ZooKeeper - Centralized service for maintaining configuration information

ZooKeeper is a centralized service for maintaining configuration information, naming, providing distributed synchronization, and providing group services. All of these kinds of services are used in some form or another by distributed applications.

The name space provided by ZooKeeper is much like that of a standard file system. A name is a sequence of path elements separated by a slash (/). Every node in ZooKeeper's name space is identified by a path. ZooKeeper supports the concept of watches. Clients can set a watch on a znodes. A watch will be triggered and removed when the znode changes. It supports replication.

In case of distributed system, there may be multiple servers interacting with each other. Few may need to share some common resources. Consider a scenario, where a application server wants to know the list of database servers, then it could communicate the Zookeeper server and fetch the required information.