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* Improved introduction (actually copied mostly from the homepage).

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<chapter xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink">
xmlns:xlink="http://www.w3.org/1999/xlink"
xml:id="chap-introduction">
<title>Introduction</title>
<para>Nix is a system for the deployment of software. Software
deployment is concerned with the creation, distribution, and
management of software components (<quote>packages</quote>). Its main
features are:
<itemizedlist>
<section><title>About Nix</title>
<listitem><para>It helps you make sure that dependency specifications
are complete. In general in a deployment system you have to specify
for each component what its dependencies are, but there are no
guarantees that this specification is complete. If you forget a
dependency, then the component will build and work correctly on
<emphasis>your</emphasis> machine if you have the dependency
installed, but not on the end user's machine if it's not
there.</para></listitem>
<para>Nix is a <emphasis>purely functional package manager</emphasis>.
This means that it treats packages like values in purely functional
programming languages such as Haskell — they are built by functions
that don’t have side-effects, and they never change after they have
been built. Nix stores packages in the <emphasis>Nix
store</emphasis>, usually the directory
<filename>/nix/store</filename>, where each package has its own unique
subdirectory such as
<listitem><para>It is possible to have <emphasis>multiple versions or
variants</emphasis> of a component installed at the same time. In
contrast, in systems such as RPM different versions of the same
package tend to install to the same location in the file system, so
installing one version will remove the other. This is especially
important if you want to use applications that have conflicting
requirements on different versions of a component (e.g., application A
requires version 1.0 of library X, while application B requires a
non-backwards compatible version 1.1).</para></listitem>
<programlisting>
/nix/store/r8vvq9kq18pz08v249h8my6r9vs7s0n3-firefox-2.0.0.1/
</programlisting>
<listitem><para>Users can have different <quote>views</quote>
(<quote>profiles</quote> in Nix parlance) on the set of installed
applications in a system. For instance, one user can have version 1.0
of some package visible, while another is using version 1.1, and a
third doesn't use it at all.</para></listitem>
where <literal>r8vvq9kq…</literal> is a unique identifier for the
package that captures all its dependencies (it’s a cryptographic hash
of the package’s build dependency graph). This enables many powerful
features.</para>
<listitem><para>It is possible to atomically
<emphasis>upgrade</emphasis> software. I.e., there is no time window
during an upgrade in which part of the old version and part of the new
version are simultaneously visible (which might well cause the
component to fail).</para></listitem>
<listitem><para>Likewise, it is possible to atomically roll back after
an install, upgrade, or uninstall action. That is, in a fast (O(1))
operation the previous configuration of the system can be restored.
This is because upgrade or uninstall actions don't actually remove
components from the system.</para></listitem>
<simplesect><title>Multiple versions</title>
<listitem><para>Unused components can be
<emphasis>garbage-collected</emphasis> automatically and safely: when
you remove an application from a profile, its dependencies will be
deleted by the garbage collector only if there are no other active
applications using them.</para></listitem>
<para>You can have multiple versions or variants of a package
installed at the same time. This is especially important when
different applications have dependencies on different versions of the
same package — it prevents the “DLL hell”. Because of the hashing
scheme, different versions of a package end up in different paths in
the Nix store, so they don’t interfere with each other.</para>
<listitem><para>Nix supports both source-based deployment models
(where you distribute <emphasis>Nix expressions</emphasis> that tell
Nix how to build software from source) and binary-based deployment
models. The latter is more-or-less transparent: installation of
components is always based on Nix expressions, but if the expressions
have been built before and Nix knows that the resulting binaries are
available somewhere, it will use those instead.</para></listitem>
<para>An important consequence is that operations like upgrading or
uninstalling an application cannot break other applications, since
these operations never “destructively” update or delete files that are
used by other packages.</para>
<listitem><para>Nix is flexible in the deployment policies that it
supports. There is a clear separation between the tools that
implement basic Nix <emphasis>mechanisms</emphasis> (e.g., building
Nix expressions), and the tools that implement various deployment
<emphasis>policies</emphasis>. For instance, there is a concept of
<quote>Nix channels</quote> that can be used to keep software
installations up-to-date automatically from a network source. This is
a policy that is implemented by a fairly short Perl script, which can
be adapted easily to achieve similar policies.</para></listitem>
</simplesect>
<listitem><para>Nix component builds aim to be <quote>pure</quote>;
that is, unaffected by anything other than the declared dependencies.
This means that if a component was built successfully once, it can be
rebuilt again on another machine and the result will be the same. We
cannot <emphasis>guarantee</emphasis> this (e.g., if the build depends
on the time-of-day), but Nix (and the tools in the Nix Packages
collection) takes special care to help achieve this.</para></listitem>
<listitem><para>Nix expressions (the things that tell Nix how to build
components) are self-contained: they describe not just components but
complete compositions. In other words, Nix expressions also describe
how to build all the dependencies. This is in contrast to component
specification languages like RPM spec files, which might say that a
component X depends on some other component Y, but since it does not
describe <emphasis>exactly</emphasis> what Y is, the result of
building or running X might be different on different machines.
Combined with purity, self-containedness ensures that a component that
<quote>works</quote> on one machine also works on another, when
deployed using Nix.</para></listitem>
<simplesect><title>Complete dependencies</title>
<listitem><para>The Nix expression language makes it easy to describe
variability in components (e.g., optional features or
dependencies).</para></listitem>
<para>Nix helps you make sure that package dependency specifications
are complete. In general, when you’re making a package for a package
management system like RPM, you have to specify for each package what
its dependencies are, but there are no guarantees that this
specification is complete. If you forget a dependency, then the
component will build and work correctly on <emphasis>your</emphasis>
machine if you have the dependency installed, but not on the end
user's machine if it's not there.</para>
<listitem><para>Nix is ideal for building build farms that do
continuous builds of software from a version management system, since
it can take care of building all the dependencies as well. Also, Nix
only rebuilds components that have changed, so there are no
unnecessary builds. In addition, Nix can transparently distribute
build jobs over different machines, including different
platforms.</para></listitem>
<para>Since Nix on the other hand doesn’t install packages in “global”
locations like <filename>/usr/bin</filename> but in package-specific
directories, the risk of incomplete dependencies is greatly reduced.
This is because tools such as compilers don’t search in per-packages
directories such as
<filename>/nix/store/5lbfaxb722zp…-openssl-0.9.8d/include</filename>,
so if a package builds correctly on your system, this is because you
specified the dependency explicitly.</para>
<listitem><para>Nix can be used not only for software deployment, but
also for <emphasis>service deployment</emphasis>, such as the
deployment of a complete web server with all its configuration files,
static pages, software dependencies, and so on. Nix's advantages for
software deployment also apply here: for instance, the ability
trivially to have multiple configurations at the same time, or the
ability to do rollbacks.</para></listitem>
<para>Runtime dependencies are found by scanning binaries for the hash
parts of Nix store paths (such as <literal>r8vvq9kq…</literal>). This
sounds risky, but it works extremely well.</para>
<listitem><para>Nix can efficiently upgrade between different versions
of a component through <emphasis>binary patching</emphasis>. If
patches are available on a server, and you try to install a new
version of some component, Nix will automatically apply a patch (or
sequence of patches), if available, to transform the installed
component into the new version.</para></listitem>
</simplesect>
</itemizedlist>
</para>
<simplesect><title>Multi-user support</title>
<para>Starting at version 0.11, Nix has multi-user support. This
means that non-privileged users can securely install software. Each
user can have a different <emphasis>profile</emphasis>, a set of
packages in the Nix store that appear in the user’s
<envar>PATH</envar>. If a user installs a package that another user
has already installed previously, the package won’t be built or
downloaded a second time. At the same time, it is not possible for
one user to inject a Trojan horse into a package that might be used by
another user.</para>
<!--
<para>More details can be found in Section 3 of our <a
href="docs/papers.html#securesharing">ASE 2005 paper</a>.</para>
-->
</simplesect>
<simplesect><title>Atomic upgrades and rollbacks</title>
<para>Since package management operations never overwrite packages in
the Nix store but just add new versions in different paths, they are
<emphasis>atomic</emphasis>. So during a package upgrade, there is no
time window in which the package has some files from the old version
and some files from the new version — which would be bad because a
program might well crash if it’s started during that period.</para>
<para>And since package aren’t overwritten, the old versions are still
there after an upgrade. This means that you can <emphasis>roll
back</emphasis> to the old version:</para>
<screen>
$ nix-env --upgrade <replaceable>some-packages</replaceable>
$ nix-env --rollback
</screen>
</simplesect>
<simplesect><title>Garbage collection</title>
<para>When you install a package like this…
<screen>
$ nix-env --uninstall firefox
</screen>
the package isn’t deleted from the system right away (after all, you
might want to do a rollback, or it might be in the profiles of other
users). Instead, unused packages can be deleted safely by running the
<emphasis>garbage collector</emphasis>:
<screen>
$ nix-collect-garbage
</screen>
This deletes all packages that aren’t in use by any user profile or by
a currently running program.</para>
</simplesect>
<simplesect><title>Functional package language</title>
<para>Packages are built from <emphasis>Nix expressions</emphasis>,
which is a simple functional language. A Nix expression describes
everything that goes into a package build action (a “derivation”):
other packages, sources, the build script, environment variables for
the build script, etc. Nix tries very hard to ensure that Nix
expressions are <emphasis>deterministic</emphasis>: building a Nix
expression twice should yield the same result.</para>
<para>Because it’s a functional language, it’s easy to support
building variants of a package: turn the Nix expression into a
function and call it any number of times with the appropriate
arguments. Due to the hashing scheme, variants don’t conflict with
each other in the Nix store.</para>
</simplesect>
<simplesect><title>Transparent source/binary deployment</title>
<para>Nix expressions generally describe how to build a package from
source, so an installation action like
<screen>
$ nix-env --install firefox
</screen>
<emphasis>could</emphasis> cause quite a bit of build activity, as not
only Firefox but also all its dependencies (all the way up to the C
library and the compiler) would have to built, at least if they are
not already in the Nix store. This is a <emphasis>source deployment
model</emphasis>. For most users, building from source is not very
pleasant as it takes far too long. However, Nix can automatically
skip building from source and download a pre-built binary instead if
it knows about it. <emphasis>Nix channels</emphasis> provide Nix
expressions along with pre-built binaries.</para>
<!--
<para>source deployment model (like <a
href="http://www.gentoo.org/">Gentoo</a>) and a binary model (like
RPM)</para>
-->
</simplesect>
<simplesect><title>Binary patching</title>
<para>In addition to downloading binaries automatically if they’re
available, Nix can download binary deltas that patch an existing
package in the Nix store into a new version. This speeds up
upgrades.</para>
</simplesect>
<simplesect><title>Nix Packages collection</title>
<para>We provide a large set of Nix expressions containing hundreds of
existing Unix packages, the <emphasis>Nix Packages
collection</emphasis> (Nixpkgs).</para>
</simplesect>
<simplesect><title>Service deployment</title>
<para>Nix can be used not only for rolling out packages, but also
complete <emphasis>configurations</emphasis> of services. This is
done by treating all the static bits of a service (such as software
packages, configuration files, control scripts, static web pages,
etc.) as “packages” that can be built by Nix expressions. As a
result, all the features above apply to services as well: for
instance, you can roll back a web server configuration if a
configuration change turns out to be undesirable, you can easily have
multiple instances of a service (e.g., a test and production server),
and because the whole service is built in a purely functional way from
a Nix expression, it is repeatable so you can easily reproduce the
service on another machine.</para>
<!--
<para>You can read more about this in our <a
href="docs/papers.html#servicecm">SCM-12 paper</a>.</para>
-->
</simplesect>
<simplesect><title>Portability</title>
<para>Nix should run on most Unix systems, including Linux, FreeBSD and
Mac OS X. It is also supported on Windows using Cygwin.</para>
</simplesect>
<simplesect><title>NixOS</title>
<para>NixOS is a Linux distribution based on Nix. It uses Nix not
just for package management but also to manage the system
configuration (e.g., to build configuration files in
<filename>/etc</filename>). This means, among other things, that it’s
possible to easily roll back the entire configuration of the system to
an earlier state. Also, users can install software without root
privileges. For more information and downloads, see the <link
xlink:href="http://nix.cs.uu.nl/nixos/">NixOS homepage</link>.</para>
</simplesect>
<!-- other features:
- build farms
- reproducibility (Nix expressions allows whole configuration to be rebuilt)
-->
</section>
<section><title>About us</title>
<para>Nix was developed at the <link
xlink:href="http://www.cs.uu.nl/">Department of Information and
Computing Sciences</link>, Utrecht University by the <link
xlink:href="http://www.cs.uu.nl/wiki/Trace/WebHome">TraCE
project</link>. The project is funded by the Software Engineering
Research Program <link
xlink:href="http://www.jacquard.nl/">Jacquard</link> to improve the
support for variability in software systems.</para>
</section>
<section><title>About this manual</title>
<para>This manual tells you how to install and use Nix and how to
write Nix expressions for software not already in the Nix Packages
collection. It also discusses some advanced topics, such as setting
up a Nix-based build farm, and doing service deployment using
Nix.</para>
up a Nix-based build farm.</para>
<note><para>Some background information on Nix can be found in a
number of papers. The ICSE 2004 paper <citetitle
</section>
<section><title>License</title>
<para>Nix is free software; you can redistribute it and/or modify it
under the terms of the <link
xlink:href="http://www.gnu.org/licenses/lgpl.html">GNU Lesser General
Public License</link> as published by the <link
xlink:href="http://www.fsf.org/">Free Software Foundation</link>;
either version 2.1 of the License, or (at your option) any later
version. Nix is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.</para>
</section>
<section><title>More information</title>
<para>Some background information on Nix can be found in a number of
papers. The ICSE 2004 paper <citetitle
xlink:href='http://www.cs.uu.nl/~eelco/pubs/immdsd-icse2004-final.pdf'>Imposing
a Memory Management Discipline on Software Deployment</citetitle>
discusses the hashing mechanism used to ensure reliable dependency
@ -141,10 +310,27 @@ gives a more general discussion of Nix from a system-administration
perspective. The CBSE 2005 paper <citetitle
xlink:href='http://www.cs.uu.nl/~eelco/pubs/eupfcdm-cbse2005-final.pdf'>Efficient
Upgrading in a Purely Functional Component Deployment Model
</citetitle> is about transparent patch deployment in Nix. Finally,
the SCM-12 paper <citetitle
</citetitle> is about transparent patch deployment in Nix. The SCM-12
paper <citetitle
xlink:href='http://www.cs.uu.nl/~eelco/pubs/servicecm-scm12-final.pdf'>
Service Configuration Management</citetitle> shows how services (e.g.,
web servers) can be deployed and managed through Nix.</para></note>
web servers) can be deployed and managed through Nix. A short
overview of NixOS is given in the HotOS XI paper <citetitle
xlink:href="http://www.cs.uu.nl/~eelco/pubs/hotos-final.pdf">Purely
Functional System Configuration Management</citetitle>. The Nix
homepage has <link
xlink:href="http://nix.cs.uu.nl/docs/papers.html">an up-to-date list
of Nix-related papers</link>.</para>
<para>Nix is the subject of Eelco Dolstra’s PhD thesis <citetitle
xlink:href="http://igitur-archive.library.uu.nl/dissertations/2006-0118-200031/index.htm">The
Purely Functional Software Deployment Model</citetitle>, which
contains most of the papers listed above.</para>
<para>Nix has a homepage at <link
xlink:href="http://nix.cs.uu.nl/"/>.</para>
</section>
</chapter>