Architecting and building a distributed system is not a trivial job, and implementing coordination systems for the distributed applications is even harder. They are often prone to errors such as race conditions and deadlocks, and such bugs are not easily detectable. Apache ZooKeeper has been developed with this objective in mind, to simplify the task of developing coordination and synchronization systems from scratch. ZooKeeper is an open source service, which enables high performance and provides highly available coordination services for distributed applications. Apache ZooKeeper is a centralized service, which exposes a simple set of primitives that distributed applications can build on, in order to implement high-level services such as naming, configuration management, synchronization, group services, and so on. ZooKeeper has been designed to be easily programmable with its simple and elegant set of APIs and client bindings for a plethora of languages. Apache ZooKeeper Essentials takes readers through an enriching practical journey of learning ZooKeeper and understanding its role in developing scalable and robust distributed applications. It starts with a crisp description of why building coordination services for distributed applications is hard, which lays the stepping stone for the need to know and learn ZooKeeper. This book then describes the installation and configuration of a ZooKeeper instance, after which readers will get a firsthand experience of using it. This book covers the core concepts of ZooKeeper internals, its administration, and the best practices for its usage. The ZooKeeper APIs and the data model are presented in the most comprehensive manner for both beginners and experts, followed by programming with ZooKeeper. Examples of developing client applications have been given in three languages: Java, C, and Python.