zookeeper 临时节点技术细节备注

临时节点多久会被删除

zookeeper的临时节点顾名思义是临时的,也就是说在一个链接session有效期内临时节点是活跃的,当链接断开后,session会天然的过时，session中过时了那么临时节点就会被删除

ZooKeeper also has the notion of ephemeral nodes. These znodes exists as long as the session that created the znode is active. When the session ends the znode is deleted.

那么session的时间默认是多少呢，答案是2xtickTime到20xtickTime之间

Ticks When using multi-server ZooKeeper, servers use ticks to define timing of events such as status uploads, session timeouts, connection timeouts between peers, etc. The tick time is only indirectly exposed through the minimum session timeout (2 times the tick time); if a client requests a session timeout less than the minimum session timeout, the server will tell the client that the session timeout is actually the minimum session timeout.

the basic time unit in milliseconds used by ZooKeeper. It is used to do heartbeats and the minimum session timeout will be twice the tickTime.

One of the parameters to the ZooKeeper client library call to create a ZooKeeper session is the session timeout in milliseconds. The client sends a requested timeout, the server responds with the timeout that it can give the client. The current implementation requires that the timeout be a minimum of 2 times the tickTime (as set in the server configuration) and a maximum of 20 times the tickTime. The ZooKeeper client API allows access to the negotiated timeout.

因此不是链接断开就会致使临时节点立刻被删除,还须要等待一点点的时间,这意味这session时间内咱们还能够救活一个临时节点,当一个链接被意外关闭或者网络缘由断开链接后.

咱们怎么救活一个临时节点?

咱们能够立刻有连上了在更新下这个临时节点,

# 建立一个临时节点
[zk: localhost:2181(CONNECTED) 4] create -e /e1 thinktik
Created /e1
# 断开链接而后从新链接，这时查询这个节点仍是能够的
[zk: localhost:2181(CONNECTED) 5] get /e1
thinktik
# 过一段时间后，这个节点会消失
[zk: localhost:2181(CONNECTED) 6] 2021-02-15 22:36:26,256 [myid:] - ERROR [main:ServiceUtils@42] - Exiting JVM with code 0

从新链接并不能救活临时节点，由于2次链接的session不同，咱们继续测试

# 建立一个临时节点
[zk: localhost:2181(CONNECTED) 4] create -e /e1 thinktik
Created /e1
# 断开链接而后从新链接，这时查询这个节点仍是能够的
[zk: localhost:2181(CONNECTED) 0] get /e1
thinktik
# 从新更新下这个临时节点，就好了
[zk: localhost:2181(CONNECTED) 1] set /e1 think

更多有关的资料请看:session

警告: 这个知识点其实没有实际价值,算是很冷门的小细节，也千万不要把这个细节用到生产环境中做为某种业务的逻辑实现来使用

参考:

• zookeeper原理篇-Zookeeper会话机制

临时节点能够有子节点吗

不行,临时节点不能够有子节点

Because of this behavior ephemeral znodes are not allowed to have children.

咱们能够这样验证

[zk: localhost:2181(CONNECTED) 17] create -e /e1 thinktik
Created /e1
# 报错,强调不能给临时节点建立子节点
[zk: localhost:2181(CONNECTED) 18] create -e /e1/se1
Ephemerals cannot have children: /e1/se1

本文原创连接: zookeeper 临时节点技术细节备注