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| -rw-r--r-- | doc/manual/introduction.xml | 127 |
1 files changed, 117 insertions, 10 deletions
diff --git a/doc/manual/introduction.xml b/doc/manual/introduction.xml index 974cdedd8fae..5eea764592bd 100644 --- a/doc/manual/introduction.xml +++ b/doc/manual/introduction.xml @@ -16,15 +16,72 @@ <para> Build management tools are used to perform <emphasis>software - builds</emphasis>, that is, the construction of derived products such - as executable programs from source code. A commonly used build tool is - Make, which is a standard tool on Unix systems. These tools have to - deal with several issues: + builds</emphasis>, that is, the construction of derived products + (<emphasis>derivates)</emphasis>) such as executable programs from + source code. A commonly used build tool is Make, which is a standard + tool on Unix systems. These tools have to deal with several issues: <itemizedlist> + + <listitem> + <para> + <emphasis>Efficiency</emphasis>. Since building large systems + can take a substantial amount of time, it is desirable that build + steps that have been performed in the past are not repeated + unnecessarily, i.e., if a new build differs from a previous build + only with respect to certain sources, then only the build steps + that (directly or indirectly) <emphasis>depend</emphasis> on + those sources should be redone. + </para> + </listitem> + + <listitem> + <para> + <emphasis>Correctness</emphasis> is this context means that the + derivates produced by a build are always consistent with the + sources, that is, they are equal to what we would get if we were + to build the derivates from those sources. This requirement is + trivially met when we do a full, unconditional build, but is far + from trivial under the requirement of efficiency, since it is not + easy to determine which derivates are affected by a change to a + source. + </para> + </listitem> + <listitem> <para> + <emphasis>Variability</emphasis> is the property that a software + system can be built in a (potentially large) number of variants. + Variation exists both in <emphasis>time</emphasis>---the + evolution of different versions of an artifact---and in + <emphasis>space</emphasis>---the artifact might have + configuration options that lead to variants that differ in the + features they support (for example, a system might be built with + or without debugging information). + </para> + + <para> + Build managers historically have had good support for variation + in time (rebuilding the system in an intelligent way when sources + change is one of the primary reasons to use a build manager), but + not always for variation in space. For example, + <command>make</command> will not automatically ensure that + variant builds are properly isolated from each other (they will + in fact overwrite each other unless special precautions are + taken). </para> </listitem> + + <listitem> + <para> + <emphasis>High-level system modelling language</emphasis>. The + language in which one describes what and how derivates are to be + produced should have sufficient abstraction facilities to make it + easy to specify the derivation of even very large systems. Also, + the language should be <emphasis>modular</emphasis> to enable + components from possible different sources to be easily combined. + </para> + </listitem> + </itemizedlist> </para> @@ -37,8 +94,8 @@ After software has been built, is must also be <emphasis>deployed</emphasis> in the intended target environment, e.g., the user's workstation. Examples include the Red Hat package manager - (RPM), Microsoft's MSI, and so on. Here also we have to deal with - several issues: + (RPM), Microsoft's MSI, and so on. Here also we have several issues to + contend with: <itemizedlist> <listitem> <para> @@ -70,24 +127,66 @@ <!--######################################################################--> <sect1> - <title>What Nix can do for you</title> + <title>What Nix provides</title> <para> - Here is a summary of what Nix provides: + Here is a summary of Nix's main features: </para> <itemizedlist> <listitem> <para> - <emphasis>Reliable dependencies.</emphasis> + <emphasis>Reliable dependencies.</emphasis> Builds of file system + objects depend on other file system object, such as source files, + tools, and so on. We would like to ensure that a build does not + refer to any objects that have not been declared as inputs for that + build. This is important for several reasons. First, if any of the + inputs change, we need to rebuild the things that depend on them to + maintain consistency between sources and derivates. Second, when we + <emphasis>deploy</emphasis> file system objects (that is, copy them + to a different system), we want to be certain that we copy everything + that we need. + </para> + + <para> + Nix ensures this by building and storing file system objects 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> </listitem> <listitem> <para> - <emphasis>Support for variability.</emphasis> + <emphasis>Support for variability.</emphasis> </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. So thanks to Nix's mechanism for reliably + dealing with dependencies, we obtain management of variants for free + (or, to quote Simon Peyton-Jone, it's not free, but it has already + been paid for). + </para> + </listitem> <listitem> @@ -120,6 +219,14 @@ </para> </listitem> + <listitem> + <para> + <emphasis>Portability.</emphasis> Nix is quite portable. Contrary + to build systems like those in, e.g., Vesta and ClearCase [sic?], it + does not rely on operating system extensions. + </para> + </listitem> + </itemizedlist> <para> |