ZooKeeper Java例子(六)

A Simple Watch Client

为了向你介绍ZooKeeper Java API,我们开发了一个非常简单的监视器客户端。ZooKeeper客户端监视一个ZooKeeper节点的改变并且通过开始和停止一个程序来作出响应。

必备条件

客户端有四个必备条件:

• 它作为参数：

1. 1. ZooKeeper服务端的地址
   2. znode的名字 - 被监视的节点
   3. 写输出内容的文件名字
   4. 带有参数的可执行文件

• 它抓取这个znode的数据并且开始这个可执行文件
• 如果znode改变，客户端重新抓取内容并且重启这个可执行文件
• 如果znode消失，客户端杀死这个可执行文件

程序设计

按照惯例，ZooKeeper应用被分为两部分，一部分维护连接，另一部分监视数据。在这个应用中，Executor的类维护ZooKeeper连接，DataMonitor的类监视ZooKeeper树中的数据。同时，Executor包含主线程和执行逻辑，它负责很少的用户交互，和你作为参数传进去的可执行程序的交互，还有那一个例子关闭和重启，根据znode的状态。

Executor 类

Executor对象是这个例子程序的主要容器。它包含ZooKeeper对象，DataMonitor，就像上面程序设计描述的那样。

   // 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) {
        }
    }

Executor的回调工作是开始和停止可执行文件，这个可执行文件的名字是你从命令行的传过来的。它做这个是为了响应被ZooKeeper对象触发的事件。就像你在上面的代码看到的那样，Executor传入一个引入给他自己作为ZooKeeper构造函数的Watcher 参数。

它也传入一个引用给它自己作为DataMonitor的构造器的DataMonitorListener参数。每一个的Executor的定义，它实现了这些接口:

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

Watcher接口被ZooKeeper Java API定义。ZooKeeper使用它和它的容器通信。它只支持一个方法，process(),并且ZooKeeper使用它和主线程感兴趣的事件通信，例如ZooKeeper通信或者ZooKeeper会话的状态。这个例子中的Executor只是简单的向下推送这些事件 给DataMonitor来决定怎么处理它。它这样做只是为了说明这一点，依照惯例，Executor或像Executor的对象"拥有"ZooKeeper连接，但是它可以自由地把事件委托给其它对象。它也使用这个作为监听事件触发的通道。

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

另一方面DataMonitorListener接口，不是ZooKeeper API的一部分，只是为了这个应用例子而设计的。DataMonitor对象使用它和它的容器通信，也就是Executor对象。DataMonitorListener接口就像这样:

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);
}

这个接口被定义在DataMonitor类并且在Executor类中实现。当Executor.exists()被调用，Executor决定是否启动或关闭每一个请求。当znode不存在的时候再次调用需要杀掉可执行程序。

当Executor.closing()被调用，Executor决定是否关掉它自己来响应ZooKeeper连接永久消失。

正如你可能已经猜到的。DataMonitor是调用这些方法的对象，来响应ZooKeeper的状态改变。

下面是Executor的DataMonitorListener.exists()和DataMonitorListener.closing的实现:

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

The DataMonitor class has the meat of the ZooKeeper logic(这句咋翻译)。它几乎上是异步和事件驱动。 DataMonitor kicks things off in the constructor with:

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()来检查znode是否存在，设置一个监视器，并且传递它自己的引用给它自己作为完成回调的对象，在这个意义上，it kicks things off，因为真实的处理在监视器被触发的时候发生。

注意

不要把完成回调和监视回调弄混淆了。ZooKeeper.exists()完成回调，它发生在DataMonitor对象的StatCallback.processResult()方法实现中，当异步的监视器set操作(被ZooKeeper.exists())在服务端完成的时候被调用。

另一方面，监视器的触发，发送一个事件给Executor对象，因为Executor作为ZooKeeper对象的监视器被注册。

此外，你可能注意到DataMonitor也能句注册它自已作为这个特定监视器事件的监听者。这是ZooKeeper3.0.0的新特性(支持多个监听者)。然而在这例子中，DataMonitor没有把它自己注册为监视器。

在ZooKeeper.exists()操作在服务端完成，ZooKeeper API在客户端调用这个完成回调函数:

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;
    }
}

这个代码首先检查Znode 是否存在，致命错误，和可恢复的错误。如果文件(或znode)存在，它从znode获取数据，并且如果状态已经改变它调用Executor的exists()回调函数。注意，它不需要为getData调用做任何Exception处理因为它有任何可能导致错误的监视器:如果在ZooKeeper.getData()方法之前这个节点被删除，通过ZooKeeper.exists()设置的监听事件会触发一个回调;如果有通信错误，当连接回来的时候会触发一个连接监听事件。

最后，注意DataMonitor是怎么处理监听事件的：

 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ZooKeeper库在会话到期(到期事件)之前可以和ZooKeeper重新建立通信通道(同步连接事件)所有会话的监视器将自动地在服务端被建立(自动重置监视器是ZooKeeper 3.0.0的新特性)。关于这点的更多信息可以参考ZooKeeper Watches。

再深入这个方法一些，当DataMonitor等到一个znode的事件，它调用ZooKeeper.exists()来查找什么被改变了。

完整的源码列表

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;
        }
    }
}