Kepler is a web framework built using Lua. Sputnik runs on top of Kepler, but you don't need to know anything about it, since Sputnik uses Kepler as an "embedded" framework. Sputnik's installer (see Installation) builds its own install of Kepler and puts all Kepler and Sputnik files in one place. If you end up installing other Kepler applications, they can use their own versions of Kepler. (If this seems wasteful, consider that Kepler+Sputnik jointly take under 1 MB.

Sputnik uses Kepler 1.1. It has been tested on the Unix builds of Kepler on different servers. It should work on Kepler for Windows too, but it hasn't been tested there yet.