Zookeeper Overview: Functions, Deployment Modes, Synchronization, and Notification Mechanism

Zookeeper is an open‑source distributed coordination service, the open‑source implementation of Google Chubby, and a key component of Hadoop and HBase, providing consistency services such as configuration maintenance, naming, distributed synchronization, and group services.

Its main functions include cluster management (monitoring node health and handling requests), leader election (automatically selecting a new leader when the current one fails), distributed locks (exclusive and shared locks for resource access), and naming service (allowing clients to resolve resource addresses by name).

Zookeeper can be deployed in three modes: single‑machine deployment, cluster deployment with multiple machines, and pseudo‑cluster deployment where multiple Zookeeper instances run on a single machine.

State synchronization between leader and followers is ensured by Zookeeper’s atomic broadcast mechanism implemented via the ZAB protocol, which operates in recovery (leader election) and broadcast (synchronization) modes; once a majority of servers have synchronized with the leader, normal operation resumes.

In a distributed environment a leader node is needed to execute certain business logic centrally, reducing duplicate computation and improving performance.

A Zookeeper ensemble can continue operating as long as a majority of servers remain alive; with three servers, the system remains functional if only one fails.

Zookeeper’s notification mechanism uses watchers: clients set a watcher on a znode, and when that znode changes, the client receives a notification and can react accordingly.

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