|Developer(s)||Apache Software Foundation|
3.8.0 / February 25, 2022
|License||Apache License 2.0|
Apache ZooKeeper is an open-source server for highly reliable distributed coordination of cloud applications. It is a project of the Apache Software Foundation.
ZooKeeper is essentially a service for distributed systems offering a hierarchical key-value store, which is used to provide a distributed configuration service, synchronization service, and naming registry for large distributed systems (see Use cases). ZooKeeper was a sub-project of Hadoop but is now a top-level Apache 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. ZooKeeper can be viewed as an atomic broadcast system, through which updates are totally ordered. The ZooKeeper Atomic Broadcast (ZAB) protocol is the core of the system.
ZooKeeper is used by companies including Yelp, Rackspace, Yahoo!, Odnoklassniki, Reddit, NetApp SolidFire, Meta, Twitter and eBay as well as open source enterprise search systems like Solr.
ZooKeeper is modeled after Google's Chubby lock service and was originally developed at Yahoo! for streamlining the processes running on big-data clusters by storing the status in local log files on the ZooKeeper servers. These servers communicate with the client machines to provide them the information. ZooKeeper was developed in order to fix the bugs that occurred while deploying distributed big-data applications.
Some of the prime features of Apache ZooKeeper are:
Some common terminologies regarding the ZooKeeper architecture:
The services in the cluster are replicated and stored on a set of servers (called an "ensemble"), each of which maintains an in-memory database containing the entire data tree of state as well as a transaction log and snapshots stored persistently. Multiple client applications can connect to a server, and each client maintains a TCP connection through which it sends requests and heartbeats and receives responses and watch events for monitoring.
Typical use cases for ZooKeeper are:
In addition to the client libraries included with the ZooKeeper distribution, a number of third-party libraries such as Apache Curator and Kazoo are available that make using ZooKeeper easier, add additional functionality, additional programming languages, etc.