Apache ZooKeeper

Apache ZooKeeper is an Apache Software Foundation project written in Java that provides coordination for distributed applications. Its topics describe it as a consensus and coordination service built around a replicated key-value style store, with uses such as service discovery and distributed configuration management.

Applications use ZooKeeper to keep shared configuration, elect leaders, track which nodes are alive and agree on state across a cluster. It relies on the ZAB protocol to keep replicas consistent. The repository documents how to build the project with Maven, produce source and binary tarballs, and generate API documentation, and release artifacts are published to Maven Central.

ZooKeeper is released under the Apache-2.0 licence and is operated by users themselves, usually as a small ensemble of servers. It is widely used as a building block beneath other distributed systems, so most teams meet it as a dependency of another tool rather than as a standalone product. Documentation and a wiki are available on the project website.