I've been looking at Zookeeper recently and wondered whether anybody was using it currently and what they were specifically using it for storing.
The most common use case is for configuration information, but what kind of data and how much data are you storing?
Norbert is a good example from a scalable production system. I general, it integrates Netty, Protocol Buffers and Zookeeper into a lightweight framework for running clustered services. Protocol Buffers are used to specify your service API, Netty implements transport-layer abstractions and Zookeeper is essentially a fault-tolerant discovery service.
Every time a service instance is started Norbert registers it as available instance of a particular service type. From implementation perspective, it creates two Zookeeper trees:
The most important property for each node is the url to use to connect to the corresponding service instance. It enables client-side load balancing - a Norbert client finds the list of urls for a given service name and attempt to connect to one of them is some order (e.g. round-robin or random).
Solr is also working to integrate ZooKeeper. Here you can see they are using for dynamic config, sharding, SPOF elimination (master/slave election), rebalancing, etc.
Here's some detail on how HBase uses ZooKeeper, including information on how they intend to use it in future. Generally they use it for eliminating SPOF on the region servers via Leader election implemented using ZooKeeper.
