Comet is an active distributed key-value store built at the University of Washington. The paper describing Comet is Comet: An active distributed key-value store, there are also slides, and a MP3 of a presentation given at OSDI '10. Here's a succinct overview of Comet:
Today's cloud storage services, such as Amazon S3 or peer-to-peer DHTs, are highly inflexible and impose a variety of constraints on their clients: specific replication and consistency schemes, fixed data timeouts, limited logging, etc. We witnessed such inflexibility first-hand as part of our Vanish work, where we used a DHT to store encryption keys temporarily. To address this issue, we built Comet, an extensible storage service that allows clients to inject snippets of code that control their data's behavior inside the storage service.
From the abstract and conclusion:
This paper described Comet, an active distributed key value store. Comet enables clients to customize a distributed storage system in application-speciﬁc ways using Comet’s active storage objects. By supporting ASOs, Comet allows multiple applications with diverse requirements to share a common storage system. We implemented Comet on the Vuze DHT using a severely restricted Lua language sandbox for handler programming. Our measurements and experience demonstrate that a broad range of behaviors and customizations are possible in a safe, but active, storage environment.
Distributed key-value storage systems are widely used incorporations and across the Internet. Our research seeks to greatly expand the application space for key-value storage systems through application-speciﬁc customization. We designed and implemented Comet, an extensible, distributed key-value store. Each Comet node stores a collection of active storage objects (ASOs) that consist of a key, a value, and a set of handlers. Comet handlers run as a result of timers or storage operations, such as get or put, allowing an ASO to take dynamic, application-speciﬁc actions to customize its behavior. Handlers are written in a simple sandboxed extension language, providing properties of safety and isolation.
We implemented a Comet prototype for the Vuze DHT, deployed Comet nodes on Vuze from PlanetLab, and built and evaluated over a dozen Comet applications. Our experience demonstrates that simple, safe, and restricted extensibility can significantly increase the power and range of applications that can run on distributed active storage systems. This approach facilitates the sharing of a single storage system by applications with diverse needs, allowing them to reap the consolidation benefits inherent in today’s massive clouds.