之前写过利用Redis实现分布式锁的文章，刚好最近在学习ZooKpeer的一些知识，想着利用Zookeeper去实现分布式锁，查阅了一些文章，发现大家一致认为高并发场景下并不适合用ZooKeeper去实现，性能存在问题。为什么呢？ZooKeeper的性能问题，知乎上有人给出了两条：
1. 本身不是为高可用性设计，master撑不住高流量容易导致系统crash。
2. Paxos算法的复杂性，选举过程或许缓慢，结合上一点，动辄需要重新选举master导致耗时过长。
有人将ZooKeeper类比于AQS(AbstractQueuedSynchronizer)。AQS是完成多线程协同一致的利器，ZooKeeper是完成跨机器多进程之间的协同一致性的利器，也确实是有那么点相似之处(这里纯属瞎扯啊，但不得不说佩服别人的联想能力)。
image.png
image.png
Zookeeper实现分布式锁的大致思路是：
1. 每个客户端都去zookeeper上创建临时的顺序节点
2. 客户端判断当前自己创建的节点是不是最小的
3. 如果是的话，就获得了执行当前任务的锁
4. 如果不是的话，就找到比自己小的节点，然后进行监听，如果被删除的话，就可以获得锁
了解原理之后，我们得去编码实现分布式锁：
package com.meituan.ZooKeeperDistributeLock;
import org.apache.zookeeper.*;
import java.io.IOException;
import java.util.concurrent.*;
/**
* @Author: taomin
* @Create_time: 2018-03-11
* @Description: zookeeper实现分布式锁
*/
public class LockTest {
private String zkQurom = "127.0.0.1:2181";
private String lockNameSpace = "/home";
private final String nodeString = lockNameSpace + "/test";
private ZooKeeper zk;
public LockTest() {
try {
zk = new ZooKeeper(zkQurom, 6000, new Watcher() {
public void process(WatchedEvent watchedEvent) {
System.out.println("Receive event " + watchedEvent);
if (Event.KeeperState.SyncConnected == watchedEvent.getState())
System.out.println("connection is established...");
}
});
} catch (IOException e) {
e.printStackTrace();
}
}
private void watchNode(final String nodeString, final Thread thread) throws InterruptedException {
try {
zk.exists(nodeString, new Watcher() {
public void process(WatchedEvent watchedEvent) {
System.out.println("==" + watchedEvent.toString());
if (watchedEvent.getType() == Event.EventType.NodeDeleted) {
System.out.println(Thread.currentThread().getName() + " released Lock ==============");
thread.interrupt();
}
try {
zk.exists(nodeString, new Watcher() {
public void process(WatchedEvent watchedEvent) {
System.out.println("==" + watchedEvent.toString());
if (watchedEvent.getType() == Event.EventType.NodeDeleted) {
System.out.printf(Thread.currentThread().getName() + " released Lock ==============");
thread.interrupt();
}
try {
zk.exists(nodeString, true);
} catch (KeeperException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
} catch (KeeperException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
} catch (KeeperException e) {
e.printStackTrace();
}
}
private void ensureRootPath() throws InterruptedException {
try {
if (zk.exists(lockNameSpace, true) == null) {
zk.create(lockNameSpace, "".getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
}
} catch (KeeperException e) {
e.printStackTrace();
}
}
public void unlock() {
try {
zk.delete(nodeString, -1);
System.out.println(Thread.currentThread().getId() + " release Lock ==============");
} catch (InterruptedException e) {
e.printStackTrace();
} catch (KeeperException e) {
e.printStackTrace();
}
}
public boolean lock() throws InterruptedException {
String path = null;
ensureRootPath();
watchNode(nodeString, Thread.currentThread());
while (true) {
try {
path = zk.create(nodeString, "".getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL);
} catch (KeeperException e) {
System.out.println(Thread.currentThread().getName() + " getting Lock but can not get ==============");
try {
Thread.sleep(5000);
} catch (InterruptedException ex) {
System.out.println("thread is notify");
}
}
if (path != null && !path.trim().equals("")) {
System.out.println(Thread.currentThread().getName() + " get Lock ==============");
return true;
}
}
}
public static void main(String args[]) throws InterruptedException {
BlockingQueue<Runnable> queue = new ArrayBlockingQueue<Runnable>(10);
ThreadPoolExecutor service = new ThreadPoolExecutor(5, 10,
60, TimeUnit.MICROSECONDS, queue);
for (int i = 0; i < 4; i++) {
service.execute(new Runnable() {
public void run() {
LockTest test = new LockTest();
try {
test.lock();
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
test.unlock();
}
});
}
service.shutdown();
}
}
输出日志，从日志里面可以清楚看到线程竞争锁的过程。
Receive event WatchedEvent state:SyncConnected type:None path:null
connection is established...
Receive event WatchedEvent state:SyncConnected type:None path:null
connection is established...
Receive event WatchedEvent state:SyncConnected type:None path:null
connection is established...
Receive event WatchedEvent state:SyncConnected type:None path:null
connection is established...
==WatchedEvent state:SyncConnected type:NodeCreated path:/home/test
==WatchedEvent state:SyncConnected type:NodeCreated path:/home/test
==WatchedEvent state:SyncConnected type:NodeCreated path:/home/test
==WatchedEvent state:SyncConnected type:NodeCreated path:/home/test
pool-1-thread-2 get Lock ==============
pool-1-thread-1 getting Lock but can not get ==============
pool-1-thread-4 getting Lock but can not get ==============
pool-1-thread-3 getting Lock but can not get ==============
==WatchedEvent state:SyncConnected type:NodeDeleted path:/home/test
==WatchedEvent state:SyncConnected type:NodeDeleted path:/home/test
pool-1-thread-3-EventThread released Lock ==============
==WatchedEvent state:SyncConnected type:NodeDeleted path:/home/test
pool-1-thread-2-EventThread released Lock ==============
==WatchedEvent state:SyncConnected type:NodeDeleted path:/home/test
thread is notify
pool-1-thread-2 release Lock ==============
pool-1-thread-4-EventThread released Lock ==============
pool-1-thread-1-EventThread released Lock ==============
thread is notify
thread is notify
Receive event WatchedEvent state:SyncConnected type:NodeCreated path:/home/test
connection is established...
Receive event WatchedEvent state:SyncConnected type:NodeCreated path:/home/test
connection is established...
pool-1-thread-3 get Lock ==============
pool-1-thread-4 getting Lock but can not get ==============
pool-1-thread-1 getting Lock but can not get ==============
Receive event WatchedEvent state:SyncConnected type:NodeDeleted path:/home/test
connection is established...
Receive event WatchedEvent state:SyncConnected type:NodeDeleted path:/home/test
connection is established...
pool-1-thread-3 release Lock ==============
pool-1-thread-4 get Lock ==============
pool-1-thread-1 getting Lock but can not get ==============
pool-1-thread-4 release Lock ==============
pool-1-thread-1 get Lock ==============
pool-1-thread-1 release Lock ==============
接触Zookeeper不久，理解不够深刻，写文章除了记录自己的所见所闻外，同时也想借此来勉励自己不放弃学习，每天一点点进步，明天会有不一样的自己。
end~
