ZooKeeper提供了分布式数据的发布/订阅功能，一个典型的发布/订阅模型系统定义了一种一对多的订阅关系，能够让多个订阅者同时监听某一个主题对象，当这个主题对象自身状态变化时，会通知所有订阅者，使他们能够做出相应的处理。在ZooKeeper中，引入了Watchert机制来实现这种分布式的通知功能。
ZooKeeper允许客户端向服务端注册一个Watcher监听，当服务端的一些事件触发了这个Watcher，那么就会向指定客户端发送一个事件通知来实现分布式的通知功能。
1、Watcher接口
在ZooKeeper中，接口类Watcher用于表示一个标准的事件处理器，其定义了事件通知相关的逻辑，包含KeeperState和EventType两个枚举类，分别代表了通知状态和事件类型，同时定义了事件的回调方法：process（WatchedEvent event）。接口代码如下：
/**
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.zookeeper;
/**
* This interface specifies the public interface an event handler class must
* implement. A ZooKeeper client will get various events from the ZooKeeper
* server it connects to. An application using such a client handles these
* events by registering a callback object with the client. The callback object
* is expected to be an instance of a class that implements Watcher interface.
*
*/
public interface Watcher {
/**
* This interface defines the possible states an Event may represent
*/
public interface Event {
/**
* zookeeper在事件类型上的通知枚举状态
*/
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
KeeperState(int intValue) {
this.intValue = intValue;
}
public int getIntValue() {
return intValue;
}
public static KeeperState fromInt(int intValue) {
switch(intValue) {
case -1: return KeeperState.Unknown;
case 0: return KeeperState.Disconnected;
case 1: return KeeperState.NoSyncConnected;
case 3: return KeeperState.SyncConnected;
case 4: return KeeperState.AuthFailed;
case 5: return KeeperState.ConnectedReadOnly;
case 6: return KeeperState.SaslAuthenticated;
case -112: return KeeperState.Expired;
default:
throw new RuntimeException("Invalid integer value for conversion to KeeperState");
}
}
}
/**
* 事件类型
*/
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
EventType(int intValue) {
this.intValue = intValue;
}
public int getIntValue() {
return intValue;
}
public static EventType fromInt(int intValue) {
switch(intValue) {
case -1: return EventType.None;
case 1: return EventType.NodeCreated;
case 2: return EventType.NodeDeleted;
case 3: return EventType.NodeDataChanged;
case 4: return EventType.NodeChildrenChanged;
default:
throw new RuntimeException("Invalid integer value for conversion to EventType");
}
}
}
}
abstract public void process(WatchedEvent event);
}
2、Watcher事件
同一个event在不同的KeeperState中所代表的含义是不一样的，如下对应表格：
| KeeperState | EventType | 触发条件 | 说明 |
|---|---|---|---|
| - | None（-1） | 客户端与服务端成功建立连接 | - |
| SyncConnected（0） | NodeCreated（1） | Watcher监听的对应数据节点被创建 | - |
| - | NodeDeleted（2） | Watcher监听的对应数据节点被删除 此时客户端和服务器处于连接状态 | - |
| - | NodeDataChanged（3） | Watcher监听的对应数据节点的数据内容发生变更 | - |
| - | NodeChildChanged（4） | Wather监听的对应数据节点的子节点列表发生变更 | - |
| Disconnected（0） | None（-1） | 客户端与ZooKeeper服务器断开连接 | 此时客户端和服务器处于断开连接状态 |
| Expired（-112） | Node（-1） | 会话超时 此时客户端会话失效，通常同时也会受到SessionExpiredException异常 | - |
| AuthFailed（4） | None（-1） | 通常有两种情况，1：使用错误的schema进行权限检查 2：SASL权限检查失败 通常同时也会收到AuthFailedException异常 | - |
3、回调方法
process方法是Watcher接口中的一个回调方法，当ZooKeeper向客户端发送一个Watcher事件通知时，客户端就会对相应的process方法进行回调，从而实现对事件的处理。方法声明如下：
abstract public void process(WatchedEvent event);
这个回调方法的定义非常简单，只有一个WatchedEvent参数,WatchedEvent包含了每一个事件的三个基本属性：通知状态（keeperState），事件类型（EventType）和节点路径（path）,WatchedEvent部分代码如下：
public class WatchedEvent {
final private KeeperState keeperState;
final private EventType eventType;
private String path;
例如，当某个节点的数据发生变更时，服务端会发送给客户端一个“ZNode数据内容变更”事件，客户端只能够接收到如下信息：
keeperState：SyncConnected
EventType：NodeDataChanged
Path：/zk-book
从上面展示的信息中我们可以看到，客户端无法直接从该事件中获取到对应数据节点的原始数据内容以及变更后的新数据内容，而是需要客户端在此主动去重新获取数据——这也是ZooKeeper Watcher机制的一个非常重要的特性。
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