Zookeeper分布式锁实现
摘要：本文要使用Zookeeper来实现一个分布式锁，是一个悲观锁。
本文源码请在这里下载：https://github.com/appleappleapple/DistributeLearning
一、锁设计
获取锁实现思路：
1. 首先创建一个作为锁目录(znode)，通常用它来描述锁定的实体，称为:/lock_node
2. 希望获得锁的客户端在锁目录下创建znode，作为锁/lock_node的子节点，并且节点类型为有序临时节点(EPHEMERAL_SEQUENTIAL)；
例如：有两个客户端创建znode，分别为/lock_node/lock-1和/lock_node/lock-2
3. 当前客户端调用getChildren（/lock_node）得到锁目录所有子节点，不设置watch，接着获取小于自己(步骤2创建)的兄弟节点
4. 步骤3中获取小于自己的节点不存在 && 最小节点与步骤2中创建的相同，说明当前客户端顺序号最小，获得锁，结束。
5. 客户端监视(watch)相对自己次小的有序临时节点状态
6. 如果监视的次小节点状态发生变化，则跳转到步骤3，继续后续操作，直到退出锁竞争。
分布锁笔者这里就不做介绍了，来看看整个代码设计的流程图如下
二、代码
接下来我们就开始编程了~
1、DistributedLock接口定义
package com.github.distribute.lock;
import java.util.concurrent.TimeUnit;
public interface DistributedLock {
/**
* 尝试获取锁,不进行等待。得到返回true,
*
* @return
* @throws Exception
*/
public boolean tryLock() throws Exception;
/**
* 阻塞等待获取锁
*
* @throws Exception
*/
public void lock() throws Exception;
/**
* 在规定时间内等待获取锁
*
* @param time
* @param unit
* @return
* @throws Exception
*/
public boolean lock(long time, TimeUnit unit) throws Exception;
/**
* 释放锁
*
* @throws Exception
*/
public void unLock() throws Exception;
}
2、部分实现BaseDistributedLock
package com.github.distribute.zookeeper;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.Watcher.Event.EventType;
import org.apache.zookeeper.ZooDefs;
import org.apache.zookeeper.ZooKeeper;
import org.apache.zookeeper.data.Stat;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import com.github.distribute.lock.DistributedLock;
public abstract class BaseDistributedLock implements DistributedLock {
private static Logger logger = LoggerFactory.getLogger(BaseDistributedLock.class);
private ZooKeeper zooKeeper;
private String rootPath;// 根路径名
private String lockNamePre;// 锁前缀
private String currentLockPath;// 用于保存某个客户端在locker下面创建成功的顺序节点，用于后续相关操作使用（如判断）
private static int MAX_RETRY_COUNT = 10;// 最大重试次数
/**
* 初始化根目录
*/
private void init() {
try {
Stat stat = zooKeeper.exists(rootPath, false);// 判断一下根目录是否存在
if (stat == null) {
zooKeeper.create(rootPath, null, ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
}
} catch (Exception e) {
logger.error("create rootPath error", e);
}
}
/**
* 取得锁的排序号
*
* @param str
* @param lockName
* @return
*/
private String getLockNodeNumber(String str, String lockName) {
int index = str.lastIndexOf(lockName);
if (index >= 0) {
index += lockName.length();
return index <= str.length() ? str.substring(index) : "";
}
return str;
}
/**
* 取得锁的排序列表
*
* @return
* @throws Exception
*/
private List getSortedChildren() throws Exception {
List children = zooKeeper.getChildren(rootPath, false);
if (children != null && !children.isEmpty()) {
Collections.sort(children, new Comparator() {
public int compare(String lhs, String rhs) {
return getLockNodeNumber(lhs, lockNamePre).compareTo(getLockNodeNumber(rhs, lockNamePre));
}
});
}
logger.info("sort childRen:{}", children);
return children;
}
/**
* 删除锁节点
*/
private void deleteLockNode() {
try {
zooKeeper.delete(currentLockPath, -1);
} catch (Exception e) {
logger.error("unLock error", e);
}
}
/**
* 该方法用于判断自己是否获取到了锁，即自己创建的顺序节点在locker的所有子节点中是否最小.如果没有获取到锁，则等待其它客户端锁的释放，
* 并且稍后重试直到获取到锁或者超时
*
* @param startMillis
* @param millisToWait
* @param ourPath
* @return
* @throws Exception
*/
private boolean waitToLock(long startMillis, Long millisToWait) throws Exception {
boolean haveTheLock = false;
boolean doDelete = false;
try {
while (!haveTheLock) {
logger.info("get Lock Begin");
// 该方法实现获取locker节点下的所有顺序节点，并且从小到大排序,
List children = getSortedChildren();
String sequenceNodeName = currentLockPath.substring(rootPath.length() + 1);
// 计算刚才客户端创建的顺序节点在locker的所有子节点中排序位置，如果是排序为0，则表示获取到了锁
int ourIndex = children.indexOf(sequenceNodeName);
/*
* 如果在getSortedChildren中没有找到之前创建的[临时]顺序节点，这表示可能由于网络闪断而导致
* Zookeeper认为连接断开而删除了我们创建的节点，此时需要抛出异常，让上一级去处理
* 上一级的做法是捕获该异常，并且执行重试指定的次数 见后面的 attemptLock方法
*/
if (ourIndex < 0) {
logger.error("not find node:{}", sequenceNodeName);
throw new Exception("节点没有找到: " + sequenceNodeName);
}
// 如果当前客户端创建的节点在locker子节点列表中位置大于0，表示其它客户端已经获取了锁
// 此时当前客户端需要等待其它客户端释放锁，
boolean isGetTheLock = ourIndex == 0;
// 如何判断其它客户端是否已经释放了锁？从子节点列表中获取到比自己次小的哪个节点，并对其建立监听
String pathToWatch = isGetTheLock ? null : children.get(ourIndex - 1);
if (isGetTheLock) {
logger.info("get the lock,currentLockPath:{}", currentLockPath);
haveTheLock = true;
} else {
// 如果次小的节点被删除了，则表示当前客户端的节点应该是最小的了，所以使用CountDownLatch来实现等待
String previousSequencePath = rootPath.concat("/").concat(pathToWatch);
final CountDownLatch latch = new CountDownLatch(1);
final Watcher previousListener = new Watcher() {
public void process(WatchedEvent event) {
if (event.getType() == EventType.NodeDeleted) {
latch.countDown();
}
}
};
// 如果节点不存在会出现异常
zooKeeper.exists(previousSequencePath, previousListener);
// 如果有超时时间，刚到超时时间就返回
if (millisToWait != null) {
millisToWait -= (System.currentTimeMillis() - startMillis);
startMillis = System.currentTimeMillis();
if (millisToWait <= 0) {
doDelete = true; // timed out - delete our node
break;
}
latch.await(millisToWait, TimeUnit.MICROSECONDS);
} else {
latch.await();
}
}
}
} catch (Exception e) {
// 发生异常需要删除节点
logger.error("waitToLock exception", e);
doDelete = true;
throw e;
} finally {
// 如果需要删除节点
if (doDelete) {
deleteLockNode();
}
}
logger.info("get Lock end,haveTheLock=" + haveTheLock);
return haveTheLock;
}
/**
* createLockNode用于在locker（basePath持久节点）下创建客户端要获取锁的[临时]顺序节点
*
* @param path
* @return
* @throws Exception
*/
private String createLockNode(String path) throws Exception {
Stat stat = zooKeeper.exists(rootPath, false);
// 判断一下根目录是否存在
if (stat == null) {
zooKeeper.create(rootPath, null, ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
}
return zooKeeper.create(path, null, ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
}
/**
* 尝试获取锁，如果不加超时时间，阻塞等待。否则，就是加了超时的阻塞等待
*
* @param time
* @param unit
* @return
* @throws Exception
*/
protected Boolean attemptLock(long time, TimeUnit unit) throws Exception {
final long startMillis = System.currentTimeMillis();
final Long millisToWait = (unit != null) ? unit.toMillis(time) : null;
boolean hasTheLock = false;
boolean isDone = false;
int retryCount = 0;
// 网络闪断需要重试一试，最大重试次数MAX_RETRY_COUNT
while (!isDone) {
isDone = true;
try {
currentLockPath = createLockNode(rootPath.concat("/").concat(lockNamePre));
hasTheLock = waitToLock(startMillis, millisToWait);
} catch (Exception e) {
if (retryCount++ < MAX_RETRY_COUNT) {
isDone = false;
} else {
throw e;
}
}
}
return hasTheLock;
}
}
waitToLock是最主要的代码
3、完整实现
package com.github.distribute.zookeeper;
import java.io.IOException;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.Watcher.Event.EventType;
import org.apache.zookeeper.ZooDefs;
import org.apache.zookeeper.ZooKeeper;
import org.apache.zookeeper.data.Stat;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import com.github.distribute.lock.DistributedLock;
public class ZookeeperDistributeLock implements DistributedLock {
private static Logger logger = LoggerFactory.getLogger(ZookeeperDistributeLock.class);
public static void main(String[] args) throws IOException {
ZooKeeper zooKeeper = new ZooKeeper("127.0.0.1:2181", 60000, null);
ZookeeperDistributeLock myLock = new ZookeeperDistributeLock(zooKeeper, "/test", "lock-");
while (true) {
try {
myLock.lock();
Thread.sleep(5000);
} catch (Exception e) {
} finally {
myLock.unLock();
}
}
}
private ZooKeeper zooKeeper;
private String rootPath;// 根路径名
private String lockNamePre;// 锁前缀
private String currentLockPath;// 用于保存某个客户端在locker下面创建成功的顺序节点，用于后续相关操作使用（如判断）
private static int MAX_RETRY_COUNT = 10;// 最大重试次数
public ZookeeperDistributeLock(ZooKeeper zookeeper, String rootPath, String lockNamePre) {
logger.info("rootPath:{},lockNamePre:{}", rootPath, lockNamePre);
this.zooKeeper = zookeeper;
this.rootPath = rootPath;
this.lockNamePre = lockNamePre;
init();
}
/**
* 初始化根目录
*/
private void init() {
try {
Stat stat = zooKeeper.exists(rootPath, false);// 判断一下根目录是否存在
if (stat == null) {
zooKeeper.create(rootPath, null, ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
}
} catch (Exception e) {
logger.error("create rootPath error", e);
}
}
/**
* 取得锁的排序号
*
* @param str
* @param lockName
* @return
*/
private String getLockNodeNumber(String str, String lockName) {
int index = str.lastIndexOf(lockName);
if (index >= 0) {
index += lockName.length();
return index <= str.length() ? str.substring(index) : "";
}
return str;
}
/**
* 取得锁的排序列表
*
* @return
* @throws Exception
*/
private List getSortedChildren() throws Exception {
List children = zooKeeper.getChildren(rootPath, false);
if (children != null && !children.isEmpty()) {
Collections.sort(children, new Comparator() {
public int compare(String lhs, String rhs) {
return getLockNodeNumber(lhs, lockNamePre).compareTo(getLockNodeNumber(rhs, lockNamePre));
}
});
}
logger.info("sort childRen:{}", children);
return children;
}
/**
* 该方法用于判断自己是否获取到了锁，即自己创建的顺序节点在locker的所有子节点中是否最小.如果没有获取到锁，则等待其它客户端锁的释放，
* 并且稍后重试直到获取到锁或者超时
*
* @param startMillis
* @param millisToWait
* @param ourPath
* @return
* @throws Exception
*/
private boolean waitToLock(long startMillis, Long millisToWait) throws Exception {
boolean haveTheLock = false;
boolean doDelete = false;
try {
while (!haveTheLock) {
logger.info("get Lock Begin");
// 该方法实现获取locker节点下的所有顺序节点，并且从小到大排序,
List children = getSortedChildren();
String sequenceNodeName = currentLockPath.substring(rootPath.length() + 1);
// 计算刚才客户端创建的顺序节点在locker的所有子节点中排序位置，如果是排序为0，则表示获取到了锁
int ourIndex = children.indexOf(sequenceNodeName);
/*
* 如果在getSortedChildren中没有找到之前创建的[临时]顺序节点，这表示可能由于网络闪断而导致
* Zookeeper认为连接断开而删除了我们创建的节点，此时需要抛出异常，让上一级去处理
* 上一级的做法是捕获该异常，并且执行重试指定的次数 见后面的 attemptLock方法
*/
if (ourIndex < 0) {
logger.error("not find node:{}", sequenceNodeName);
throw new Exception("节点没有找到: " + sequenceNodeName);
}
// 如果当前客户端创建的节点在locker子节点列表中位置大于0，表示其它客户端已经获取了锁
// 此时当前客户端需要等待其它客户端释放锁，
boolean isGetTheLock = ourIndex == 0;
// 如何判断其它客户端是否已经释放了锁？从子节点列表中获取到比自己次小的哪个节点，并对其建立监听
String pathToWatch = isGetTheLock ? null : children.get(ourIndex - 1);
if (isGetTheLock) {
logger.info("get the lock,currentLockPath:{}", currentLockPath);
haveTheLock = true;
} else {
// 如果次小的节点被删除了，则表示当前客户端的节点应该是最小的了，所以使用CountDownLatch来实现等待
String previousSequencePath = rootPath.concat("/").concat(pathToWatch);
final CountDownLatch latch = new CountDownLatch(1);
final Watcher previousListener = new Watcher() {
public void process(WatchedEvent event) {
if (event.getType() == EventType.NodeDeleted) {
latch.countDown();
}
}
};
// 如果节点不存在会出现异常
zooKeeper.exists(previousSequencePath, previousListener);
// 如果有超时时间，刚到超时时间就返回
if (millisToWait != null) {
millisToWait -= (System.currentTimeMillis() - startMillis);
startMillis = System.currentTimeMillis();
if (millisToWait <= 0) {
doDelete = true; // timed out - delete our node
break;
}
latch.await(millisToWait, TimeUnit.MICROSECONDS);
} else {
latch.await();
}
}
}
} catch (Exception e) {
// 发生异常需要删除节点
logger.error("waitToLock exception", e);
doDelete = true;
throw e;
} finally {
// 如果需要删除节点
if (doDelete) {
unLock();
}
}
logger.info("get Lock end,haveTheLock=" + haveTheLock);
return haveTheLock;
}
/**
* createLockNode用于在locker（basePath持久节点）下创建客户端要获取锁的[临时]顺序节点
*
* @param path
* @return
* @throws Exception
*/
private String createLockNode(String path) throws Exception {
Stat stat = zooKeeper.exists(rootPath, false);
// 判断一下根目录是否存在
if (stat == null) {
zooKeeper.create(rootPath, null, ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
}
return zooKeeper.create(path, null, ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
}
/**
* 尝试获取锁，如果不加超时时间，阻塞等待。否则，就是加了超时的阻塞等待
*
* @param time
* @param unit
* @return
* @throws Exception
*/
private Boolean attemptLock(long time, TimeUnit unit) throws Exception {
final long startMillis = System.currentTimeMillis();
final Long millisToWait = (unit != null) ? unit.toMillis(time) : null;
boolean hasTheLock = false;
boolean isDone = false;
int retryCount = 0;
// 网络闪断需要重试一试，最大重试次数MAX_RETRY_COUNT
while (!isDone) {
isDone = true;
try {
currentLockPath = createLockNode(rootPath.concat("/").concat(lockNamePre));
hasTheLock = waitToLock(startMillis, millisToWait);
} catch (Exception e) {
if (retryCount++ < MAX_RETRY_COUNT) {
isDone = false;
} else {
throw e;
}
}
}
return hasTheLock;
}
public boolean tryLock() throws Exception {
logger.info("tryLock Lock Begin");
// 该方法实现获取locker节点下的所有顺序节点，并且从小到大排序,
List children = getSortedChildren();
String sequenceNodeName = currentLockPath.substring(rootPath.length() + 1);
// 计算刚才客户端创建的顺序节点在locker的所有子节点中排序位置，如果是排序为0，则表示获取到了锁
int ourIndex = children.indexOf(sequenceNodeName);
if (ourIndex < 0) {
logger.error("not find node:{}", sequenceNodeName);
throw new Exception("节点没有找到: " + sequenceNodeName);
}
// 如果当前客户端创建的节点在locker子节点列表中位置大于0，表示其它客户端已经获取了锁
return ourIndex == 0;
}
public void lock() throws Exception {
// -1,null表示阻塞等待，不设置超时时间
attemptLock(-1, null);
}
public boolean lock(long time, TimeUnit unit) throws Exception {
if (time <= 0) {
throw new Exception("Lock wait for time must greater than 0");
}
if (unit == null) {
throw new Exception("TimeUnit can not be null");
}
return attemptLock(time, unit);
}
public void unLock() {
try {
zooKeeper.delete(currentLockPath, -1);
} catch (Exception e) {
logger.error("unLock error", e);
}
}
}
三、对比
在文章Redis分布式锁----悲观锁实现,以秒杀系统为例，我们用redis也实现了分布式锁。zk的方案最大的优势在于避免结点挂掉后导致的死锁；redis的方案最大的优势在于性能超强；在实际生产过程中，结合自身情况来决定最适合的分布式锁。
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