zookeeper的watcher观察者机制

Zookeeper提供了数据的发布/订阅功能，多个订阅者可同时监听某一特定主题对象，当该主题对象的自身状态发生变化时(例如节点内容改变、节点下的子节点列表改变等)，会实时、主动通知所有订阅者。

watcher实现原理

框架实现如下图：

1、zk客户端向zk服务器注册watcher的同时，会将watcher对象存储在客户端的watchManager。
2、Zk服务器触发watcher事件后，会向客户端发送通知，客户端线程从watchManager中回调watcher执行相应的功能。

watcher触发的事件

两种watch类型：结点的watch和子结点的watch

4种watch触发事件类型：NodeCreated、NodeDeleted、NodeDataChanged、NodeChildrenChanged

	创建结点	创建子结点	删除结点	删除子结点	设置值
exists	NodeCreated		NodeDeleted		NodeDataChanged
getData			NodeDeleted		NodeDataChanged
getChildren		NodeChildrenChanged	NodeDeleted	NodeChildrenChanged

watch只能触发一次，如果需要多次收到通知，客户端需要重新注册watch

• 当watch的znode被创建、删除或数据被更新时（无论数据是否改变），设置在exists上的事件被触发
• 当watch的znode被删除或数据被更新时。设置在getData上的事件被触发
• 当watch的childrenznode被创建或删除时，设置在getChildren上的事件被触发

测试exists操作

[zk: zk02:2181(CONNECTED) 0] ls /
[zookeeper]
[zk: zk02:2181(CONNECTED) 1] stat -w /abc
Node does not exist: /abc
[zk: zk02:2181(CONNECTED) 2] create /abc "a"

WATCHER::Created /abc


WatchedEvent state:SyncConnected type:NodeCreated path:/abc
[zk: zk02:2181(CONNECTED) 3] set /abc 'a'
[zk: zk02:2181(CONNECTED) 4] stat -w /abc
cZxid = 0x10000000a
ctime = Sun Mar 07 06:24:56 EST 2021
mZxid = 0x10000000b
mtime = Sun Mar 07 06:51:18 EST 2021
pZxid = 0x10000000a
cversion = 0
dataVersion = 1
aclVersion = 0
ephemeralOwner = 0x0
dataLength = 1
numChildren = 0
[zk: zk02:2181(CONNECTED) 5] set /abc 'a'

WATCHER::

WatchedEvent state:SyncConnected type:NodeDataChanged path:/abc
[zk: zk02:2181(CONNECTED) 6] stat -w /abc
cZxid = 0x10000000a
ctime = Sun Mar 07 06:24:56 EST 2021
mZxid = 0x10000000c
mtime = Sun Mar 07 06:51:32 EST 2021
pZxid = 0x10000000a
cversion = 0
dataVersion = 2
aclVersion = 0
ephemeralOwner = 0x0
dataLength = 1
numChildren = 0
[zk: zk02:2181(CONNECTED) 8] stat -w /abc
cZxid = 0x10000000a
ctime = Sun Mar 07 06:24:56 EST 2021
mZxid = 0x10000000c
mtime = Sun Mar 07 06:51:32 EST 2021
pZxid = 0x10000000a
cversion = 0
dataVersion = 2
aclVersion = 0
ephemeralOwner = 0x0
dataLength = 1
numChildren = 0
[zk: zk02:2181(CONNECTED) 9] set /abc 'a'

WATCHER::

WatchedEvent state:SyncConnected type:NodeDataChanged path:/abc
[zk: zk02:2181(CONNECTED) 10] stat -w /abc
cZxid = 0x10000000a
ctime = Sun Mar 07 06:24:56 EST 2021
mZxid = 0x10000000d
mtime = Sun Mar 07 06:52:46 EST 2021
pZxid = 0x10000000a
cversion = 0
dataVersion = 3
aclVersion = 0
ephemeralOwner = 0x0
dataLength = 1
numChildren = 0

  [zk: zk02:2181(CONNECTED) 11] delete /abc

  WATCHER::

  WatchedEvent state:SyncConnected type:NodeDeleted path:/abc

测试getData

[zk: zk02:2181(CONNECTED) 37] get -w /a
a
[zk: zk02:2181(CONNECTED) 38] set /a 'a'
[zk: zk02:2181(CONNECTED) 39] 
WATCHER::

WatchedEvent state:SyncConnected type:NodeDataChanged path:/a

测试getChildren

[zk: zk02:2181(CONNECTED) 42] ls -w /a
[]
[zk: zk02:2181(CONNECTED) 43] create /a/b 'b'

WATCHER::

WatchedEvent state:SyncConnected type:NodeChildrenChanged path:/a
Created /a/b

[zk: zk02:2181(CONNECTED) 44] delete /a/b

WATCHER::

WatchedEvent state:SyncConnected type:NodeDeleted path:/a/b

WATCHER::

WatchedEvent state:SyncConnected type:NodeChildrenChanged path:/a