ZooKeeper: Distributed Process Coordination Book by Benjamin Reed and Flavio Junqueira

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ZooKeeper was designed to be a robust service that enables application developers to focus mainly on their application logic rather than coordination. It exposes a simple API, inspired by the filesystem API, that allows developers to implement common coordination tasks, such as electing a master server, managing group membership, and managing metadata. ZooKeeper is an application library with two principal implementations of the APIs—Java and C—and a service component implemented in Java that runs on an ensemble of dedicated servers. Having an ensemble of servers enables Zoo‐Keeper to tolerate faults and scale throughput.

Let’s look at some examples where ZooKeeper has been useful to get a better sense of where it is applicable:

Apache HBase

HBase is a data store typically used alongside Hadoop. In HBase, ZooKeeper is used to elect a cluster master, to keep track of available servers, and to keep cluster metadata.

Apache Kafka

Kafka is a pub-sub messaging system. It uses ZooKeeper to detect crashes, to implement topic discovery, and to maintain production and consumption state for topics.

Apache Solr

Solr is an enterprise search platform. In its distributed form, called SolrCloud, it uses ZooKeeper to store metadata about the cluster and coordinate the updates to this metadata.

Yahoo! Fetching Service

Part of a crawler implementation, the Fetching Service fetches web pages efficiently by caching content while making sure that web server policies, such as those in robots.txt files, are preserved. This service uses ZooKeeper for tasks such as master election, crash detection, and metadata storage.

Facebook Messages

This is a Facebook application that integrates communication channels: email, SMS, Facebook Chat, and the existing Facebook Inbox. It uses ZooKeeper as a controller for implementing sharding and failover, and also for service discovery.

There are a lot more examples out there; this is a just a sample. Given this sample, let’s now bring the discussion to a more abstract level. When programming with ZooKeeper, developers design their applications as a set of clients that connect to ZooKeeper servers and invoke operations on them through the ZooKeeper client API. Among the strengths of the ZooKeeper API, it provides:

• Strong consistency, ordering, and durability guarantees
• The ability to implement typical synchronization primitives
• A simpler way to deal with many aspects of concurrency that often lead to incorrect behavior in real distributed systems

ZooKeeper, however, is not magic; it will not solve all problems out of the box. It is important to understand what ZooKeeper provides and to be aware of its tricky aspects.