ZooKeeper监听机制

前言：Zookeeper的监听机制很多人都踩过坑，感觉实现了watcher 接口，后面节点的变化都会一一推送过来，然而并非如此。

Watch机制官方声明：一个Watch事件是一个一次性的触发器，当被设置了Watch的数据发生了改变的时候，则服务器将这个改变发送给设置了Watch的客户端，以便通知它们。

Zookeeper机制的特点：

   1.一次性触发  数据发生改变时，一个watcher event会被发送到client，但是client只会收到一次这样的信息。

   2.watcher event异步发送   watcher 的通知事件从server发送到client是异步的，这就存在一个问题，不同的客户端和服务器之间通过socket进行通信，由于网络延迟或其他因素导致客户端在不通的时刻监听到事件，由于Zookeeper本身提供了ordering guarantee，即客户端监听事件后，才会感知它所监视znode发生了变化。

   3.数据监视   Zookeeper有数据监视和子数据监视   getdata() and exists() 设置数据监视，getchildren()设置了子节点监视

   4.注册watcher   getData、exists、getChildren

    5. 触发watcher    create、delete、setData

watcher里面的相关事件
	event For "/path"	event For "/path/child"
create("/path")	EventType.NodeCreated	NA
delete("/path")	EventType.NodeDeleted	NA
setData("/path")	EventType.NodeDataChanged	NA
create("/path/child")	EventType.NodeChildrenChanged	EventType.NodeCreated
delete("/path/child")	EventType.NodeChildrenChanged	EventType.NodeDeleted
setData("/path/child")	NA	EventType.NodeDataChanged

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

各种操作触发的watcher事件
	"/path"			"/path/child"
	exists	getData	getChildren	exists		getData	getChildren
create("/path")	√	√
delete("/path")	√	√	√
setData("/path")	√	√
create("/path/child")			√	√		√
delete("/path/child")			√	√		√	√
setData("/path/child")				√		√

操作与watcher关系
"/path"				"/path/child"
	exists	getData	getChildren	exists		getData	getChildren
create("/path")	√	√
delete("/path")	√	√	√
setData("/path")	√	√
create("/path/child")			√	√		√
delete("/path/child")			√	√		√	√
setData("/path/child")				√		√

各种watch测试

/**
 * watch test
 *
 */
public class App implements Watcher {

    private static CountDownLatch connectedSemaphore = new CountDownLatch(1);
    public static Stat stat = new Stat();
    ZooKeeper zooKeeper;

    public static void main(String[] args) throws IOException, InterruptedException, KeeperException {
        String p = "/testaa";
        ZooKeeper zooKeeper = new ZooKeeper("192.168.1.10:2181", 5000, new App());
        connectedSemaphore.await();
        //exists register watch
        zooKeeper.exists(p, true);
        String path = zooKeeper.create(p, "456".getBytes(), Ids.OPEN_ACL_UNSAFE,
                CreateMode.PERSISTENT);
        //get register watch
        zooKeeper.getData(path, true, stat);
        zooKeeper.setData(path, "hhhh".getBytes(), -1);
        zooKeeper.exists(path, true);
        //exists register watch
        zooKeeper.delete(path, -1); 

    }

    public void process(WatchedEvent event) {
        if (KeeperState.SyncConnected == event.getState()) {
            if (EventType.None == event.getType() && null == event.getPath()) {
                connectedSemaphore.countDown();
                System.out.println("Zookeeper session established");
            } else if (EventType.NodeCreated == event.getType()) {
                System.out.println("success create znode");

            } else if (EventType.NodeDataChanged == event.getType()) {
                System.out.println("success change znode: " + event.getPath());

            } else if (EventType.NodeDeleted == event.getType()) {
                System.out.println("success delete znode");

            } else if (EventType.NodeChildrenChanged == event.getType()) {
                System.out.println("NodeChildrenChanged");

            }

        }
    }
}