Apache ZooKeeper

Apache ZooKeeper is a software project of the Apache Software Foundation. It is essentially a distributed hierarchical key-value store, which is used to provide a distributed configuration service, synchronization service, and naming registry for large distributed systems. ZooKeeper was a sub-project of Hadoop but is now a top-level project in its own right.

ZooKeeper's architecture supports high availability through redundant services. The clients can thus ask another ZooKeeper leader if the first fails to answer. ZooKeeper nodes store their data in a hierarchical name space, much like a file system or a tree data structure. Clients can read from and write to the nodes and in this way have a shared configuration service. Updates are totally ordered.

ZooKeeper is used by companies including Rackspace, Yahoo!, Odnoklassniki, Reddit and eBay as well as open source enterprise search systems like Solr.