151413416006-0000000277 is leader: true
 id: x-94912557367427130-0000000278 is leader: true
 id: x-94912557367427131-0000000279 is leader: true
 id: x-239027745716109361-0000000280 is leader: true
 id: x-239027745716109362-0000000281 is leader: true
 id: x-166970151413416007-0000000282 is leader: true
 id: x-94912557367427132-0000000283 is leader: true
 id: x-22854963329433652-0000000284 is leader: true
 id: x-166970151413416008-0000000285 is leader: true
 id: x-239027745716109363-0000000286 is leader: true
 id: x-239027745716109364-0000000287 is leader: true
 id: x-166970151413416009-0000000288 is leader: true
 id: x-166970151413416010-0000000289 is leader: true
 id: x-239027745716109365-0000000290 is leader: true
 id: x-94912557367427133-0000000291 is leader: true
 id: x-239027745716109366-0000000292 is leader: true
 id: x-94912557367427134-0000000293 is leader: true
 id: x-22854963329433653-0000000294 is leader: true
 id: x-94912557367427135-0000000295 is leader: true
 id: x-239027745716109367-0000000296 is leader: true
 id: x-239027745716109368-0000000297 is leader: true

5 升级版

实现了一个分布式lock后，可以解决多进程之间的同步问题，但设计多线程+多进程的lock控制需求，单jvm中每个线程都和zookeeper进行网络交互成本就有点高了，所以基于DistributedLock，实现了一个分布式二层锁。

大致原理就是ReentrantLock 和 DistributedLock的一个结合：

1. 单jvm的多线程竞争时，首先需要先拿到第一层的ReentrantLock的锁；
2. 拿到锁之后这个线程再去和其他JVM的线程竞争锁，最后拿到之后锁之后就开始处理任务；

锁的释放过程是一个反方向的操作，先释放DistributedLock，再释放ReentrantLock。 可以思考一下，如果先释放ReentrantLock，假如这个JVM ReentrantLock竞争度比较高，一直其他JVM的锁竞争容易被饿死。

1. DistributedReentrantLock.java源码：多进程+多线程分布式锁

 package com.king.lock; import java.text.MessageFormat;
 import java.util.concurrent.locks.ReentrantLock; import org.apache.zookeeper.KeeperException; /**
  * 多进程+多线程分布式锁
  */
 public class DistributedReentrantLock extends DistributedLock {     private static final String ID_FORMAT = "Thread[{0}] Distributed[{1}]";
     private ReentrantLock reentrantLock = new ReentrantLock();     public DistributedReentrantLock(String root) {
         super(root);
     }     public void lock() throws Exception {
         reentrantLock.lock();//多线程竞争时，先拿到第一层锁
         super.lock();
     }     public boolean tryLock() throws Exception {
         //多线程竞争时，先拿到第一层锁
         return reentrantLock.tryLock() && super.tryLock();
     }     public void unlock() throws KeeperException {
         super.unlock();
         reentrantLock.unlock();//多线程竞争时，释放最外层锁
     }     @Override
     public String getId() {
         return MessageFormat.format(ID_FORMAT, Thread.currentThread().getId(), super.getId());
     }     @Override
     public boolean isOwner() {
         return reentrantLock.isHeldByCurrentThread() && super.isOwner();
     }
 }

2. 测试代码：

 package com.king.lock; import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors; import org.apache.zookeeper.KeeperException; /**
  * @author taomk
  * @version 1.0
  * @since 15-11-23 下午12:15
  */
 public class DistributedReentrantLockTest {     public static void main(String [] args) {
         ExecutorService executor = Executors.newCachedThreadPool();
         final int count = 50;
         final CountDownLatch latch = new CountDownLatch(count);         final DistributedReentrantLock lock = new DistributedReentrantLock("/locks"); //单个锁
         for (int i = 0; i < count; i++) {
             executor.submit(new Runnable() {
                 public void run() {
                     try {
                         Thread.sleep(1000);
                         lock.lock();
                         Thread.sleep(100);                         System.out.println("id: " + lock.getId() + " is leader: " + lock.isOwner());
                     } catch (Exception e) {
                         e.printStackTrace();
                     } finally {
                         latch.countDown();
                         try {
                             lock.unlock();
                         } catch (KeeperException e) {
                             e.printStackTrace();
                         }
                     }
                 }
             });
         }         try {
             latch.await();
         } catch (InterruptedException e) {
             e.printStackTrace();
         }         executor.shutdown();
     }
 }

6 最后

其实再可以发散一下，实现一个分布式的read/write lock，也差不多就是这个理了。大致思路：

1. 竞争资源标示： read_自增id , write_自增id；
2. 首先按照自增id进行排序，如果队列的前边都是read标识，对应的所有read都获得锁。如果队列的前边是write标识，第一个write节点获取锁；
3. watcher监听： read监听距离自己最近的一个write节点的exist，write监听距离自己最近的一个节点(read或者write节点)；