<chapter xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="chap-introduction"> <title>Introduction</title> <section><title>About Nix</title> <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 <programlisting> /nix/store/r8vvq9kq18pz08v249h8my6r9vs7s0n3-firefox-2.0.0.1/ </programlisting> 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> <simplesect><title>Multiple versions</title> <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> <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> </simplesect> <simplesect><title>Complete dependencies</title> <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 package 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> <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> <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> </simplesect> <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://nixos.org/">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 originally 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> (2003-2008). The project was 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. Further funding is now provided by the NIRICT LaQuSo Build Farm project.</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.</para> </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.st.ewi.tudelft.nl/~dolstra/pubs/immdsd-icse2004-final.pdf'>Imposing a Memory Management Discipline on Software Deployment</citetitle> discusses the hashing mechanism used to ensure reliable dependency identification and non-interference between different versions and variants of packages. The LISA 2004 paper <citetitle xlink:href='http://www.st.ewi.tudelft.nl/~dolstra/pubs/nspfssd-lisa2004-final.pdf'>Nix: A Safe and Policy-Free System for Software Deployment</citetitle> gives a more general discussion of Nix from a system-administration perspective. The CBSE 2005 paper <citetitle xlink:href='http://www.st.ewi.tudelft.nl/~dolstra/pubs/eupfcdm-cbse2005-final.pdf'>Efficient Upgrading in a Purely Functional Component Deployment Model </citetitle> is about transparent patch deployment in Nix. The SCM-12 paper <citetitle xlink:href='http://www.st.ewi.tudelft.nl/~dolstra/pubs/servicecm-scm12-final.pdf'> Service Configuration Management</citetitle> shows how services (e.g., 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.st.ewi.tudelft.nl/~dolstra/pubs/hotos-final.pdf">Purely Functional System Configuration Management</citetitle>. The Nix homepage has <link xlink:href="http://nixos.org/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://nixos.org/"/>.</para> </section> </chapter>