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