zookeeper典型应用场景之一就是利用发布订阅模式实现配置动态管理。基本原理就是将配置信息存在zk的某个节点中，客户端启动时从这个节点读取配置信息，并Watcher，一旦配置发生变化，客户端会接收到变化通知，便可以再次读取节点内容。

样例

/**
 * A simple example program to use DataMonitor to start and
 * stop executables based on a znode. The program watches the
 * specified znode and saves the data that corresponds to the
 * znode in the filesystem. It also starts the specified program
 * with the specified arguments when the znode exists and kills
 * the program if the znode goes away.
 */
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooKeeper;

public class Executor
        implements Watcher, Runnable, DataMonitor.DataMonitorListener
{
    String znode;

    DataMonitor dm;

    ZooKeeper zk;

    String filename;

    String exec[];

    Process child;

    public Executor(String hostPort, String znode, String filename,
                    String exec[]) throws KeeperException, IOException {
        this.filename = filename;
        this.exec = exec;
        zk = new ZooKeeper(hostPort, 3000, this);
        dm = new DataMonitor(zk, znode, null, this);
    }

    /**
     * @param args
     */
    public static void main(String[] args) {
        if (args.length < 4) {
            System.err
                    .println("USAGE: Executor hostPort znode filename program [args ...]");
            System.exit(2);
        }
        String hostPort = args[0];
        String znode = args[1];
        String filename = args[2];
        String exec[] = new String[args.length - 3];
        System.arraycopy(args, 3, exec, 0, exec.length);
        try {
            new Executor(hostPort, znode, filename, exec).run();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    /***************************************************************************
     * We do process any events ourselves, we just need to forward them on.
     *
     * @see org.apache.zookeeper.Watcher#process(org.apache.zookeeper.proto.WatcherEvent)
     */
    @Override
    public void process(WatchedEvent event) {
        dm.process(event);
    }

    @Override
    public void run() {
        try {
            synchronized (this) {
                while (!dm.dead) {
                    wait();
                }
            }
        } catch (InterruptedException e) {
        }
    }

    public void closing(int rc) {
        synchronized (this) {
            notifyAll();
        }
    }

    static class StreamWriter extends Thread {
        OutputStream os;

        InputStream is;

        StreamWriter(InputStream is, OutputStream os) {
            this.is = is;
            this.os = os;
            start();
        }

        public void run() {
            byte b[] = new byte[80];
            int rc;
            try {
                while ((rc = is.read(b)) > 0) {
                    os.write(b, 0, rc);
                }
            } catch (IOException e) {
            }

        }
    }

    public void exists(byte[] data) {
        if (data == null) {
            if (child != null) {
                System.out.println("Killing process");
                child.destroy();
                try {
                    child.waitFor();
                } catch (InterruptedException e) {
                }
            }
            child = null;
        } else {
            if (child != null) {
                System.out.println("Stopping child");
                child.destroy();
                try {
                    child.waitFor();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            try {
                FileOutputStream fos = new FileOutputStream(filename);
                fos.write(data);
                fos.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
            try {
                System.out.println("Starting child");
                child = Runtime.getRuntime().exec(exec);
                new StreamWriter(child.getInputStream(), System.out);
                new StreamWriter(child.getErrorStream(), System.err);
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

/**
 * A simple class that monitors the data and existence of a ZooKeeper
 * node. It uses asynchronous ZooKeeper APIs.
 */
import java.util.Arrays;

import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooKeeper;
import org.apache.zookeeper.AsyncCallback.StatCallback;
import org.apache.zookeeper.KeeperException.Code;
import org.apache.zookeeper.data.Stat;

public class DataMonitor implements Watcher, StatCallback {

    ZooKeeper zk;

    String znode;

    Watcher chainedWatcher;

    boolean dead;

    DataMonitorListener listener;

    byte prevData[];

    public DataMonitor(ZooKeeper zk, String znode, Watcher chainedWatcher,
                       DataMonitorListener listener) {
        this.zk = zk;
        this.znode = znode;
        this.chainedWatcher = chainedWatcher;
        this.listener = listener;
        // Get things started by checking if the node exists. We are going
        // to be completely event driven
        zk.exists(znode, true, this, null);
    }

    /**
     * Other classes use the DataMonitor by implementing this method
     */
    public interface DataMonitorListener {
        /**
         * The existence status of the node has changed.
         */
        void exists(byte data[]);

        /**
         * The ZooKeeper session is no longer valid.
         *
         * @param rc
         *                the ZooKeeper reason code
         */
        void closing(int rc);
    }

    public void process(WatchedEvent event) {
        String path = event.getPath();
        if (event.getType() == Event.EventType.None) {
            // We are are being told that the state of the
            // connection has changed
            switch (event.getState()) {
                case SyncConnected:
                    // In this particular example we don't need to do anything
                    // here - watches are automatically re-registered with
                    // server and any watches triggered while the client was
                    // disconnected will be delivered (in order of course)
                    break;
                case Expired:
                    // It's all over
                    dead = true;
                    listener.closing(KeeperException.Code.SessionExpired);
                    break;
            }
        } else {
            if (path != null && path.equals(znode)) {
                // Something has changed on the node, let's find out
                zk.exists(znode, true, this, null);
            }
        }
        if (chainedWatcher != null) {
            chainedWatcher.process(event);
        }
    }

    public void processResult(int rc, String path, Object ctx, Stat stat) {
        boolean exists;
        switch (rc) {
            case Code.Ok:
                exists = true;
                break;
            case Code.NoNode:
                exists = false;
                break;
            case Code.SessionExpired:
            case Code.NoAuth:
                dead = true;
                listener.closing(rc);
                return;
            default:
                // Retry errors
                zk.exists(znode, true, this, null);
                return;
        }

        byte b[] = null;
        if (exists) {
            try {
                b = zk.getData(znode, false, null);
            } catch (KeeperException e) {
                // We don't need to worry about recovering now. The watch
                // callbacks will kick off any exception handling
                e.printStackTrace();
            } catch (InterruptedException e) {
                return;
            }
        }
        if ((b == null && b != prevData)
                || (b != null && !Arrays.equals(prevData, b))) {
            listener.exists(b);
            prevData = b;
        }
    }
}

测试

以上两个类来自zookeeper官网，下面测试一下这两个类是如何工作的。

• 创建zk节点

  设置初始值为Hello_world

[zk: localhost:2181(CONNECTED) 0] create /zk_tutorials Hello_world
Created /zk_tutorials
[zk: localhost:2181(CONNECTED) 1] get /zk_tutorials
Hello_world
cZxid = 0x300000017
ctime = Mon May 22 08:28:10 CST 2017
mZxid = 0x300000017
mtime = Mon May 22 08:28:10 CST 2017
pZxid = 0x300000017
cversion = 0
dataVersion = 0
aclVersion = 0
ephemeralOwner = 0x0
dataLength = 11
numChildren = 0
```

• 创建测试脚本count.sh
  output.txt保存的是/zk_tutorials的值，所以这个脚本的作用是每5秒输出一次此节点的值。

count=0
while [ true ]; do
let count+=1
echo Count: $count using cat output.txt
sleep 5
done
```

• 编译Executor.java和DataMonitor

javac Executor.java DataMonitor.java -classpath /usr/hdp/2.5.0.0-1245/zookeeper/zookeeper-3.4.6.2.5.0.0-1245.jar
```

• 运行测试

  export ZK_HOME=/usr/hdp/2.5.0.0-1245/zookeeper
  export CLASSPATH=.:$ZK_HOME/zookeeper-3.4.6.2.5.0.0-1245.jar:$ZK_HOME/lib/slf4j-api-1.6.1.jar:$ZK_HOME/lib/slf4j-log4j12-1.6.1.jar:$ZK_HOME/lib/log4j-1.2.16.jar:$ZK_HOME/conf
  java Executor localhost:3181 /zk_tutorials output.txt ./count.sh
  

  会看到每5秒输出一次结果

  Count: 1 using Hello_world
  Count: 2 using Hello_world
  Count: 3 using Hello_world
  

  这时在另一终端修改/zk_tutorials的值

  set /zk_tutorials Hello_zk
  

  可以看到结果已经发生了改变

Count: 43 using Hello_world
Stopping child
Starting child
Count: 1 using Hello_zk
Count: 2 using Hello_zk
Count: 3 using Hello_zk
Count: 4 using Hello_zk
Count: 5 using Hello_zk
```

总结

通过上面的方法，利用zookeeper可以实现配置的动态管理，更多内容请参考zookeeper-v3.4.10官网