ZooKeeper Java 示例

一个简单的监视客户端

介绍ZooKeeper Java API，我们在这里开发一个非常简单的观看客户端。该ZooKeeper客户端通过启动或停止程序来监视ZooKeeper节点的更改并进行响应。

需求

客户端有四个需求:

需要的参数:
- zookpeer的服务地址
- znode的名字
- 将输出写入到一个文件的名称
- 一个可执行的参数。
获取znode相关的数据并开始执行程序。
如果znode发生变化,重启客户端重新提取内容和可执行文件。
如果znode消失,客户端可进行线程销毁。

程序设计

一般来说,zookpeer应用被分解成两个部分,一个保持连接,另负责监控数据。在这个应用程序中,Executor(执行者)保持zookpeer联系,另一个类DataMonitor(数据监视者)监控树中的数据。此外,Executor包含主线程和包含执行逻辑。它负责小用户交互是什么,以及交互exectuable计划你在作为参数,该示例根据znode状态进行关闭和重新启动。

// from the Executor class...
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();
    }
}

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);
}

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

回想一下,执行程序的工作是启动和停止我传递的名字。它是zookpeer事件的相应对象。正如你所看到的在上面的代码中,执行者通过引用Zookpeer本的构造函数。它还通过引用DataMonitor DataMonitorListener参数的构造函数。没当程序执行的时候,就实现了这两个接口:

public class Executor implements Watcher, Runnable, DataMonitor.DataMonitorListener {
...

Watcher接口是Zookpeer的Java API中定义的。Zookpeer使用它与容器进行通信。它支持一个方法process()。执行程序在这个例子简单地将这些事件转发到DataMonitor决定如何处理它们。它只是为了说明这一点,按照惯例,遗嘱执行人或一些Executor-like对象“拥有”Zookpeer的连接,(后面详细讨论)。

public void process(WatchedEvent event) {
    dm.process(event);
}

DataMonitorListener接口不是Zookpeer API的一部分。它是一个自定义的interface,为这个示例应用程序而设计的。DataMonitor对象通信使用它回到它的容器,这也是执行程序对象。DataMonitorListener界面如下所示:

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    * The ZooKeeper session is no longer valid.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);
}

DataMonitor中定义该接口类和执行程序中实现类。当Executor.exists()调用,执行程序决定是否启动或关闭的要求。

当Executor.closing()调用,执行程序决定是否关闭自己的Zookpeer连接。您可能已经猜到, DataMonitor的对象调用这些方法,以应对变化的Zookpeer状态。

以下是Executor的DataMonitorListener.exists实现()和DataMonitorListener.closing:

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();
        }
    }
}

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

DataMonitor类

DataMonitor类是Zookpeer的主要逻辑。它主要是异步和事件驱动的。DataMonitor构造函数:

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);
}

调用ZooKeeper.exists()检查znode的存在,设置一个坚挺,和通过引用本身(这)作为完成回调对象。在这个意义上,它开始做事了,因为真正的watch被触发。

不要将完成回调与Watch回调混淆。当监视操作的异步设置（由ZooKeeper.exists（））在服务器上完成时，ZooKeeper.exists（）完成回调恰好是在DataMonitor对象中实现的StatCallback.processResult（）方法。

另一方面，触发Watch会向Executor对象发送一个事件，因为Executor注册为ZooKeeper对象的Watcher。

除此之外，您可能会注意到，DataMonitor也可以将自己注册为此特定观看事件的观察者。这是ZooKeeper 3.0.0（支持多个观察者）的新功能。但是在这个例子中，DataMonitor不会注册为Watcher。

当ZooKeeper.exists()操作在服务器上完成,Zookpeer API回调客户端:

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;
    }
}

代码首先检查znode存在的错误代码,致命错误,可恢复错误。如果文件(或znode)存在,它从znode获取数据,如果状态改变,调用执行者的exist()。注意,它没有任何异常处理getData调用,因为watches等待任何可能导致一个错误:如果节点被删除之前调用ZooKeeper.getData(),观察事件的ZooKeeper.exists()触发回调;如果有一个通信错误,当连接返回后一个连接监听事件触发。

最后,注意DataMonitor过程观察事件:

 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);
        }
    }

如果客户端Zookpeer库可以重建通信通道(SyncConnected事件)会话过期前Zookpeer(过期事件)的所有会话的监听会自动重新建立。

完整源代码清单

Executor.java

/**
 * 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)
     */
    public void process(WatchedEvent event) {
        dm.process(event);
    }

    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();
            }
        }
    }
}

DataMonitor.java

/**
 * 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;
        }
    }
}