学步园

curator简介

Netflix curator 是Netflix公司开源的一个Zookeeper client library，用于简化zookeeper客户端编程，包含一下几个模块：

• curator-client - zookeeper client封装，用于取代原生的zookeeper客户端，提供一些非常有用的客户端特性
• curator-framework - zookeeper api的高层封装，大大简化zookeeper客户端编程，添加了例如zookeeper连接管理、重试机制等
• curator-recipes -
  zookeeper recipes 基于curator-framework的实现（除2PC以外）

maven dependency:

		<dependency>
			<groupId>com.netflix.curator</groupId>
			<artifactId>curator-recipes</artifactId>
			<version>0.6.4</version>
		</dependency>

注意：在www.mvnrepository.com中认为0.32为最新版本，其实迄今为止最新版本为0.64，github trunk中的版本现在是0.65-SNAPSHOT

curator framework 使用

示例代码：

                String path = "/test_path";
		CuratorFramework client = CuratorFrameworkFactory.builder()
				.connectString("test:2181").namespace("/test1")
				.retryPolicy(new RetryNTimes(Integer.MAX_VALUE, 1000))
				.connectionTimeoutMs(5000).build();
		//create a node
		client.create().forPath("/head", new byte[0]);
		
		//delete a node in background
		client.delete().inBackground().forPath("/head");
		
		// create a EPHEMERAL_SEQUENTIAL
		client.create().withMode(CreateMode.EPHEMERAL_SEQUENTIAL).forPath("/head/child", new byte[0]);
		
		// get the data 
		client.getData().watched().inBackground().forPath("/test");
		
		// check the path exits
		client.checkExists().forPath(path);

curator framework使用builder模式和类似nio的chain api，代码非常简洁

curator recipes 使用

InterProcessMutex

用途：进程间互斥锁

示例代码：

		String lockName = "/lock1";
		InterProcessLock lock1 = new InterProcessMutex(this.curator, lockName);
		InterProcessLock lock2 = new InterProcessMutex(this.curator, lockName);
		lock1.acquire();
		boolean result = lock2.acquire(1, TimeUnit.SECONDS);
		assertFalse(result);
		lock1.release();
		result = lock2.acquire(1, TimeUnit.SECONDS);
		assertTrue(result);

原理：每次调用acquire在/lock1节点节点下使用CreateMode.EPHEMERAL_SEQUENTIAL 创建新的ephemeral节点，然后getChildren获取所有的children，判断刚刚创建的临时节点是否为第一个，如果是，则获取锁成功；如果不是，则删除刚刚创建的临时节点。

注意： 每次accquire操作，成功，则请求zk server 2次（一次写，一次getChildren）；如果失败，则请求zk server 3次（一次写，一次getChildren，一次delete）

InterProcessReadWriteLock

示例代码：

	@Test
	public void testReadWriteLock() throws Exception{
		String readWriteLockPath = "/RWLock";
		InterProcessReadWriteLock readWriteLock1 = new InterProcessReadWriteLock(this.curator, readWriteLockPath);
		InterProcessMutex writeLock1 = readWriteLock1.writeLock();
		InterProcessMutex readLock1 = readWriteLock1.readLock();
		
		InterProcessReadWriteLock readWriteLock2 = new InterProcessReadWriteLock(this.curator, readWriteLockPath);
		InterProcessMutex writeLock2 = readWriteLock2.writeLock();
		InterProcessMutex readLock2 = readWriteLock2.readLock();
		writeLock1.acquire();
		
		// same with WriteLock, can read
		assertTrue(readLock1.acquire(1, TimeUnit.SECONDS));
		
		// different lock, can't read while writting
		assertFalse(readLock2.acquire(1, TimeUnit.SECONDS));
		
		// different write lock, can't write
		assertFalse(writeLock2.acquire(1, TimeUnit.SECONDS));
		
		// release the write lock
		writeLock1.release();
		
		//both read lock can read
		assertTrue(readLock1.acquire(1, TimeUnit.SECONDS));
		assertTrue(readLock2.acquire(1, TimeUnit.SECONDS));
	}

原理： 同InterProcessMutext，在ephemeral node的排序算法上做trick，write lock的排序在前。

注意： 同一个InterProcessReadWriteLock如果已经获取了write lock，则获取read lock也会成功

LeaderSelector

示例代码：

	@Test
	public void testLeader() throws Exception{
		LeaderSelectorListener listener = new LeaderSelectorListener(){


			@Override
			public void takeLeadership(CuratorFramework client)
					throws Exception {
				System.out.println("i'm leader");
			}

			@Override
			public void handleException(CuratorFramework client,
					Exception exception) {
				
			}

			@Override
			public void notifyClientClosing(CuratorFramework client) {
				
			}};
		String leaderPath = "/leader";
		LeaderSelector selector1 = new LeaderSelector(this.curator, leaderPath, listener);
		selector1.start();
		LeaderSelector selector2 = new LeaderSelector(this.curator, leaderPath, listener);
		selector2.start();
		assertFalse(selector2.hasLeadership());
	}

原理：内部基于InterProcessMutex实现，具体细节参见shared lock一节

总结

    curator还提供了很多其他的实现，具体参见https://github.com/Netflix/curator/wiki/Recipes

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