A few weeks ago Jan Lehnardt of CouchDB came by to Ibuildings UK and gave a talk about the project. CouchDB is a database that's designed to be highly scalable, in terms of, well pretty much everything really: the amount of data it can handle, and the number of CPUs the distributed server can efficiently use in parallel, the number of concurrent clients. (Note though that CouchDB sits at version 0.8.1 at the moment, and many of the scalability features either haven't been implemented, or haven't been tested.) What follows is a short description of how CouchDB works, including the unusual MapReduce-powered database query technique, as gleaned from the talk and a few days' worth of playing with it.