144 lines
7 KiB
XML
144 lines
7 KiB
XML
<chapter><title>Introduction</title>
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<epigraph><para><quote>The number of Nix installations in the world
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has grown to 5, with more expected.</quote></para></epigraph>
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<para>Nix is a system for the deployment of software. Software
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deployment is concerned with the creation, distribution, and
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management of software components (<quote>packages</quote>). Its main
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features are:
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<itemizedlist>
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<listitem><para>It makes sure that dependency specifications are
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complete. In general in a deployment system you have to specify for
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each component what its dependencies are, but there are no guarantees
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that this specification is complete. If you forget a dependency, then
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the component will build and work correctly on
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<emphasis>your</emphasis> machine if you have the dependency
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installed, but not on the end user's machine if it's not
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there.</para></listitem>
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<listitem><para>It is possible to have <emphasis>multiple versions or
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variants</emphasis> of a component installed at the same time. In
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contrast, in systems such as RPM different versions of the same
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package tend to install to the same location in the file system, so
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you installing one version will remove the other. This is especially
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important if you want to have use applications that have conflicting
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requirements on different versions of a component (e.g., application A
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requires version 1.0 of library X, while application B requires a
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non-backwards compatible version 1.1).</para></listitem>
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<listitem><para>Users can have different <quote>views</quote>
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(<quote>profiles</quote> in Nix parlance) on the set of installed
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applications in a system. For instance, one user can have version 1.0
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of some package visible, while another is using version 1.1, and a
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third doesn't use it at all.</para></listitem>
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<listitem><para>It is possible to atomically
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<emphasis>upgrade</emphasis> software. I.e., there is no time window
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during an upgrade in which part of the old version and part of the new
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version are simultaneously visible (which might well cause the
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component to fail).</para></listitem>
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<listitem><para>Likewise, it is possible to atomically roll-back after
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an install, upgrade, or uninstall action. That is, in a fast (O(1))
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operation the previous configuration of the system will be
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restored. This is because upgrade or uninstall actions doesn't
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actually remove components from the system.</para></listitem>
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<listitem><para>Unused components can be
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<emphasis>garbage-collected</emphasis> automatically and safely.
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I.e., when you remove an application from a profile, its dependencies
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will be deleted by the garbage collector if there are no other active
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applications that are using it.</para></listitem>
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<listitem><para>Nix supports both source-based deployment models
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(where you distribute <emphasis>Nix expressions</emphasis> that tell
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Nix how to build software from source) and binary-based deployment
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models. The latter is more-or-less transparent: installation of
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components is always based on Nix expressions, but if those
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expressions have been built before and Nix knows that the resulting
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binaries are available somewhere, it will use those
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instead.</para></listitem>
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<listitem><para>Nix is flexible in the deployment policies that it
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supports. There is a clear separation between the tools that
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implement basic Nix <emphasis>mechanisms</emphasis> (e.g., building
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Nix expressions), and the tools that implement various deployment
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<emphasis>policies</emphasis>. For instance, there is a concept of
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<quote>Nix channels</quote> that can be used to keep software
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installations up-to-date automatically from a network source. This is
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a policy that is implemented by a fairly short Perl script, which can
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be adapted easily to achieve similar policies.</para></listitem>
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<listitem><para>Nix component builds aim to be <quote>pure</quote>;
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that is, unaffected by anything other than the declared dependencies.
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This means that if a component was built succesfully once, it can be
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rebuilt again on another machine and the result will be the same. We
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cannot <emphasis>guarantee</emphasis> this (e.g., if the build depends
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on the time-of-day), but Nix (and the tools in the Nix Packages
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collection) takes special measures to help achieve
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this.</para></listitem>
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<listitem><para>Nix expressions (the things that tell Nix how to build
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components) are self-contained: they describe not just components but
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complete compositions. In other words, Nix expressions also describe
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how to build all the dependencies. This is contrast to component
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specification languages like RPM spec files, which might say that a
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component X depends on some other component Y, but since it does not
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describe <emphasis>exactly</emphasis> what Y is, the result of
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building or running X might be different on different machines.
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Combined with purity, self-containedness ensures that a component that
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<quote>works</quote> on one machine also works on another, when
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deployed using Nix.</para></listitem>
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<listitem><para>The Nix expression language makes it easy to describe
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variability in components (e.g., optional features or
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dependencies).</para></listitem>
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<listitem><para>Nix is ideal for building build farms that do
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continuous builds of software from a version management system, since
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it can take care of building all the dependencies as well. Also, Nix
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only rebuilds components that have changed, so there are no
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unnecessary builds. In addition, Nix can transparently distribute
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build jobs over different machines, including different
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platforms.</para></listitem>
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<listitem><para>Nix can be used not only for software deployment, but
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also for <emphasis>service deployment</emphasis>, such as the
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deployment of a complete web server with all its configuration files,
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static pages, software dependencies, and so on. Nix's advantages for
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software deployment also apply here, for instance, the ability
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trivially to have multiple configurations at the same time, or the
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ability to do roll-backs.</para></listitem>
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</itemizedlist>
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</para>
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<para>This manual tells you how to install and use Nix and how to
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write Nix expressions for software not already in the Nix Packages
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collection. It also discusses some advanced topics, such as setting
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up a Nix-based build farm, and doing service deployment using
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Nix.</para>
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<warning><para>This manual is a work in progress. It's quite likely
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to be incomplete, inconsistent with the current implementation, or
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simply wrong.</para></warning>
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<note><para>Some background information on Nix can be found in two
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papers. The ICSE 2004 paper <ulink
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url='http://www.cs.uu.nl/~eelco/pubs/immdsd-icse2004-final.pdf'><citetitle>Imposing
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a Memory Management Discipline on Software
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Deployment</citetitle></ulink> discusses the hashing mechanism used to
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ensure reliable dependency identification and non-interference between
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different versions and variants of packages. The LISA 2004 paper
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<ulink
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url='http://www.cs.uu.nl/~eelco/pubs/nspfssd-lisa2004-final.pdf'><citetitle>Nix:
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A Safe and Policy-Free System for Software
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Deployment</citetitle></ulink> gives a more general discussion of Nix
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from a system-administration perspective.</para></note>
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</chapter>
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