The number of Nix installations in the world has grown to 4, with more expected. Nix is a system for software deployment. It supports the creation and distribution of software packages, as well as the installation and subsequent management of these on target machines (i.e., it is also a package manager). Nix solves some large problems that exist in most current deployment and package management systems. Dependency determination is a big one: the correct installation of a software component requires that all dependencies of that component (i.e., other components used by it) are also installed. Most systems have no way to verify that the specified dependencies of a component are actually sufficient. Another big problem is the lack of support for concurrent availability of multiple variants of a component. It must be possible to have several versions of a component installed at the same time, or several instances of the same version built with different parameters. Unfortunately, components are in general not properly isolated from each other. For instance, upgrading a component that is a dependency for some other component might break the latter. Nix solves these problems by building and storing packages in paths that are infeasible to predict in advance. For example, the artifacts of a package X might be stored in /nix/store/d58a0606ed616820de291d594602665d-X, rather than in, say, /usr/lib. The path component d58a... is actually a cryptographic hash of all the inputs (i.e., sources, requisites, and build flags) used in building X, and as such is very fragile: any change to the inputs will change the hash. Therefore it is not sensible to hard-code such a path into the build scripts of a package Y that uses X (as does happen with fixed paths such as /usr/lib). Rather, the build script of package Y is parameterised with the actual location of X, which is supplied by the Nix system. As stated above, the path name of a file system object contain a cryptographic hash of all inputs involved in building it. A change to any of the inputs will cause the hash to change--and by extension, the path name. These inputs include both sources (variation in time) and configuration options (variation in space). Therefore variants of the same package don't clash---they can co-exist peacefully within the same file system. Other features: Transparent source/binary deployment. Unambiguous identification of configuration. Automatic storage management. Atomic upgrades and rollbacks. Support for many simultaneous configurations. Portability. Nix is quite portable. Contrary to build systems like those in, e.g., Vesta and ClearCase, it does not rely on operating system extensions.