Paper

Comet - An Example of the New Key-Code Databases

High Scalability

Jan 27, 2011 — 2 min read

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.

I found this paper quite interesting because it takes the initial steps of collocating code with a key-value store, which turns it into what might called a key-code store. This is something I've been exploring as a way of moving behavior to data in order to overcome network limitations in the cloud and provide other benefits. An innovator in this area is the Alchemy Database, which has already combined Redis and Lua. A good platform for this sort of thing might be Node.js integrated with V8. This would allow complex Javascript programs to run in an efficient evented container. There are a lot of implications of this sort of architecture, more about that later, but the Comet paper describes a very interesting start.

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.