Nix and NixOS releases are now built in Hydra, the new Nix-based continuous build system. Hydra replaces our old Nix-based build farm, which will be phased out soon. There are several advantages over the old build farm: the build tasks for a project are scheduled and published separately, so that for instance a (fast) tarball build doesn’t have to wait for a (slow) Cygwin build; build results are stored in a database, which will enable all sorts of interesting queries; better error reporting; a better web interface; and much more. We have written a draft paper about Hydra. There are some instructions available about how to set up your own Hydra server.