ZooKeeper監聽機制

前言：Zookeeper的監聽機制很多人都踩過坑，感覺實現了watcher 介面，後面節點的變化都會一一推送過來，然而並非如此。

Watch機制官方聲明：一個Watch事件是一個一次性的觸發器，當被設置了Watch的數據發生了改變的時候，則伺服器將這個改變發送給設置了Watch的客戶端，以便通知它們。

Zookeeper機制的特點：

   1.一次性觸發  數據發生改變時，一個watcher event會被髮送到client，但是client只會收到一次這樣的信息。

   2.watcher event非同步發送   watcher 的通知事件從server發送到client是非同步的，這就存在一個問題，不同的客戶端和伺服器之間通過socket進行通信，由於網路延遲或其他因素導致客戶端在不通的時刻監聽到事件，由於Zookeeper本身提供了ordering guarantee，即客戶端監聽事件後，才會感知它所監視znode發生了變化。

   3.數據監視   Zookeeper有數據監視和子數據監視   getdata() and exists() 設置數據監視，getchildren()設置了子節點監視

   4.註冊watcher   getData、exists、getChildren

    5. 觸發watcher    create、delete、setData

各種watch測試

/**
 * 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");

            }

        }
    }
}