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<chapter>
<title>Introduction</title>
<para>
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).
</para>
<para>
Nix solves some large problems that exist in most current deployment and
package management systems. <emphasis>Dependency determination</emphasis>
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.
</para>
<para>
Another big problem is the lack of support for concurrent availability of
multiple <emphasis>variants</emphasis> 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.
</para>
<para>
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 <literal>X</literal> might be stored in
<filename>/nix/store/d58a0606ed616820de291d594602665d-X</filename>, rather
than in, say, <filename>/usr/lib</filename>. The path component
<filename>d58a...</filename> is actually a cryptographic hash of all the
inputs (i.e., sources, requisites, and build flags) used in building
<literal>X</literal>, and as such is very fragile: any change to the inputs
will change the hash. Therefore it is not sensible to
<emphasis>hard-code</emphasis> such a path into the build scripts of a
package <literal>Y</literal> that uses <literal>X</literal> (as does happen
with <quote>fixed</quote> paths such as <filename>/usr/lib</filename>).
Rather, the build script of package <literal>Y</literal> is parameterised
with the actual location of <literal>X</literal>, which is supplied by the
Nix system.
</para>
<para>
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.
</para>
<para>
Other features:
</para>
<para>
<emphasis>Transparent source/binary deployment.</emphasis>
</para>
<para>
<emphasis>Unambiguous identification of configuration.</emphasis>
</para>
<para>
<emphasis>Automatic storage management.</emphasis>
</para>
<para>
<emphasis>Atomic upgrades and rollbacks.</emphasis>
</para>
<para>
<emphasis>Support for many simultaneous configurations.</emphasis>
</para>
<para>
<emphasis>Portability.</emphasis> Nix is quite portable. Contrary to
build systems like those in, e.g., Vesta and ClearCase, it does not rely on
operating system extensions.
</para>
</chapter>
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