分布式技术专题「Zookeeper中间件」给大家学习一下Zookeeper的”开发伴侣”—Curator-Framework（组件篇）

Posted 2022-12-13 洛神灬殇

CuratorFramework

Curator-Framework是ZooKeeper Client更高的抽象API，最佳核心的功能就是自动连接管理:

1. 当ZooKeeper客户端内部出现异常, 将自动进行重连或重试, 该过程对外几乎完全透明
2. 监控节点数据变化事件NodeDataChanged，需要时调用updateServerList()方法
3. Curator recipes自动移除监控

CuratorFramework版本

目前Curator有2.x.x和3.x.x两个系列的版本，支持不同版本的Zookeeper。其中Curator 2.x.x兼容Zookeeper的3.4.x和3.5.x。而Curator 3.x.x只兼容Zookeeper 3.5.x，并且提供了一些诸如动态重新配置、watch删除等新特性。

更加清晰的API

简化了ZooKeeper原生的方法, 事件等, 提供流式fluent的接口，提供Recipes实现 : 选举，共享锁， 路径cache， 分布式队列，分布式优先队列等。

maven配置依赖

<!-- https://mvnrepository.com/artifact/org.apache.curator/curator-recipes -->
<dependency>
    <groupId>org.apache.curator</groupId>
    <artifactId>curator-recipes</artifactId>
    <version>2.12.0</version>
</dependency>

事务管理

/** 事务管理：碰到异常，事务会回滚
 * 使用transaction()来控制事务
 * @throws Exception
 */
public void testTransaction() throws Exception
    //定义几个基本操作
    CuratorOp createOp = client.transactionOp().create()
            .forPath("/curator/one_path","some data".getBytes());
    CuratorOp setDataOp = client.transactionOp().setData()
            .forPath("/curator","other data".getBytes());
    CuratorOp deleteOp = client.transactionOp().delete()
            .forPath("/curator");
    //事务执行结果
    List<CuratorTransactionResult> results = client.transaction()
            .forOperations(createOp,setDataOp,deleteOp);
    //遍历输出结果
    for(CuratorTransactionResult result : results)
        System.out.println("执行结果是： " + result.getForPath() + "--" + result.getType());
    

//因为节点“/curator”存在子节点，所以在删除的时候将会报错，事务回滚

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。

/**
 * 在注册监听器的时候，如果传入此参数，当事件触发时，逻辑由线程池处理
 */
ExecutorService pool = Executors.newFixedThreadPool(2);
​
/**
 * 监听数据节点的变化情况
 */
final NodeCache nodeCache = new NodeCache(client, "/zk-huey/cnode", false);
    nodeCache.start(true);
            nodeCache.getListenable().addListener(
            new NodeCacheListener() 
    @Override
    public void nodeChanged() throws Exception 
        System.out.println("Node data is changed, new data: " +
        new String(nodeCache.getCurrentData().getData()));
    , pool);
​
/**
 * 监听子节点的变化情况
 */
final PathChildrenCache childrenCache = new PathChildrenCache(client, "/zk-huey", true);
        childrenCache.start(StartMode.POST_INITIALIZED_EVENT);
        childrenCache.getListenable().addListener(
        new PathChildrenCacheListener() 
        @Override
        public void childEvent(CuratorFramework client, PathChildrenCacheEvent event) throws Exception 
            switch (event.getType()) 
                case CHILD_ADDED:
                    System.out.println("CHILD_ADDED: " + event.getData().getPath());
                    break;
                case CHILD_REMOVED:
                    System.out.println("CHILD_REMOVED: " + event.getData().getPath());
                    break;
                case CHILD_UPDATED:
                    System.out.println("CHILD_UPDATED: " + event.getData().getPath());
                    break;
                default:
                    break;
                
            
            ,
        pool
        );
        client.setData().forPath("/zk-huey/cnode", "world".getBytes());
        Thread.sleep(10 * 1000);
        pool.shutdown();
        client.close();

分布式锁思路

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，每次获得锁时会生成这种串，释放锁时清空数据。

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。

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();
    

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。