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When you’re running a service distributed across a large cluster of machines, even tasks like reading configuration information, which are simple on single-machine systems, can be hard to implement reliably. The ZooKeeper framework was originally built at Yahoo! to make it easy for the company’s applications to access configuration information in a robust and easy-to-understand way, but it has since grown to offer a lot of features that help coordinate work across distributed clusters. One way to think of it is as a very specialized key/value store, with an interface that looks a lot like a filesystem and supports operations like watching callbacks, write consensus, and transaction IDs that are often needed for coordinating distributed algorithms.

This has allowed it to act as a foundation layer for services like LinkedIn’s Norbert, a flexible framework for managing clusters of machines. ZooKeeper itself is built to run in a distributed way across a number of machines, and it’s designed to offer very fast reads, at the expense of writes that get slower the more servers are used to host the service.

Source of Information : Big data Glossary