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* Some work on the introduction.

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Eelco Dolstra 2003-08-22 11:29:20 +00:00
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127
doc/manual/introduction.xml
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@ -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>