Apache ZooKeeper is an effort to develop and maintain an open-source server which enables highly reliable distributed coordination.

ZooKeeper supports locking, synchronization, queues, hierarchical naming service and much more, out of the box.

ZooKeeper runs on a cluster of servers. Data is replicated on nodes and is kept in memory. Clients can connect to any node to read and write, however, writes are passed to the cluster `leader`.

ZooKeeper is eventually consistent: the writes are guaranteed to be executed in the order they were received from the client, updates are atomic, and will eventually be replicated to other nodes.