ZooKeeper客户端事件串行化处理

为了提升系统的性能,进一步提高系统的吞吐能力,最近公司很多系统都在进行异步化改造。在异步化改造的过程中,肯定会比以前碰到更多的多线程问题,上周就碰到ZooKeeper客户端异步化过程中的一个死锁问题,这里说明下。

通常ZooKeeper对于同一个API,提供了同步和异步两种调用方式。
同步接口很容易理解,使用方法如下:

ZooKeeper zk = new ZooKeeper(...);
List children = zk.getChildren( path, true );
异步接口就相对复杂一点,使用方法如下:
ZooKeeper zk = new ZooKeeper(...);
zk.getChildren( path, true, new AsyncCallback.Children2Callback() {
            @Override
            public void processResult( int rc, String path, Object ctx, List children, Stat stat ) {
                System.out.println( "Recive the response." );
            }
}, null);

我们可以看到,异步调用中,需要注册一个Children2Callback,并实现回调方法:processResult。

上周碰到这样的问题:应用注册了对某znode子节点列表变化的监听,逻辑是在接受到ZooKeeper服务器节点列表变更通知(EventType.NodeChildrenChanged)的时候,会重新获取一次子节点列表。之前,他们是使用同步接口,整个应用可以正常运行,但是这次异步化改造后,出现了诡异现象,能够收到子节点的变更通知,但是无法重新获取子节点列表了。

下面,我首先把应用之前使用同步接口的逻辑代码,用一个简单的demo来演示下,如下:

package book.chapter05;

import java.io.IOException;
import java.util.List;
import java.util.concurrent.CountDownLatch;

import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.Watcher.Event.EventType;
import org.apache.zookeeper.ZooDefs.Ids;
import org.apache.zookeeper.ZooKeeper;
import org.apache.zookeeper.Watcher.Event.KeeperState;

/**
 * ZooKeeper API 获取子节点列表,使用同步(sync)接口。
 * @author [银时](mailto:nileader@gmail.com)
 */
public class ZooKeeper_GetChildren_API_Sync_Usage implements Watcher {

    private CountDownLatch connectedSemaphore = new CountDownLatch( 1 );
    private static CountDownLatch _semaphore = new CountDownLatch( 1 );
    private ZooKeeper zk;

    ZooKeeper createSession( String connectString, int sessionTimeout, Watcher watcher ) throws IOException {
        ZooKeeper zookeeper = new ZooKeeper( connectString, sessionTimeout, watcher );
        try {
            connectedSemaphore.await();
        } catch ( InterruptedException e ) {
        }
        return zookeeper;
    }

    /** create path by sync */
    void createPath_sync( String path, String data, CreateMode createMode ) throws IOException, KeeperException, InterruptedException {

        if ( zk == null ) {
            zk = this.createSession( "domain1.book.zookeeper:2181", 5000, this );
        }
        zk.create( path, data.getBytes(), Ids.OPEN_ACL_UNSAFE, createMode );
    }

    /** Get children znodes of path and set watches */
    List getChildren( String path ) throws KeeperException, InterruptedException, IOException{

        System.out.println( "===Start to get children znodes.===" );
        if ( zk == null ) {
            zk = this.createSession( "domain1.book.zookeeper:2181", 5000, this );
        }
        return zk.getChildren( path, true );
    }

    public static void main( String[] args ) throws IOException, InterruptedException {

        ZooKeeper_GetChildren_API_Sync_Usage sample = new ZooKeeper_GetChildren_API_Sync_Usage();
        String path = "/get_children_test";

        try {

            sample.createPath_sync( path, "", CreateMode.PERSISTENT );
            sample.createPath_sync( path + "/c1", "", CreateMode.PERSISTENT );
            List childrenList = sample.getChildren( path );
            System.out.println( childrenList );
            //Add a new child znode to test watches event notify.
            sample.createPath_sync( path + "/c2", "", CreateMode.PERSISTENT );
            _semaphore.await();
        } catch ( KeeperException e ) {
            System.err.println( "error: " + e.getMessage() );
            e.printStackTrace();
        }
    }

    /**
     * Process when receive watched event
     */
    @Override
    public void process( WatchedEvent event ) {
        System.out.println( "Receive watched event:" + event );
        if ( KeeperState.SyncConnected == event.getState() ) {

            if( EventType.None == event.getType() && null == event.getPath() ){
                connectedSemaphore.countDown();
            }else if( event.getType() == EventType.NodeChildrenChanged ){
                //children list changed
                try {
                    System.out.println( this.getChildren( event.getPath() ) );
                    _semaphore.countDown();
                } catch ( Exception e ) {}
            }

        }
    }
}
输出结果如下:
Receive watched event:WatchedEvent state:SyncConnected type:None path:null
===Start to get children znodes.===
[c1]
Receive watched event:WatchedEvent state:SyncConnected type:NodeChildrenChanged path:/get_children_test
===Start to get children znodes.===
[c1, c2]
在上面这个程序中,我们首先创建了一个父节点: /get_children_test,以及一个子节点:/get_children_test/c1。然后调用getChildren的同步接口来获取/get_children_test节点下的所有子节点,调用的同时注册一个watches。之后,我们继续向/get_children_test节点创建子节点:/get_children_test/c2,这个时候,因为我们之前我们注册了一个watches,因此,一旦此时有子节点被创建,ZooKeeper Server就会向客户端发出“子节点变更”的通知,于是,客户端可以再次调用getChildren方法来获取新的子节点列表。 这个例子当然是能够正常运行的。现在,我们进行异步化改造,如下:
package book.chapter05;

import java.io.IOException;
import java.util.List;
import java.util.concurrent.CountDownLatch;

import org.apache.zookeeper.AsyncCallback;
import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.Watcher.Event.EventType;
import org.apache.zookeeper.ZooDefs.Ids;
import org.apache.zookeeper.data.Stat;
import org.apache.zookeeper.ZooKeeper;
import org.apache.zookeeper.Watcher.Event.KeeperState;

/**
 * ZooKeeper API 获取子节点列表,使用异步(ASync)接口。
 * @author [银时](mailto:nileader@gmail.com)
 */
public class ZooKeeper_GetChildren_API_ASync_Usage_Deadlock implements Watcher {

    private CountDownLatch connectedSemaphore = new CountDownLatch( 1 );
    private static CountDownLatch _semaphore = new CountDownLatch( 1 );
    private ZooKeeper zk;

    ZooKeeper createSession( String connectString, int sessionTimeout, Watcher watcher ) throws IOException {
        ZooKeeper zookeeper = new ZooKeeper( connectString, sessionTimeout, watcher );
        try {
            connectedSemaphore.await();
        } catch ( InterruptedException e ) {
        }
        return zookeeper;
    }

    /** create path by sync */
    void createPath_sync( String path, String data, CreateMode createMode ) throws IOException, KeeperException, InterruptedException {

        if ( zk == null ) {
            zk = this.createSession( "domain1.book.zookeeper:2181", 5000, this );
        }
        zk.create( path, data.getBytes(), Ids.OPEN_ACL_UNSAFE, createMode );
    }

    /** Get children znodes of path and set watches */
    void getChildren( String path ) throws KeeperException, InterruptedException, IOException{

        System.out.println( "===Start to get children znodes.===" );
        if ( zk == null ) {
            zk = this.createSession( "domain1.book.zookeeper:2181", 5000, this );
        }

        final CountDownLatch _semaphore_get_children = new CountDownLatch( 1 );

        zk.getChildren( path, true, new AsyncCallback.Children2Callback() {
            @Override
            public void processResult( int rc, String path, Object ctx, List children, Stat stat ) {

                System.out.println( "Get Children znode result: [response code: " + rc + ", param path: " + path + ", ctx: " + ctx + ", children list: "
                        + children + ", stat: " + stat );
                _semaphore_get_children.countDown();
            }
        }, null);
        _semaphore_get_children.await();
    }

    public static void main( String[] args ) throws IOException, InterruptedException {

        ZooKeeper_GetChildren_API_ASync_Usage_Deadlock sample = new ZooKeeper_GetChildren_API_ASync_Usage_Deadlock();
        String path = "/get_children_test";

        try {
            sample.createPath_sync( path, "", CreateMode.PERSISTENT );
            sample.createPath_sync( path + "/c1", "", CreateMode.PERSISTENT );
            //Get children and register watches.
            sample.getChildren( path );
            //Add a new child znode to test watches event notify.
            sample.createPath_sync( path + "/c2", "", CreateMode.PERSISTENT );

            _semaphore.await();
        } catch ( KeeperException e ) {
            System.err.println( "error: " + e.getMessage() );
            e.printStackTrace();
        }
    }

    /**
     * Process when receive watched event
     */
    @Override
    public void process( WatchedEvent event ) {
        System.out.println( "Receive watched event:" + event );
        if ( KeeperState.SyncConnected == event.getState() ) {

            if( EventType.None == event.getType() && null == event.getPath() ){
                connectedSemaphore.countDown();
            }else if( event.getType() == EventType.NodeChildrenChanged ){
                //children list changed
                try {
                    this.getChildren( event.getPath() );
                    _semaphore.countDown();
                } catch ( Exception e ) {
                    e.printStackTrace();
                }
            }

        }
    }
}
输出结果如下:
Receive watched event:WatchedEvent state:SyncConnected type:None path:null
===Start to get children znodes.===
Get Children znode result: [response code: 0, param path: /get_children_test, ctx: null, children list: [c1], stat: 555,555,1373931727380,1373931727380,0,1,0,0,0,1,556

Receive watched event:WatchedEvent state:SyncConnected type:NodeChildrenChanged path:/get_children_test
===Start to get children znodes.===

在上面这个demo中,执行逻辑和之前的同步版本基本一致,唯一有区别的地方在于获取子节点列表的过程异步化了。这样一改造,问题就出来了,整个程序在进行第二次获取节点列表的时候,卡住了。和应用方确认了,之前同步版本从来没有出现过这个现象的,所以开始排查这个异步化中哪里会阻塞。

这里,我们重点讲解在ZooKeeper客户端中,需要处理来自服务端的两类事件通知:一类是Watches时间通知,另一类则是异步接口调用的响应。值得一提的是,在ZooKeeper的客户端线程模型中,这两个事件由同一个线程处理,并且是串行处理。具体可以自己查看事件处理的核心类:org.apache.zookeeper.ClientCnxn.EventThread。

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