【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）

【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） 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:

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监控节点数据变化事件NodeDataChanged，需要时调用updateServerList()方法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

    org.apache.curator
    curator-recipes
    2.12.0

事务管理

public void testTransaction() throws Exception{
    //定义几个基本 *** 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 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。

ExecutorService pool = Executors.newFixedThreadPool(2);
​

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);
​

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;

public class CuratorDistrLockTest {
    
    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;
​

public class CuratorLeaderTest {
​
    
    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。

参考资料

https://www.cnblogs.com/qingyunzong/p/8666288.html