Kepler is a web framework built using Lua. Sputnik runs on top of Kepler, but you might not have to deal with that. Sputnik's installer (see Installation) builds its own install of Kepler.

Spunik uses Kepler 1.1, which have not been officially released yet but has been quite stable on Unix since early 2007. Sputnik is expected to run on Windows whenever Kepler for Windows becomes available.