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The Zookeeper service, which is a sub-project of Hadoop, is provided by a cluster of servers to avoid a single point of failure. Zookeeper uses a distributed consensus protocol to determine which node in the ZooKeeper service is the leader at any given time.
The leader assigns a timestamp to each update to keep order. Once a majority of nodes have acknowledged receipt of a time-stamped update, the leader can declare a quorum, which means that any data contained in the update can be coordinated with elements of the data store. The use of a quorum ensures that the service always returns consistent answers.
According to the Hadoop developer's wiki, the service is named zookeeper because "coordinating distributing services is a zoo."