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社区首页 >专栏 >ZooKeeper 分布式锁实现

ZooKeeper 分布式锁实现

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用户1263954
发布2018-06-22 12:24:15
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发布2018-06-22 12:24:15
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文章被收录于专栏:IT技术精选文摘IT技术精选文摘

#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

  1. 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

  1. 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)

  1. 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() {								public void process(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;
		}		private boolean innerState() {			return getState() == 1;
		}		private void innerLock() throws InterruptedException {
			acquireSharedInterruptibly(0);
		}		private void innerLock(long nanosTimeout) throws InterruptedException, TimeoutException {			if (!tryAcquireSharedNanos(0, nanosTimeout))				throw new TimeoutException();
		}		private void innerSetTrue() {			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
				}
			}
		}		private void innerSetFalse() {			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: trueid: x-22854963329433645-0000000249 is leader: trueid: x-22854963329433646-0000000250 is leader: trueid: x-166970151413415997-0000000251 is leader: trueid: x-166970151413415998-0000000252 is leader: trueid: x-166970151413415999-0000000253 is leader: trueid: x-166970151413416000-0000000254 is leader: trueid: x-166970151413416001-0000000255 is leader: trueid: x-166970151413416002-0000000256 is leader: trueid: x-22854963329433647-0000000257 is leader: trueid: x-239027745716109355-0000000258 is leader: trueid: x-166970151413416003-0000000259 is leader: trueid: x-94912557367427124-0000000260 is leader: trueid: x-22854963329433648-0000000261 is leader: trueid: x-239027745716109356-0000000262 is leader: trueid: x-239027745716109357-0000000263 is leader: trueid: x-166970151413416004-0000000264 is leader: trueid: x-239027745716109358-0000000265 is leader: trueid: x-239027745716109359-0000000266 is leader: trueid: x-22854963329433649-0000000267 is leader: trueid: x-22854963329433650-0000000268 is leader: trueid: x-94912557367427125-0000000269 is leader: trueid: x-22854963329433651-0000000270 is leader: trueid: x-94912557367427126-0000000271 is leader: trueid: x-239027745716109360-0000000272 is leader: trueid: x-94912557367427127-0000000273 is leader: trueid: x-94912557367427128-0000000274 is leader: trueid: x-166970151413416005-0000000275 is leader: trueid: x-94912557367427129-0000000276 is leader: trueid: x-166970151413416006-0000000277 is leader: trueid: x-94912557367427130-0000000278 is leader: trueid: x-94912557367427131-0000000279 is leader: trueid: x-239027745716109361-0000000280 is leader: trueid: x-239027745716109362-0000000281 is leader: trueid: x-166970151413416007-0000000282 is leader: trueid: x-94912557367427132-0000000283 is leader: trueid: x-22854963329433652-0000000284 is leader: trueid: x-166970151413416008-0000000285 is leader: trueid: x-239027745716109363-0000000286 is leader: trueid: x-239027745716109364-0000000287 is leader: trueid: x-166970151413416009-0000000288 is leader: trueid: x-166970151413416010-0000000289 is leader: trueid: x-239027745716109365-0000000290 is leader: trueid: x-94912557367427133-0000000291 is leader: trueid: x-239027745716109366-0000000292 is leader: trueid: x-94912557367427134-0000000293 is leader: trueid: x-22854963329433653-0000000294 is leader: trueid: x-94912557367427135-0000000295 is leader: trueid: x-239027745716109367-0000000296 is leader: trueid: 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existwrite监听距离自己最近的一个节点(read或者write节点)

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