ZooKeeper watcher机制

watcher机制

• 针对每个节点的操作，都会有一个监督者->watcher
• 当监控的某个对象(znode)发生了变化，则触发watcher事件
• zk中的watcher是一次性的，触发后立即销毁
• 父节点、子节点增删改都能够触发其watcher
• 针对不同类型的操作，则触发的watcher事件也不同
  1. (子)节点创建事件
  2. (子)节点删除事件
  3. (子)节点数据变化事件

watcher常用命令使用

创建父节点触发: NodeCreated

为/lzc创建一个watcher事件，当/lzc被创建时就会触发

[zk: localhost:2181(CONNECTED) 6] stat /lzc watcher
Node does not exist: /lzc

忽略此时的错误，因为此时/lzc确实不存在，但是watcher还是会被设置

接下来创建/lzc

[zk: localhost:2181(CONNECTED) 7] create /lzc lzc-data

WATCHER::

WatchedEvent state:SyncConnected type:NodeCreated path:/lzc
Created /lzc

我们可以看到WATCHER事件已经触发，触发事件类型为NodeCreated，针对的路径为/lzc

修改父节点触发: NodeDataChanged

前面已经使用了stat来设置watcher事件，这次使用get来设置watcher事件。

[zk: localhost:2181(CONNECTED) 17] get /lzc watcher
lzc-data
cZxid = 0x1a
ctime = Sat Mar 16 23:17:20 CST 2019
mZxid = 0x1e
mtime = Sat Mar 16 23:24:02 CST 2019
pZxid = 0x1a
cversion = 0
dataVersion = 4
aclVersion = 0
ephemeralOwner = 0x0
dataLength = 8
numChildren = 0

重新为/lzc设置数据

[zk: localhost:2181(CONNECTED) 18] set /lzc new-lzc-data

WATCHER::

WatchedEvent state:SyncConnected type:NodeDataChanged path:/lzc
cZxid = 0x1a
ctime = Sat Mar 16 23:17:20 CST 2019
mZxid = 0x1f
mtime = Sat Mar 16 23:24:54 CST 2019
pZxid = 0x1a
cversion = 0
dataVersion = 5
aclVersion = 0
ephemeralOwner = 0x0
dataLength = 12
numChildren = 0

我们可以看到WATCHER事件已经触发，触发事件类型为NodeDataChanged，针对的路径为/lzc

删除父节点触发: NodeDeleted

由于watcher事件是一次性的，因此我们需要重新为/lzc设置watcher事件

[zk: localhost:2181(CONNECTED) 24] get /lzc watcher
new-lzc-data
cZxid = 0x21
ctime = Sat Mar 16 23:29:53 CST 2019
mZxid = 0x21
mtime = Sat Mar 16 23:29:53 CST 2019
pZxid = 0x21
cversion = 0
dataVersion = 0
aclVersion = 0
ephemeralOwner = 0x0
dataLength = 12

执行删除/lzc操作

[zk: localhost:2181(CONNECTED) 25] delete /lzc

WATCHER::

WatchedEvent state:SyncConnected type:NodeDeleted path:/lzc

我们可以看到WATCHER事件已经触发，触发事件类型为NodeDeleted，针对的路径为/lzc

ls为父节点设置watcher，创建子节点触发: NodeChildrenChanged

查看当前节点

[zk: localhost:2181(CONNECTED) 28] ls /
[zookeeper, lzc]

使用ls对/lzc设置watcher事件

[zk: localhost:2181(CONNECTED) 30] ls /lzc watcher
[]

为/lzc创建一个子节点

[zk: localhost:2181(CONNECTED) 32] create /lzc/abc abc-data

WATCHER::

WatchedEvent state:SyncConnected type:NodeChildrenChanged path:/lzc
Created /lzc/abc

我们可以看到触发事件为NodeChildrenChanged

ls为父节点设置watcher，删除子节点触发: NodeChildrenChanged

[zk: localhost:2181(CONNECTED) 38] ls /lzc/abc watcher
[]
[zk: localhost:2181(CONNECTED) 39] delete /lzc/abc

WATCHER::

WatchedEvent state:SyncConnected type:NodeDeleted path:/lzc/abc

我们可以看到触发事件为NodeDeleted

ls为父节点设置watcher，修改子节点不触发事件

[zk: localhost:2181(CONNECTED) 51] ls /
[zookeeper, lzc]
[zk: localhost:2181(CONNECTED) 52] ls /lzc
[abc]
[zk: localhost:2181(CONNECTED) 53] ls /lzc watcher
[abc]
[zk: localhost:2181(CONNECTED) 54] set /lzc/abc aabbaabb
cZxid = 0x29
ctime = Sat Mar 16 23:41:36 CST 2019
mZxid = 0x2b
mtime = Sat Mar 16 23:45:37 CST 2019
pZxid = 0x29
cversion = 0
dataVersion = 2
aclVersion = 0
ephemeralOwner = 0x0
dataLength = 8
numChildren = 0