doc/dependency_resolution/task_scheduling.docbook

<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
	in advance, rather than through replacement. The following constraints
	protect inappropriate packages from being chosen for automatic
	uninstallation:
	<itemizedlist>
	<listitem>
	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.
	</listitem>
	<listitem>
	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.
	</listitem>
	<listitem>
	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.
	</listitem>
	</itemizedlist>
	</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>
	TODO: Spawn an appropriate number of tasks in parallel when desired.
	</para>
</sect1>
</chapter>