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<chapter id='dependency-resolution-package-modeling'>
<title>Package Modeling</title>
<sect1 id='dependency-resolution-package-modeling-constraint-satisfaction'>
<title>Constraint Satisfaction</title>
<sect2 id='dedependency-resolution-package-modeling-constraint-satisfaction-constraint-types'>
<title>Constraint Types</title>
<para>
Dependency resolution involves satisfaction of
many constraints:
</para>
<itemizedlist>
<listitem>
<para>Persistent configuration parameters, like those that come from
make.profile, make.conf, and the /etc/portage directory.</para>
</listitem>
<listitem>
<para>Current command parameters, which may include options, atoms, or sets.</para>
</listitem>
<listitem>
<para><link linkend='dependency-resolution-package-modeling-constraint-satisfaction-package-dependencies'>
Package Dependencies</link></para>
</listitem>
</itemizedlist>
</sect2>
<sect2 id='dependency-resolution-package-modeling-constraint-satisfaction-package-dependencies'>
<title>Package Dependencies</title>
<para>
Common types of package dependencies:
</para>
<itemizedlist>
<listitem>
<para>Files required for building or installing. Downloads may
be necessary to satisfy these.</para>
</listitem>
<listitem>
<para>Other packages required to be installed for
buildtime or runtime.</para>
</listitem>
<listitem>
<para>Blockers that prevent conflicting packages from being installed
simultaneously.</para>
</listitem>
</itemizedlist>
</sect2>
</sect1>
<sect1 id='dependency-resolution-package-modeling-conflicts'>
<title>Conflicts</title>
<sect2 id='dependency-resolution-package-modeling-blocker-conflicts'>
<title>Blocker Conflicts</title>
<para>
If one package blocks another package, the two packages
conflict such that they cannot be installed simultaneously.
These conflicts are often due to file collisions. In some
cases, packages that block each other can be temporarily
installed simultaneously. In order to resolve file collisions
that occur between two blocking packages that are installed
simultaneously, the overlapping files must be removed from
the contents list of the package which was installed first.
</para>
<para>
Some cases may exist such that temporary simultaneous installation
of blocking packages will cause some sort of problem. However,
this type of solution will only be chosen for blockers that can not
be satisfied in any other way, such as by <link
linkend='dependency-resolution-task-scheduling-conflict-avoidance'>
simple adjustment of merge order.</link> In addition, this type of
solution will not be chosen if a blocking package will overwrite
files belonging to packages from the system set, or packages that are
runtime dependencies of Portage itself. These constraints serve to
limit the probability that a chosen solution will cause an unforeseen
problem.
</para>
</sect2>
<sect2 id='dependency-resolution-package-modeling-slot-conflicts'>
<title>Slot Conflicts</title>
<para>
If two different packages that occupy the same slot are chosen
to satisfy dependencies, a slot conflict occurs. The two packages
cannot be installed simultaneously and therefore the respective
dependencies will not be satisfied simultaneously.
</para>
</sect2>
<sect2 id='dependency-resolution-package-modeling-indirect-conflicts'>
<title>Indirect Conflicts</title>
<para>
If the dependencies of two parent packages cannot be installed
simultaneously, it creates an indirect conflict between the parent
packages since their respective dependencies cannot be satisfied
simultaneously.
</para>
</sect2>
</sect1>
<sect1 id='dependency-resolution-package-modeling-dependency-neglection'>
<title>Dependency Neglection</title>
<para>
In order to significantly reduce the resources consumed by
the modeling process, the dependencies of
installed packages may be neglected.
</para>
<para>
If a more complete dependency calculation is desired,
there is a --complete-graph option which will ensure that the
dependencies of installed packages are properly considered.
</para>
</sect1>
</chapter>
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