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diff --git a/doc/manual/introduction/about-nix.xml b/doc/manual/introduction/about-nix.xml new file mode 100644 index 000000000000..38dd7e6657a3 --- /dev/null +++ b/doc/manual/introduction/about-nix.xml @@ -0,0 +1,253 @@ +<chapter xmlns="http://docbook.org/ns/docbook" + xmlns:xlink="http://www.w3.org/1999/xlink" + xmlns:xi="http://www.w3.org/2001/XInclude" + version="5.0" + xml:id="ch-about-nix"> + +<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/nlc4z5y1hm8w9s8vm6m1f5hy962xjmp5-firefox-12.0 +</programlisting> + +where <literal>nlc4z5…</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>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 uninstall 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) + +--> + +</chapter> \ No newline at end of file |