forked from lix-project/lix
* Some work on the introduction.
This commit is contained in:
parent
956801fcc2
commit
56b98c3857
|
@ -16,15 +16,72 @@
|
||||||
|
|
||||||
<para>
|
<para>
|
||||||
Build management tools are used to perform <emphasis>software
|
Build management tools are used to perform <emphasis>software
|
||||||
builds</emphasis>, that is, the construction of derived products such
|
builds</emphasis>, that is, the construction of derived products
|
||||||
as executable programs from source code. A commonly used build tool is
|
(<emphasis>derivates)</emphasis>) such as executable programs from
|
||||||
Make, which is a standard tool on Unix systems. These tools have to
|
source code. A commonly used build tool is Make, which is a standard
|
||||||
deal with several issues:
|
tool on Unix systems. These tools have to deal with several issues:
|
||||||
<itemizedlist>
|
<itemizedlist>
|
||||||
|
|
||||||
<listitem>
|
<listitem>
|
||||||
<para>
|
<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>
|
</para>
|
||||||
</listitem>
|
</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>
|
</itemizedlist>
|
||||||
</para>
|
</para>
|
||||||
|
|
||||||
|
@ -37,8 +94,8 @@
|
||||||
After software has been built, is must also be
|
After software has been built, is must also be
|
||||||
<emphasis>deployed</emphasis> in the intended target environment, e.g.,
|
<emphasis>deployed</emphasis> in the intended target environment, e.g.,
|
||||||
the user's workstation. Examples include the Red Hat package manager
|
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
|
(RPM), Microsoft's MSI, and so on. Here also we have several issues to
|
||||||
several issues:
|
contend with:
|
||||||
<itemizedlist>
|
<itemizedlist>
|
||||||
<listitem>
|
<listitem>
|
||||||
<para>
|
<para>
|
||||||
|
@ -70,24 +127,66 @@
|
||||||
<!--######################################################################-->
|
<!--######################################################################-->
|
||||||
|
|
||||||
<sect1>
|
<sect1>
|
||||||
<title>What Nix can do for you</title>
|
<title>What Nix provides</title>
|
||||||
|
|
||||||
<para>
|
<para>
|
||||||
Here is a summary of what Nix provides:
|
Here is a summary of Nix's main features:
|
||||||
</para>
|
</para>
|
||||||
|
|
||||||
<itemizedlist>
|
<itemizedlist>
|
||||||
|
|
||||||
<listitem>
|
<listitem>
|
||||||
<para>
|
<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>
|
</para>
|
||||||
</listitem>
|
</listitem>
|
||||||
|
|
||||||
<listitem>
|
<listitem>
|
||||||
<para>
|
<para>
|
||||||
<emphasis>Support for variability.</emphasis>
|
<emphasis>Support for variability.</emphasis>
|
||||||
</para>
|
</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>
|
||||||
|
|
||||||
<listitem>
|
<listitem>
|
||||||
|
@ -120,6 +219,14 @@
|
||||||
</para>
|
</para>
|
||||||
</listitem>
|
</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>
|
</itemizedlist>
|
||||||
|
|
||||||
<para>
|
<para>
|
||||||
|
|
Loading…
Reference in a new issue