之前寫過利用Redis實現(xiàn)分布式鎖的文章，剛好最近在學(xué)習(xí)ZooKpeer的一些知識，想著利用Zookeeper去實現(xiàn)分布式鎖，查閱了一些文章，發(fā)現(xiàn)大家一致認為高并發(fā)場景下并不適合用ZooKeeper去實現(xiàn)，性能存在問題。為什么呢？ZooKeeper的性能問題，知乎上有人給出了兩條：
1. 本身不是為高可用性設(shè)計，master撐不住高流量容易導(dǎo)致系統(tǒng)crash。
2. Paxos算法的復(fù)雜性，選舉過程或許緩慢，結(jié)合上一點，動輒需要重新選舉master導(dǎo)致耗時過長。
有人將ZooKeeper類比于AQS(AbstractQueuedSynchronizer)。AQS是完成多線程協(xié)同一致的利器，ZooKeeper是完成跨機器多進程之間的協(xié)同一致性的利器，也確實是有那么點相似之處(這里純屬瞎扯啊，但不得不說佩服別人的聯(lián)想能力)。
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Zookeeper實現(xiàn)分布式鎖的大致思路是：
1. 每個客戶端都去zookeeper上創(chuàng)建臨時的順序節(jié)點
2. 客戶端判斷當(dāng)前自己創(chuàng)建的節(jié)點是不是最小的
3. 如果是的話，就獲得了執(zhí)行當(dāng)前任務(wù)的鎖
4. 如果不是的話，就找到比自己小的節(jié)點，然后進行監(jiān)聽，如果被刪除的話，就可以獲得鎖
了解原理之后，我們得去編碼實現(xiàn)分布式鎖：
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實現(xiàn)分布式鎖
*/
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();
}
}
輸出日志，從日志里面可以清楚看到線程競爭鎖的過程。
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 ==============
接觸Zookeeper不久，理解不夠深刻，寫文章除了記錄自己的所見所聞外，同時也想借此來勉勵自己不放棄學(xué)習(xí)，每天一點點進步，明天會有不一樣的自己。
end~
