ZooKeeper is a distributed, open-source coordination service for
distributed applications. It exposes a simple set of primitives that
distributed applications can build upon to implement higher level
services for synchronization, configuration maintenance, and groups
and naming. It is designed to be easy to program to, and uses a data
model styled after the familiar directory tree structure of file
systems. It runs in Java and has bindings for both Java and C.

Coordination services are notoriously hard to get right. They are
especially prone to errors such as race conditions and deadlock. The
motivation behind ZooKeeper is to relieve distributed applications the
responsibility of implementing coordination services from scratch.

WWW: http://zookeeper.apache.org/