基于Zookeeper的分布式共享锁

首先，说说我们的场景，订单服务是做成集群的，当两个以上结点同时收到一个相同订单的创建指令，这时并发就产生了，系统就会重复创建订单。等等......场景。这时，分布式共享锁就闪亮登场了。

共享锁在同一个进程中是很容易实现的，但在跨进程或者在不同Server之间就不好实现了。Zookeeper就很容易实现。具体的实现原理官网和其它网站也有翻译，这里就不在赘述了。

官网资料： http://zookeeper.apache.org/doc/r3.4.5/recipes.html

中文资料： https://www.ibm.com/developerworks/cn/opensource/os-cn-zookeeper

详见Locks章节。

原理都知道了，网上一搜索Apache上面已经有提供了，既然已经有轮子了，哪我们也没必要重复造轮子了吧！直接使用 Curator。但是，我们在测试中发现，用于共享锁的结点无法自动回收，除了最末一级的临时结点会在锁释放和session超时的时候能自动回收外，其它结点均无法自动回收。我们的订单一天有好几万，遇到618和双十一的时候每天的订单量超50W，如果结点长期不回收的话，肯定会影响Zookeeper的性能。这时，我们就想到了一句话“自己动手，丰衣足食”。下面直接上代码：

首先，创建一个Maven工程，在pom文件里导入下面的包：

	<dependencies>
		<dependency>
			<groupId>org.apache.zookeeper</groupId>
			<artifactId>zookeeper</artifactId>
			<version>3.4.6</version>
		</dependency>
		<dependency>
			<groupId>org.apache.curator</groupId>
			<artifactId>curator-client</artifactId>
			<version>2.8.0</version>
		</dependency>
		<dependency>
			<groupId>org.apache.curator</groupId>
			<artifactId>curator-recipes</artifactId>
			<version>2.8.0</version>
		</dependency>
		<dependency>
			<groupId>org.apache.curator</groupId>
			<artifactId>curator-framework</artifactId>
			<version>2.8.0</version>
		</dependency>
		<dependency>
			<groupId>commons-beanutils</groupId>
			<artifactId>commons-beanutils</artifactId>
			<version>1.9.2</version>
		</dependency>
		<dependency>
			<groupId>commons-logging</groupId>
			<artifactId>commons-logging</artifactId>
			<version>1.2</version>
		</dependency>
		<dependency>
			<groupId>commons-lang</groupId>
			<artifactId>commons-lang</artifactId>
			<version>2.6</version>
		</dependency>
	</dependencies>

LockZookeeperClient接口：

package com.XXX.framework.lock;

import org.apache.curator.framework.CuratorFramework;

/**
 * 
 * description
 * 
 * @author Roadrunners
 * @version 1.0, 2015年7月9日
 */
public interface LockZookeeperClient {
	/**
	 * 
	 * @return
	 */
	CuratorFramework getCuratorFramework();

	/**
	 * 
	 * @return
	 */
	String getBasePath();

	/**
	 * garbage collector
	 * 
	 * @param gcPath
	 */
	void gc(String gcPath);
}

LockZookeeperClient接口的实现LockZookeeperClientFactory：

package com.XXX.framework.lock;

import java.util.Date;
import java.util.List;
import java.util.Timer;
import java.util.TimerTask;
import java.util.concurrent.ConcurrentSkipListSet;

import org.apache.commons.collections.CollectionUtils;
import org.apache.commons.lang.StringUtils;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.retry.ExponentialBackoffRetry;

/**
 * 
 * description
 *  
 * @author Roadrunners
 * @version 1.0, 2015年7月9日
 */
public class LockZookeeperClientFactory implements LockZookeeperClient {
	private static final Log LOG = LogFactory.getLog(LockZookeeperClientFactory.class);

	private boolean hasGc = true;
	private Timer gcTimer;
	private TimerTask gcTimerTask;
	private ConcurrentSkipListSet<String> gcPaths = new ConcurrentSkipListSet<String>();
	private int gcIntervalSecond = 60;

	private CuratorFramework curatorFramework;
	private String zookeeperIpPort = "localhost:2181";
	private int sessionTimeoutMs = 10000;
	private int connectionTimeoutMs = 10000;
	private String basePath = "/locks";

	public void setHasGc(boolean hasGc) {
		this.hasGc = hasGc;
	}

	public void setGcIntervalSecond(int gcIntervalSecond) {
		this.gcIntervalSecond = gcIntervalSecond;
	}

	public void setZookeeperIpPort(String zookeeperIpPort) {
		this.zookeeperIpPort = zookeeperIpPort;
	}

	public void setSessionTimeoutMs(int sessionTimeoutMs) {
		this.sessionTimeoutMs = sessionTimeoutMs;
	}

	public void setConnectionTimeoutMs(int connectionTimeoutMs) {
		this.connectionTimeoutMs = connectionTimeoutMs;
	}

	public void setBasePath(String basePath) {
		basePath = basePath.trim();
		if (basePath.endsWith("/")) {
			basePath = basePath.substring(0, basePath.length() - 1);
		}

		this.basePath = basePath;
	}

	public void init() {
		if(StringUtils.isBlank(zookeeperIpPort)){
			throw new NullPointerException("zookeeperIpPort");
		}

		ExponentialBackoffRetry retryPolicy = new ExponentialBackoffRetry(1000, 3);
		curatorFramework = CuratorFrameworkFactory.newClient(zookeeperIpPort.trim(), sessionTimeoutMs, connectionTimeoutMs, retryPolicy);
		curatorFramework.start();
		LOG.info("CuratorFramework initialise succeed.");

		if (hasGc) {
			gc();
		}
	}

	public void destroy() {
		gcPaths.clear();
		gcPaths = null;
		gcStop();
		curatorFramework.close();
		curatorFramework = null;
	}

	@Override
	public void gc(String gcPath) {
		if (hasGc && StringUtils.isNotBlank(gcPath)) {
			gcPaths.add(gcPath.trim());
		}
	}

	@Override
	public CuratorFramework getCuratorFramework() {
		return this.curatorFramework;
	}

	@Override
	public String getBasePath() {
		return this.basePath;
	}

	private synchronized void gc() {
		gcStop();

		try {
			scanningGCNodes();
		} catch (Throwable e) {
			LOG.warn(e);
		}

		gcTimerTask = new TimerTask() {
			@Override
			public void run() {
				doingGc();
			}
		};

		Date begin = new Date();
		begin.setTime(begin.getTime() + (10 * 1000L));
		gcTimer = new Timer("lock-gc", true);
		gcTimer.schedule(gcTimerTask, begin, gcIntervalSecond * 1000L);
	}

	private synchronized void gcStop() {
		if (null != gcTimer) {
			gcTimer.cancel();
			gcTimer = null;
		}

		if (null != gcTimerTask) {
			gcTimerTask.cancel();
			gcTimerTask = null;
		}
	}

	private synchronized void scanningGCNodes() throws Exception {
		if (null == curatorFramework.checkExists().forPath(basePath)) {
			return;
		}

		List<String> paths = curatorFramework.getChildren().forPath(basePath);
		if (CollectionUtils.isEmpty(paths)) {
			gcPaths.add(basePath);
			return;
		}

		for (String path : paths) {
			try{
				String tmpPath = basePath + "/" + path;
				if (null == curatorFramework.checkExists().forPath(tmpPath)) {
					continue;
				}
				
				gcPaths.add(tmpPath);
			} catch(Throwable e){
				LOG.warn("scanning gc nodes error.", e);
			}
		}
	}
	
	private synchronized void doingGc() {
		LOG.debug("GC beginning.");

		if (CollectionUtils.isNotEmpty(gcPaths)) {
			for (String path : gcPaths) {
				try {
					if (null != curatorFramework.checkExists().forPath(path)) {
						if (CollectionUtils.isEmpty(curatorFramework.getChildren().forPath(path))) {
							curatorFramework.delete().forPath(path);
							gcPaths.remove(path);
							LOG.debug("GC " + path);
						}
					} else {
						gcPaths.remove(path);
					}
				} catch (Throwable e) {
					gcPaths.remove(path);
					LOG.warn(e);
				}
			}
		}

		LOG.debug("GC ended.");
	}

}

SharedLock共享锁：

package com.XXX.framework.lock.shared;

import java.util.concurrent.TimeUnit;

import org.apache.commons.lang.StringUtils;
import org.apache.curator.framework.recipes.locks.InterProcessLock;
import org.apache.curator.framework.recipes.locks.InterProcessMutex;

import com.XXX.framework.lock.LockZookeeperClient;

/**
 * 
 * description
 * 
 * @author Roadrunners
 * @version 1.0, 2015年7月9日
 */
public class SharedLock {
	private InterProcessLock interProcessLock;

	public SharedLock(LockZookeeperClient lockZookeeperClient, String resourceId) {
		super();
		
		if (StringUtils.isBlank(resourceId)) {
			throw new NullPointerException("resourceId");
		}
		String path = lockZookeeperClient.getBasePath();
		path += ("/" + resourceId.trim());

		interProcessLock = new InterProcessMutex(lockZookeeperClient.getCuratorFramework(), path);
		lockZookeeperClient.gc(path);
	}
	
    /**
     * Acquire the mutex - blocking until it's available. Each call to acquire must be balanced by a call
     * to {@link #release()}
     *
     * @throws Exception ZK errors, connection interruptions
     */
	public void acquire() throws Exception {
		interProcessLock.acquire();
	}

    /**
     * Acquire the mutex - blocks until it's available or the given time expires. Each call to acquire that returns true must be balanced by a call
     * to {@link #release()}
     *
     * @param time time to wait
     * @param unit time unit
     * @return true if the mutex was acquired, false if not
     * @throws Exception ZK errors, connection interruptions
     */
	public boolean acquire(long time, TimeUnit unit) throws Exception {
		return interProcessLock.acquire(time, unit);
	}

    /**
     * Perform one release of the mutex.
     *
     * @throws Exception ZK errors, interruptions, current thread does not own the lock
     */
	public void release() throws Exception {
		interProcessLock.release();
	}
	
    /**
     * Returns true if the mutex is acquired by a thread in this JVM
     *
     * @return true/false
     */
	public boolean isAcquiredInThisProcess() {
		return interProcessLock.isAcquiredInThisProcess();
	}
}

到此代码已经完成。下面写一个简单的Demo：

		//LockZookeeperClientFactory通常是通过Spring配置注入的，此处是为了Demo的简单明了才这样写的，不建议这样写
		LockZookeeperClientFactory lzc = new LockZookeeperClientFactory();
		lzc.setZookeeperIpPort("10.100.15.1:8900");
		lzc.setBasePath("/locks/sharedLock/");
		lzc.init();

		SharedLock sharedLock = new SharedLock(lzc, "sharedLock1");
		try {
			if (sharedLock.acquire(100, TimeUnit.MILLISECONDS)) {
				System.out.println("sharedLock1 get");
			}
		} catch (Exception e) {
			e.printStackTrace();
		} finally {
			try {
				sharedLock.release();
			} catch (Exception e) {
				e.printStackTrace();
			}
		}

		lzc.destroy();

就这样，系统就会每隔一分钟去回收一次没有使用的结点。