Apache ZooKeeper is an open-source server that provides critical infrastructure support for distributed systems. Hosted on GitHub at https://github.com/apache/zookeeper, it primarily focuses on maintaining configuration information, naming, providing distributed synchronization, and group services. Developed by Apache Software Foundation under the Apache License 2.0, ZooKeeper serves as a centralized repository to manage the coordination among distributed applications. Its core functionality is built around a hierarchy of nodes (znodes) that store data in a structured format resembling a file system.

ZooKeeper utilizes a client-server model where clients can perform operations like create, delete, and update on znodes while maintaining a consistent view across all servers. This feature ensures high availability and reliability, crucial for complex distributed environments such as databases, messaging systems, and cloud computing platforms. The design leverages a consensus protocol known as Zab (ZooKeeper Atomic Broadcast) to ensure data consistency and replication across multiple nodes in the cluster.

A distinguishing trait of ZooKeeper is its simplicity and flexibility; it allows developers to use an abstraction layer for coordination tasks without worrying about the underlying complexity. This has led to its adoption by several large-scale projects, including Apache HBase, Kafka, and Mesos, which utilize ZooKeeper's capabilities for leader election, configuration management, and maintaining state across a cluster.

The repository on GitHub is a hub of active development and collaboration, featuring contributions from both core developers and the wider open-source community. It includes comprehensive documentation, setup guides, and extensive testing frameworks to ensure robustness and stability. Developers can explore various branches for features or fixes, engage in discussions through issues and pull requests, and contribute by adhering to the project’s contribution guidelines.

In essence, Apache ZooKeeper provides a reliable coordination service that is vital for ensuring consistency and synchronization across distributed applications. Its design principles of simplicity, flexibility, and robustness make it an essential tool in building scalable and fault-tolerant systems.