ZK watcher 实现节点的监听

zk: localhost:2181(CONNECTED) 14] help
ZooKeeper -server host:port cmd args
        stat path [watch]
        set path data [version]
        ls path [watch]
        delquota [-n|-b] path
        ls2 path [watch]
        setAcl path acl
        setquota -n|-b val path
        history
        redo cmdno
        printwatches on|off
        delete path [version]
        sync path
        listquota path
        rmr path
        get path [watch]
        create [-s] [-e] path data acl
        addauth scheme auth
        quit
        getAcl path
        close
        connect host:port

监听器的原理：

watcher实现节点的监听

客户端A创建节点

[zk: localhost:2181(CONNECTED) 18] create /testwatcher 123
Created /testwatcher
[zk: localhost:2181(CONNECTED) 19] ls /
[controller_epoch, brokers, zookeeper, hellozk, testwatcher, admin, consumers, zktmp_1230000000021, config, my_zktmp0000000022, zktmp]

客户端B监听节点testwatch 变化 

[zk: 192.168.13.138(CONNECTED) 57] ls /
[controller_epoch, brokers, zookeeper, hellozk, testwatcher, admin, consumers, zktmp_1230000000021, config, my_zktmp0000000022, zktmp]
[zk: 192.168.13.138(CONNECTED) 58] get /testwatcher watch
123
cZxid = 0xd0000057a
ctime = Sun Mar 31 23:18:51 CST 2019
mZxid = 0xd0000057a
mtime = Sun Mar 31 23:18:51 CST 2019
pZxid = 0xd0000057a
cversion = 0
dataVersion = 0
aclVersion = 0
ephemeralOwner = 0x0
dataLength = 3
numChildren = 0
[zk: 192.168.13.138(CONNECTED) 59]
WATCHER::

WatchedEvent state:SyncConnected type:NodeDataChanged path:/testwatcher

客户端A 重新set 修改数据

[zk: localhost:2181(CONNECTED) 20] set /testwatcher 456
cZxid = 0xd0000057a
ctime = Sun Mar 31 23:18:51 CST 2019
mZxid = 0xd0000057b
mtime = Sun Mar 31 23:19:36 CST 2019
pZxid = 0xd0000057a
cversion = 0
dataVersion = 1
aclVersion = 0
ephemeralOwner = 0x0
dataLength = 3
numChildren = 0

客户端B收到了数据变化的监听

WatchedEvent state:SyncConnected type:NodeDataChanged path:/testwatcher

客户端A再次修改数据，客户端B不会再接受数据，因为watch机制是 只会监听一次变化，再次节点变化，则监听不到，只能再次设置监听任务