Apache ZooKeeper is a distributed coordination service that simplifies the implementation of complex patterns in distributed systems by providing a robust set of basic operations. It functions similarly to a distributed file system, with information organized in nodes called zNodes, which can be ephemeral or sequential, supporting use cases like leader election and distributed locks. ZooKeeper also acts as a message queue through watchers, enabling clients to monitor changes, though it emphasizes correctness and consistency over speed. At Found, ZooKeeper is extensively used for discovery, resource allocation, leader election, and high-priority notifications, playing a critical role in managing Elasticsearch clusters. Despite its versatility, ZooKeeper is not suited for all tasks, such as handling large data files or application logs due to its limitations in size and throughput. The service operates under the CAP theorem as a CP system, prioritizing consistency and partition tolerance. The core consensus algorithm used by ZooKeeper, ZAB, focuses on total ordering of changes and requires a quorum to function, ensuring protection against network partitions. While it is a significant part of Found's infrastructure, ZooKeeper is not used for all communication needs, and care is taken not to exceed its capabilities. The article also highlights Curator, a project that provides well-tested implementations of common patterns on top of ZooKeeper, which is crucial for handling exceptions and network issues in distributed systems.