The principle of watcher in Zookeeper

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Zookeeper provides an efficient and reliable distributed coordination service for distributed applications. Its implementation depends on the ZAB protocol and implements an architecture of active and standby mode to maintain the consistency of data in the cluster. Zookeeper enables distributed applications to be coordinated through a shared tree-structured namespace. It also stores all the data in memory. Any machine in the Zookeeper cluster can respond to the read operations of the client, and all the data exists in memory, so Zookeeper is more suitable for read-based application scenarios.

There are three roles in the cluster: leader,follower,observer. Leader is a machine determined by election, which provides read and write function for the client. Follower and observer can improve the read performance of the cluster without affecting the write performance. The number of Zookeeper cluster nodes is odd. There are three types of nodes: temporary node, persistent node and sequential node. Each node has a stat structure. The most important function is the watcher function.

Watcher:watcher is a core function in ZK. Watcher can monitor changes in the data of directory nodes and changes in subdirectories. Once these states change, the server will notify all watcher set on this directory node, so that each client will quickly know that the state of the directory node it is concerned about has changed and respond accordingly.

Its application scenarios include: unified naming service, cluster management, master election, distributed barrier and so on.

Open source clients are: zkclient, curator.

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