简介

Curator是Netflix公司开源的一套zookeeper客户端框架，解决了很多Zookeeper客户端非常底层的细节开发工作，包括连接重连、反复注册Watcher和NodeExistsException异常等等。Patrixck Hunt（Zookeeper）以一句“Guava is to Java that Curator to Zookeeper” 给Curator予高度评价。
Curator的maven依赖: 一般直接使用curator-recipes就行了，如果需要自己封装一些底层些的功能的话，例如增加连接管理重试机制等，则可以引入curator-framework包

<!-- 对zookeeper的底层api的一些封装 -->
        <dependency>
            <groupId>org.apache.curator</groupId>
            <artifactId>curator-framework</artifactId>
            <version>2.12.0</version>
        </dependency>
        <!-- 封装了一些高级特性，如：Cache事件监听、选举、分布式锁、分布式Barrier -->
        <dependency>
            <groupId>org.apache.curator</groupId>
            <artifactId>curator-recipes</artifactId>
            <version>2.12.0</version>
        </dependency>

基本API

创建会话

使用静态工程方法创建

 

        RetryPolicy retryPolicy = new ExponentialBackoffRetry(1000, 3);
        CuratorFramework client = CuratorFrameworkFactory.newClient("192.168.128.129:2181",
                5000, 5000, retryPolicy);

其中RetryPolicy为重试策略，第一个参数为baseSleepTimeMs初始的sleep时间，用于计算之后的每次重试的sleep时间。第二个参数为maxRetries，最大重试次数。

使用Fluent风格api创建

 

        RetryPolicy retryPolicy = new ExponentialBackoffRetry(1000, 3);
        CuratorFramework client = CuratorFrameworkFactory.builder()
                .connectString("192.168.128.129:2181")
                .sessionTimeoutMs(5000)  // 会话超时时间
                .connectionTimeoutMs(5000) // 连接超时时间
                .retryPolicy(retryPolicy)
                .namespace("base") // 包含隔离名称
                .build();
        client.start();

创建数据节点

 

client.create().creatingParentContainersIfNeeded() // 递归创建所需父节点
                .withMode(CreateMode.PERSISTENT) // 创建类型为持久节点
                .forPath("/nodeA", "init".getBytes()); // 目录及内容

删除数据节点

 

          client.delete() 
                .guaranteed()  // 强制保证删除
                .deletingChildrenIfNeeded() // 递归删除子节点
                .withVersion(10086) // 指定删除的版本号
                .forPath("/nodeA"); 

读取数据节点

读数据，返回值为byte[]

 

byte[] bytes = client.getData().forPath("/nodeA"); 
        System.out.println(new String(bytes));

读stat

 

Stat stat = new Stat();
        client.getData()
                .storingStatIn(stat)
                .forPath("/nodeA");

修改数据节点

 

client.setData()
                .withVersion(10086) // 指定版本修改
                .forPath("/nodeA", "data".getBytes());

事务

 

client.inTransaction().check().forPath("/nodeA")
                .and()
                .create().withMode(CreateMode.EPHEMERAL).forPath("/nodeB", "init".getBytes())
                .and()
                .create().withMode(CreateMode.EPHEMERAL).forPath("/nodeC", "init".getBytes())
                .and()
                .commit();

其他

 

      client.checkExists() // 检查是否存在
                .forPath("/nodeA"); 
        client.getChildren().forPath("/nodeA"); // 获取子节点的路径

异步回调

异步创建节点

 

Executor executor = Executors.newFixedThreadPool(2);
        client.create()
                .creatingParentsIfNeeded()
                .withMode(CreateMode.EPHEMERAL)
                .inBackground((curatorFramework, curatorEvent) -> {
                    System.out.println(String.format("eventType:%s,resultCode:%s",curatorEvent.getType(),curatorEvent.getResultCode()));
                },executor)
                .forPath("path");

 

监听器

Curator提供了三种Watcher(Cache)来监听结点的变化：

• Path Cache：监视一个路径下1）孩子结点的创建、2）删除，3）以及结点数据的更新。产生的事件会传递给注册的PathChildrenCacheListener。
• Node Cache：监视一个结点的创建、更新、删除，并将结点的数据缓存在本地。
• Tree Cache：Path Cache和Node Cache的“合体”，监视路径下的创建、更新、删除事件，并缓存路径下所有孩子结点的数据。

package com.cdai.codebase.bigdata.hadoop.zookeeper.curator;

import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.framework.recipes.cache.ChildData;
import org.apache.curator.framework.recipes.cache.PathChildrenCache;
import org.apache.curator.framework.recipes.cache.PathChildrenCache.StartMode;
import org.apache.curator.retry.RetryNTimes;

/**
 * Curator framework watch test.
 */
public class CuratorWatcherTest {

    /** Zookeeper info */
    private static final String ZK_ADDRESS = "192.168.1.100:2181";
    private static final String ZK_PATH = "/zktest";

    public static void main(String[] args) throws Exception {
        // 1.Connect to zk
        CuratorFramework client = CuratorFrameworkFactory.newClient(
                ZK_ADDRESS,
                new RetryNTimes(10, 5000)
        );
        client.start();
        System.out.println("zk client start successfully!");

        // 2.Register watcher
        PathChildrenCache watcher = new PathChildrenCache(
                client,
                ZK_PATH,
                true    // if cache data
        );
        watcher.getListenable().addListener((client1, event) -> {
            ChildData data = event.getData();
            if (data == null) {
                System.out.println("No data in event[" + event + "]");
            } else {
                System.out.println("Receive event: "
                        + "type=[" + event.getType() + "]"
                        + ", path=[" + data.getPath() + "]"
                        + ", data=[" + new String(data.getData()) + "]"
                        + ", stat=[" + data.getStat() + "]");
            }
        });
        watcher.start(StartMode.BUILD_INITIAL_CACHE);
        System.out.println("Register zk watcher successfully!");

        Thread.sleep(Integer.MAX_VALUE);
    }

}

Curator“菜谱”

既然Maven包叫做curator-recipes，那说明Curator有它独特的“菜谱”：

• 锁：包括共享锁、共享可重入锁、读写锁等。
• 选举：Leader选举算法。
• Barrier：阻止分布式计算直至某个条件被满足的“栅栏”，可以看做JDK Concurrent包中Barrier的分布式实现。
• 缓存：前面提到过的三种Cache及监听机制。
• 持久化结点：连接或Session终止后仍然在Zookeeper中存在的结点。
• 队列：分布式队列、分布式优先级队列等。

分布式锁

分布式编程时，比如最容易碰到的情况就是应用程序在线上多机部署，于是当多个应用同时访问某一资源时，就需要某种机制去协调它们。例如，现在一台应用正在rebuild缓存内容，要临时锁住某个区域暂时不让访问；又比如调度程序每次只想一个任务被一台应用执行等等。

下面的程序会启动两个线程t1和t2去争夺锁，拿到锁的线程会占用5秒。运行多次可以观察到，有时是t1先拿到锁而t2等待，有时又会反过来。Curator会用我们提供的lock路径的结点作为全局锁，这个结点的数据类似这种格式：[_c_64e0811f-9475-44ca-aa36-c1db65ae5350-lock-0000000005]，每次获得锁时会生成这种串，释放锁时清空数据。

package com.cdai.codebase.bigdata.hadoop.zookeeper.curator;

import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.framework.recipes.locks.InterProcessMutex;
import org.apache.curator.retry.RetryNTimes;

import java.util.concurrent.TimeUnit;

/**
 * Curator framework's distributed lock test.
 */
public class CuratorDistrLockTest {

    /** Zookeeper info */
    private static final String ZK_ADDRESS = "192.168.1.100:2181";
    private static final String ZK_LOCK_PATH = "/zktest";

    public static void main(String[] args) throws InterruptedException {
        // 1.Connect to zk
        CuratorFramework client = CuratorFrameworkFactory.newClient(
                ZK_ADDRESS,
                new RetryNTimes(10, 5000)
        );
        client.start();
        System.out.println("zk client start successfully!");

        Thread t1 = new Thread(() -> {
            doWithLock(client);
        }, "t1");
        Thread t2 = new Thread(() -> {
            doWithLock(client);
        }, "t2");

        t1.start();
        t2.start();
    }

    private static void doWithLock(CuratorFramework client) {
        InterProcessMutex lock = new InterProcessMutex(client, ZK_LOCK_PATH);
        try {
            if (lock.acquire(10 * 1000, TimeUnit.SECONDS)) {
                System.out.println(Thread.currentThread().getName() + " hold lock");
                Thread.sleep(5000L);
                System.out.println(Thread.currentThread().getName() + " release lock");
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                lock.release();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }

    }

}

 

Leader选举

当集群里的某个服务down机时，我们可能要从slave结点里选出一个作为新的master，这时就需要一套能在分布式环境中自动协调的Leader选举方法。Curator提供了LeaderSelector监听器实现Leader选举功能。同一时刻，只有一个Listener会进入takeLeadership()方法，说明它是当前的Leader。注意：当Listener从takeLeadership()退出时就说明它放弃了“Leader身份”，这时Curator会利用Zookeeper再从剩余的Listener中选出一个新的Leader。autoRequeue()方法使放弃Leadership的Listener有机会重新获得Leadership，如果不设置的话放弃了的Listener是不会再变成Leader的。

package com.cdai.codebase.bigdata.hadoop.zookeeper.curator;

import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.framework.recipes.leader.LeaderSelector;
import org.apache.curator.framework.recipes.leader.LeaderSelectorListener;
import org.apache.curator.framework.state.ConnectionState;
import org.apache.curator.retry.RetryNTimes;
import org.apache.curator.utils.EnsurePath;

/**
 * Curator framework's leader election test.
 * Output:
 *  LeaderSelector-2 take leadership!
 *  LeaderSelector-2 relinquish leadership!
 *  LeaderSelector-1 take leadership!
 *  LeaderSelector-1 relinquish leadership!
 *  LeaderSelector-0 take leadership!
 *  LeaderSelector-0 relinquish leadership! 
 *      ...
 */
public class CuratorLeaderTest {

    /** Zookeeper info */
    private static final String ZK_ADDRESS = "192.168.1.100:2181";
    private static final String ZK_PATH = "/zktest";

    public static void main(String[] args) throws InterruptedException {
        LeaderSelectorListener listener = new LeaderSelectorListener() {
            @Override
            public void takeLeadership(CuratorFramework client) throws Exception {
                System.out.println(Thread.currentThread().getName() + " take leadership!");

                // takeLeadership() method should only return when leadership is being relinquished.
                Thread.sleep(5000L);

                System.out.println(Thread.currentThread().getName() + " relinquish leadership!");
            }

            @Override
            public void stateChanged(CuratorFramework client, ConnectionState state) {
            }
        };

        new Thread(() -> {
            registerListener(listener);
        }).start();

        new Thread(() -> {
            registerListener(listener);
        }).start();

        new Thread(() -> {
            registerListener(listener);
        }).start();

        Thread.sleep(Integer.MAX_VALUE);
    }

    private static void registerListener(LeaderSelectorListener listener) {
        // 1.Connect to zk
        CuratorFramework client = CuratorFrameworkFactory.newClient(
                ZK_ADDRESS,
                new RetryNTimes(10, 5000)
        );
        client.start();

        // 2.Ensure path
        try {
            new EnsurePath(ZK_PATH).ensure(client.getZookeeperClient());
        } catch (Exception e) {
            e.printStackTrace();
        }

        // 3.Register listener
        LeaderSelector selector = new LeaderSelector(client, ZK_PATH, listener);
        selector.autoRequeue();
        selector.start();
    }

}