Apache Curator入门实战

Curator是Netflix公司开源的一个Zookeeper客户端，与Zookeeper提供的原生客户端相比，Curator的抽象层次更高，简化了Zookeeper客户端的开发量。

1.Zookeeper安装部署

Zookeeper的部署很简单，如果已经有Java运行环境的话，下载tarball解压后即可运行。

[root@vm Temp]$ wget http://mirror.bit.edu.cn/apache/zookeeper/zookeeper-3.4.6/zookeeper-3.4.6.tar.gz
[root@vm Temp]$ tar zxvf zookeeper-3.4.6.tar.gz
[root@vm Temp]$ cd zookeeper-3.4.6

[root@vm zookeeper-3.4.6]$ cp conf/zoo_sample.cfg conf/zoo.cfg
[root@vm zookeeper-3.4.6]$ export ZOOKEEPER_HOME=/usr/local/src/zookeeper-3.4.5
[root@vm zookeeper-3.4.6]$ export PATH=$ZOOKEEPER_HOME/bin:$PATH

[root@vm zookeeper-3.4.6]$ bin/zkServer.sh start
[root@vm zookeeper-3.4.6]$ bin/zkCli.sh -server 127.0.0.1:2181

2.客户端常用操作

用zkCli.sh连接上Zookeeper服务后，用help能列出所有命令：

[root@BC-VM-edce4ac67d304079868c0bb265337bd4 zookeeper-3.4.6]# bin/zkCli.sh -127.0.0.1:2181
Connecting to localhost:2181
2015-06-11 10:55:14,387 [myid:] - INFO  [main:Environment@100] - Client environment:zookeeper.version=3.4.6-1569965, built on 02/20/2014 09:09 GMT
    ...

[zk: localhost:2181(CONNECTED) 5] help
ZooKeeper -server host:port cmd args
        connect host:port
        get path [watch]
        ls path [watch]
        set path data [version]
        rmr path
        delquota [-n|-b] path
        quit 
        printwatches on|off
        create [-s] [-e] path data acl
        stat path [watch]
        close 
        ls2 path [watch]
        history 
        listquota path
        setAcl path acl
        getAcl path
        sync path
        redo cmdno
        addauth scheme auth
        delete path [version]
        setquota -n|-b val path

下面就试验一下常用的命令：

• create：创建路径结点。
• ls：查看路径下的所有结点。
• get：获得结点上的值。
• set：修改结点上的值。
• delete：删除结点。

[zk: localhost:2181(CONNECTED) 6] create /zktest mydata
Created /zktest
[zk: localhost:2181(CONNECTED) 12] ls /
[zktest, zookeeper]
[zk: localhost:2181(CONNECTED) 7] ls /zktest
[]
[zk: localhost:2181(CONNECTED) 13] get /zktest
mydata
cZxid = 0x1c
ctime = Thu Jun 11 10:58:06 CST 2015
mZxid = 0x1c
mtime = Thu Jun 11 10:58:06 CST 2015
pZxid = 0x1c
cversion = 0
dataVersion = 0
aclVersion = 0
ephemeralOwner = 0x0
dataLength = 6
numChildren = 0
[zk: localhost:2181(CONNECTED) 14] set /zktest junk
cZxid = 0x1c
ctime = Thu Jun 11 10:58:06 CST 2015
mZxid = 0x1f
mtime = Thu Jun 11 10:59:08 CST 2015
pZxid = 0x1c
cversion = 0
dataVersion = 1
aclVersion = 0
ephemeralOwner = 0x0
dataLength = 4
numChildren = 0
[zk: localhost:2181(CONNECTED) 15] delete /zktest
[zk: localhost:2181(CONNECTED) 16] ls /
[zookeeper]

3.用Curator管理Zookeeper

Curator的Maven依赖如下，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。

    <dependency>
        <groupId>org.apache.curator</groupId>
        <artifactId>curator-recipes</artifactId>
        <version>2.7.0</version>
    </dependency>

3.1 Client操作

利用Curator提供的客户端API，可以完全实现上面原生客户端的功能。值得注意的是，Curator采用流式风格API。

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

import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.retry.RetryNTimes;

/**
 * Curator framework‘s client test.
 * Output:
 *  $ create /zktest hello 
 *  $ ls / 
 *  [zktest, zookeeper]
 *  $ get /zktest 
 *  hello
 *  $ set /zktest world 
 *  $ get /zktest 
 *  world
 *  $ delete /zktest 
 *  $ ls / 
 *  [zookeeper]
 */
public class CuratorClientTest {

    /** 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.Client API test
        // 2.1 Create node
        String data1 = "hello";
        print("create", ZK_PATH, data1);
        client.create().
                creatingParentsIfNeeded().
                forPath(ZK_PATH, data1.getBytes());

        // 2.2 Get node and data
        print("ls", "/");
        print(client.getChildren().forPath("/"));
        print("get", ZK_PATH);
        print(client.getData().forPath(ZK_PATH));

        // 2.3 Modify data
        String data2 = "world";
        print("set", ZK_PATH, data2);
        client.setData().forPath(ZK_PATH, data2.getBytes());
        print("get", ZK_PATH);
        print(client.getData().forPath(ZK_PATH));

        // 2.4 Remove node
        print("delete", ZK_PATH);
        client.delete().forPath(ZK_PATH);
        print("ls", "/");
        print(client.getChildren().forPath("/"));
    }

    private static void print(String... cmds) {
        StringBuilder text = new StringBuilder("$ ");
        for (String cmd : cmds) {
            text.append(cmd).append(" ");
        }
        System.out.println(text.toString());
    }

    private static void print(Object result) {
        System.out.println(
                result instanceof byte[]
                    ? new String((byte[]) result)
                        : result);
    }

}

3.2 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。

下面就测试一下最简单的Path Watcher：

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);
    }

}

下面是在zkCli.sh中操作时Java程序的输出：

Java: zk client start successfully!
Java: Register zk watcher successfully!

zkCli: [zk: localhost:2181(CONNECTED) 11] create /zktest/hello mydata
Java: Receive event: type=[CHILD_ADDED], path=[/zktest/hello], data=[mydata], stat=[121,121,1434001221097,1434001221097,0,0,0,0,6,0,121]

zkCli: [zk: localhost:2181(CONNECTED) 12] set /zktest/hello otherdata
Java: Receive event: type=[CHILD_UPDATED], path=[/zktest/hello], data=[otherdata], stat=[121,122,1434001221097,1434001228467,1,0,0,0,9,0,121]

zkCli: [zk: localhost:2181(CONNECTED) 13] delete /zktest/hello
Java: Receive event: type=[CHILD_REMOVED], path=[/zktest/hello], data=[otherdata], stat=[121,122,1434001221097,1434001228467,1,0,0,0,9,0,121]

4.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。

4.1 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();
            }
        }

    }

}

4.2 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();
    }

}