Thursday, September 4, 2014

ZooKeeper - The King of Coordination

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ZooKeeper is a coordination service for distributed systems. By providing a robust implementation of a few basic operations, ZooKeeper simplifies the implementation of many advanced patterns in distributed systems.

ZooKeeper as a Distributed File System
One way of getting to know ZooKeeper is to think of it as a distributed file system. In fact, the way information in ZooKeeper is organized is quite similar to a file system. At the top there is a root simply referred to as /. Below the root there are nodes referred to as zNodes, short for ZooKeeper Node, but mostly a term used to avoid confusion with computer nodes. A zNode may act as both a file containing binary data and a directory with more zNodes as sub nodes. As most file systems, each zNode has some meta data. This meta data includes read and write permissions and version information.

Unlike an ordinary distributed file system, ZooKeeper supports the concepts of ephemeral zNodes and sequential zNodes. An ephemeral zNode is a node that will disappear when the session of its owner ends. A typical use case for ephemeral nodes is when using ZooKeeper for discovery of hosts in your distributed system. Each server can then publish its IP address in an ephemeral node, and should a server loose connectivity with ZooKeeper and fail to reconnect within the session timeout, then its information is deleted.

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