What is Apache Zookeeper?

Apache Zookeeper is a coordination service for distributed application that enables synchronization across a cluster. Zookeeper in Hadoop can be viewed as centralized repository where distributed applications can put data and get data out of it. It is used to keep the distributed system functioning together as a single unit, using its synchronization, serialization and coordination goals. For simplicity's sake Zookeeper can be thought of as a file system where we have znodes that store data instead of files or directories storing data. Zookeeper is a Hadoop Admin tool used for managing the jobs in the cluster.