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<chapter id='dependency-resolution-task-scheduling'>
<title>Task Scheduling</title>
<sect1 id='dependency-resolution-task-scheduling-dependencies'>
<title>Task Dependencies</title>
<para>
All tasks are executed in an order such
that a task's dependencies are satisfied
when it is executed. Dependency relationships between tasks
form a directed graph.
</para>
</sect1>
<sect1 id='dependency-resolution-task-scheduling-conflict-avoidance'>
<title>Conflict Avoidance</title>
<para>
Sometimes a package installation order exists such that it is
possible to avoid having two conflicting packages installed
simultaneously. If a currently installed package conflicts with a
new package that is planned to be installed, it may be possible to
solve the conflict by replacing the installed package with a
different package that occupies the same slot.
</para>
<para>
In order to avoid a conflict, a package may need to be uninstalled
rather than replaced. The following constraints
protect inappropriate packages from being chosen for automatic
uninstallation:
</para>
<itemizedlist>
<listitem>
<para>Installed packages that have been pulled into the current dependency
graph will not be uninstalled. Due to
<link linkend='dependency-resolution-package-modeling-dependency-neglection'>
dependency neglection</link> and special properties of packages
in the "system" set, other checks may be necessary in order
to protect inappropriate packages from being uninstalled.</para>
</listitem>
<listitem>
<para>An installed package that is matched by a dependency atom from the
"system" set will not be uninstalled in advance since it might not
be safe. Such a package will only be uninstalled through replacement.</para>
</listitem>
<listitem>
<para>An installed package that is matched by a dependency atom from the
"world" set will not be uninstalled if the dependency graph does not
contain a replacement package that is matched by the same dependency
atom.</para>
</listitem>
</itemizedlist>
<para>
In order to ensure that package files remain installed in a usable state
whenever possible, uninstallation operations are not executed
until after all associated conflicting packages have been installed.
When file collisions occur between conflicting packages, the contents
entries for those files are removed from the packages
that are scheduled for uninstallation. This prevents
uninstallation operations from removing overlapping files that
have been claimed by conflicting packages.
</para>
</sect1>
<sect1 id='dependency-resolution-task-scheduling-circular-dependencies'>
<title>Circular Dependencies</title>
<para>
TODO: Automatically solve circular dependencies by temporarily disabling
conditional dependencies and then rebuilding packages with the conditional
dependencies enabled.
</para>
</sect1>
<sect1 id='dependency-resolution-task-scheduling-parallel'>
<title>Parallel Scheduling</title>
<para>
The algorithm used to choose packages that will execute concurrently
with other packages is as conservative as possible in the sense that
a given package will not be executed if the subgraph composed of its
direct and indirect dependencies contains any scheduled merges. By
ensuring that the subgraph of deep dependencies is fully up to date
in this way, potential problems are avoided which could be triggered
by other build orders that are less optimal.
</para>
</sect1>
</chapter>
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