Zookeeper（五）Watcher机制

最新推荐文章于 2025-03-15 08:51:58 发布

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最新推荐文章于 2025-03-15 08:51:58 发布 · 818 阅读

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文章标签：

#zookeeper #watcher

本文深入探讨了Zookeeper的Watcher机制，从Watcher的结构和参数传递开始，详细阐述了客户端如何向服务端发送数据，包括header部分和request请求内容。接着，分析了客户端接收服务端数据的过程，特别是数据变更通知和数据结果返回的处理，尤其是EventThread在处理服务端返回数据时的角色。

0.Watcher结构和传参

先看看Watcher定义的结构：

public interface Watcher {

    /**
     * This interface defines the possible states an Event may represent
     */
    public interface Event {
        /**
         * Enumeration of states the ZooKeeper may be at the event
         */
        public enum KeeperState {
            /** Unused, this state is never generated by the server */
            @Deprecated
            Unknown (-1),

            /** The client is in the disconnected state - it is not connected
             * to any server in the ensemble. */
            Disconnected (0),

            /** Unused, this state is never generated by the server */
            @Deprecated
            NoSyncConnected (1),

            /** The client is in the connected state - it is connected
             * to a server in the ensemble (one of the servers specified
             * in the host connection parameter during ZooKeeper client
             * creation). */
            SyncConnected (3),

            /**
             * Auth failed state
             */
            AuthFailed (4),

            /**
             * The client is connected to a read-only server, that is the
             * server which is not currently connected to the majority.
             * The only operations allowed after receiving this state is
             * read operations.
             * This state is generated for read-only clients only since
             * read/write clients aren't allowed to connect to r/o servers.
             */
            ConnectedReadOnly (5),

            /**
              * SaslAuthenticated: used to notify clients that they are SASL-authenticated,
              * so that they can perform Zookeeper actions with their SASL-authorized permissions.
              */
            SaslAuthenticated(6),

            /** The serving cluster has expired this session. The ZooKeeper
             * client connection (the session) is no longer valid. You must
             * create a new client connection (instantiate a new ZooKeeper
             * instance) if you with to access the ensemble. */
            Expired (-112);

            private final int intValue;     // Integer representation of value
                                            // for sending over wire
        }

        /**
         * Enumeration of types of events that may occur on the ZooKeeper
         */
        public enum EventType {
            None (-1),
            NodeCreated (1),
            NodeDeleted (2),
            NodeDataChanged (3),
            NodeChildrenChanged (4);

            private final int intValue;     // Integer representation of value
                                            // for sending over wire           
        }
    }

    abstract public void process(WatchedEvent event);
}

Watcher机制包含对事件(Event)和通知方法(process)的定义。
而事件Event包含通知状态(KeeperState)和事件类型(EventType)。
通知方法process的参数WatchedEvent event封装了通知状态(KeeperState)和事件类型(EventType)

public class WatchedEvent {
    final private KeeperState keeperState;
    final private EventType eventType;
    private String path;
    //省略其他代码
}

所以服务端通知客户端时，客户端能够知道数据变更通知的状态和类型。

当创建一个ZooKeeper对象时，可以通过向构造函数传Watcher对象，它会保存在org.apache.zookeeper.ZooKeeper.ZKWatchManager.defaultWatcher，即是下面源码的“1处”，也就是说客户端保存了Watcher对象。

public ZooKeeper(String connectString, int sessionTimeout, Watcher watcher, boolean canBeReadOnly) throws IOException{
    LOG.info("Initiating client connection, connectString=" + connectString
            + " sessionTimeout=" + sessionTimeout + " watcher=" + watcher);
    //1处
    watchManager.defaultWatcher = watcher;
    //2处
    ConnectStringParser connectStringParser = new ConnectStringParser(connectString);
    HostProvider hostProvider = new StaticHostProvider(connectStringParser.getServerAddresses());
    //3处
    cnxn = new ClientCnxn(connectStringParser.getChrootPath(),
            hostProvider, sessionTimeout, this, watchManager,
            getClientCnxnSocket(), canBeReadOnly);
    cnxn.start();
}

此外，还可以通过ZooKeeper的register(Watcher watcher)方法来设置默认Watcher。
Zookeeper除了可以通过构造函数来设置默认的Watcher，还可以通过以下方法来设置Watcher。
public byte[] getData(final String path, Watcher watcher, Stat stat);
public Stat exists(final String path, Watcher watcher);
public List<String> getChildren(final String path, Watcher watcher, Stat stat)
并分别存在dataWatches、existWatches和childWatches中，其中key是指path。

private static class ZKWatchManager implements ClientWatchManager {
    private final Map<String, Set<Watcher>> dataWatches = new HashMap<String, Set<

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