The number of Nix installations in the world has grown to 5, with more expected. Nix is a system for the deployment of software. Software deployment is concerned with the creation, distribution, and management of software components (packages). There are many tools for this, but they tend to ignore some important requirements for deployment: Correctness. The basic goal of software deployment is to transfer software from one machine (e.g., the developer's, where it presumably works) to another machine (e.g., the end user's). The software should work exactly the same on the target machine as on the source machine. But this in practice turns out to be rather difficult due to dependencies between components and interference between components. If we deploy a component that depends on other components, then we should deploy those dependencies as well. If they are missing on the target system, the component probably won't work. If they are present but are not the right version, the component might not work. And if even if they are the right version, they may have been built with different flags or options, which can cause incompatibilities. Interference occurs when components collide with each other in the file system. For instance, different versions of the same package tend to overwrite each other, so they cannot be installed at the same time. But always picking the latest version might break components that only work with some older version. Variability. Many package management tools have difficulty supporting the installation of multiple versions or variants of the same component. This is bad because as ... Here are some of Nix's main features: Nix can quite reliably figure out the dependencies between components. This manual is a work in progress. It's quite likely to be incomplete, inconsistent with the current implementation, or simply wrong. Some background information on Nix can be found in two papers. The ICSE 2004 paper Imposing a Memory Management Discipline on Software Deployment discusses the hashing mechanism used to ensure reliable dependency identification and non-interference between different versions and variants of packages. The LISA 2004 paper Nix: A Safe and Policy-Free System for Software Deployment gives a more general discussion of Nix from a system-administration perspective. 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.