Since version 0.20.x, HBase has been using ZooKeeper as its distributed coordination service. This includes tracking of region servers, where the root region is hosted, and more. Version 0.90.x introduced a new master implementation which has an even tighter integration with ZooKeeper. It enables HBase to remove critical heartbeat messages that needed to be sent between the master and the region servers. These are now moved into ZooKeeper, which informs either party of changes whenever they occur, as opposed to the fixed intervals that were used before.
ZooKeeper also has the following characteristics:
ZooKeeper is simple
ZooKeeper is, at its core, a stripped-down filesystem that exposes a few simple operations, and some extra abstractions such as ordering and notifications.
ZooKeeper is expressive
The ZooKeeper primitives are a rich set of building blocks that can be used to build a large class of coordination data structures and protocols. Examples include: distributed queues, distributed locks, and leader election among a group of peers.
ZooKeeper is highly available
ZooKeeper runs on a collection of machines and is designed to be highly available, so applications can depend on it. ZooKeeper can help you avoid introducing single points of failure into your system, so you can build a reliable application.
ZooKeeper facilitates loosely coupled interactions
ZooKeeper interactions support participants that do not need to know about one another. For example, ZooKeeper can be used as a rendezvous mechanism so that processes that otherwise don’t know of each other’s existence (or network details) can discover and interact with each other. Coordinating parties may not even be contemporaneous, since one process may leave a message in ZooKeeper that is read by another after the first has shut down.
ZooKeeper is a library
ZooKeeper provides an open source, shared repository of implementations and recipes of common coordination patterns. Individual programmers are spared the burden of writing common protocols themselves (which are often difficult to get right). Over time, the community can add to and improve the libraries, which is to everyone’s benefit.
ZooKeeper is highly performant, too. At Yahoo!, where it was created, the throughput for a ZooKeeper cluster has been benchmarked at over 10,000 operations per second for write-dominant workloads generated by hundreds of clients. For workloads where reads dominate, which is the norm, the throughput is several times higher.
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