[curator] Netflix Curator 使用

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