An Apache Zookeeper developer works on the open-source Apache Zookeeper project, a service used for maintaining configuration information, naming, providing distributed synchronization, and group services. They design, develop, and maintain the system, ensuring it remains fault-tolerant and scalable. They work on improving its performance, reliability, and efficiency. They also fix bugs and handle patches from the community. Furthermore, they work on the API and client libraries, ensuring they are robust and easy to use. They also interact with the user community, helping them understand the system and solve any issues they encounter.