Make pkgCache::Priority() static, it does not need the instance

[apt.git] / doc / design.dbk

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<book lang="en">

<title>The APT project design document</title>

<bookinfo>

<authorgroup>
  <author>
    <personname>Manoj Srivastava</personname><email>srivasta@debian.org</email>
  </author>
</authorgroup>

<releaseinfo>Version &apt-product-version;</releaseinfo>

<abstract>
<para>
This document is an overview of the specifications and design goals of the APT
project. It also attempts to give a broad description of the implementation
as well.
</para>
</abstract>

<copyright><year>1997</year><holder>Manoj Srivastava</holder></copyright>

<legalnotice>
<title>License Notice</title>
<para>
APT, including this document, is free software; you may redistribute it and/or
modify it under the terms of the GNU General Public License as published by the
Free Software Foundation; either version 2, or (at your option) any later
version.
</para>
<para>
This is distributed in the hope that it will be useful, but <emphasis>without
any warranty</emphasis>; without even the implied warranty of merchantability
or fitness for a particular purpose. See the GNU General Public License for
more details.
</para>
<para>
You should have received a copy of the GNU General Public License with your
Debian system, in <literal>/usr/share/common-licenses/GPL</literal>, or with
the <command>debiandoc-sgml</command> source package as the file
<literal>COPYING</literal>. If not, write to the Free Software Foundation,
Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
</para>
</legalnotice>

</bookinfo>

<chapter id="introduction"><title>Introduction</title>
<para>
APT is supposed to be a replacement for dselect, and not a replacement for
dpkg. However, since addition functionality has been required for APT, and
given the fact that this is very closely related to dpkg, it is not
unreasonable to expect that additional functionality in the underlying dpkg
would also be requested.
</para>
<para>
Deity/dselect are the first introduction that people have to Debian, and
unfortunately this first impression contributes greatly to the public
perception of the distribution. It is imperative that this be a showcase for
Debian, rather than frighten novices away (which has been an accusation often
levelled at the current system)
</para>
</chapter>

<chapter id="ch2"><title>Requirements</title>
<orderedlist numeration="arabic">
<listitem>
<para>
APT should be a replacement for dselect. Therefore it should have all the
functionality that dselect has currently. This is the primary means of
interaction between the user and the package management system, and it should
be able to handle all tasks involved in installing, upgrading, and routine
management without having the users take recourse to the underlying management
system.
</para>
</listitem>
<listitem>
<para>
It should be easier to use and less confusing for novice users. The primary
stimulus for the creation of APT was the perceived intractability, complexity,
and non-intuitive behavior of the existing user interface, and as such, human
factors must be a primary mandate of APT.
</para>
</listitem>
<listitem>
<para>
It should be able to group packages more flexibly, and possibly allow
operations based on a group. One should be able to select, or deselect,
a coherent group of related packages simultaneously, allowing one to add,
remove, or upgrade functionality to a machine as one step.
</para>
</listitem>
<listitem>
<para>
This would allow APT to handle <emphasis>standard installations</emphasis>,
namely, one could then install a set of packages to enable a machine to
fulfill specific tasks. Define a few standard installations, and which
packages are included therein. The packages should be internally consistent.
</para>
</listitem>
<listitem>
<para>
Make use of a keywords field in package headers; provide a standard list of
keywords for people to use. This could be the underpinning to allow the
previous two requirements to work (though the developers are not constrained
to implement the previous requirements using keywords)
</para>
</listitem>
<listitem>
<para>
Use dependencies, conflicts, and reverse dependencies to properly order
packages for installation and removal. This has been a complaint in the past
that the installation methods do not really understand dependencies, causing
the upgrade process to break, or allowing the removal of packages that left the
system in an untenable state by breaking the dependencies on packages that were
dependent on the package being removed. A special emphasis is placed on
handling pre-dependencies correctly; the target of a predependency has to be
fully configured before attempting to install the pre-dependent package. Also,
<emphasis>configure immediately</emphasis> requests mentioned below should be
handled.
</para>
</listitem>
<listitem>
<para>
Handle replacement of a package providing a virtual package with another (for
example, it has been very difficult replacing <command>sendmail</command> with
<command>smail</command>, or vice versa), making sure that the dependencies are
still satisfied.
</para>
</listitem>
<listitem>
<para>
Handle source lists for updates from multiple sources. APT should also be able
to handle diverse methods of acquiring new packages; local filesystem,
mountable CD-ROM drives, FTP accessible repositories are some of the methods
that come to mind. Also, the source lists can be separated into categories,
such as main, contrib, non-us, non-local, non-free, my-very-own, etc. APT
should be set up to retrieve the Packages files from these multiple source
lists, as well as retrieving the packages themselves.
</para>
</listitem>
<listitem>
<para>
Handle base of source and acquire all Packages files underneath. (possibly
select based on architecture), this should be a simple extension of the
previous requirement.
</para>
</listitem>
<listitem>
<para>
Handle remote installation (to be implemented maybe in a future version, it
still needs to be designed). This would ease the burden of maintaining
multiple Debian machines on a site. In the authors opinion this is a killer
difference for the distribution, though it may be too hard a problem to be
implemented with the initial version of APT. However, some thought must be
given to this to enable APT to retain hooks for future functionality, or at
least to refrain from methods that may preclude remote activity. It is
desirable that adding remote installation not require a redesign of APT from
the ground up.
</para>
</listitem>
<listitem>
<para>
Be scalable. Dselect worked a lot better with 400 packages, but at last count
the number of packages was around twelve hundred and climbing. This also
requires APT to pay attention to the needs of small machines which are low on
memory (though this requirement shall diminish as we move towards bigger
machines, it would still be nice if Debian worked on all old machines where
Linux itself would work).
</para>
</listitem>
<listitem>
<para>
Handle install immediately requests. Some packages, like watchdog, are
required to be working for the stability of the machine itself. There are
others which may be required for the correct functioning of a production
machine, or which are mission critical applications. APT should, in these
cases, upgrade the packages with minimal downtime; allowing these packages to
be one of potentially hundreds of packages being upgraded concurrently may
not satisfy the requirements of the package or the site. (Watchdog, for
example, if not restarted quickly, may cause the machine to reboot in the
midst of installation, which may cause havoc on the machine)
</para>
</listitem>
</orderedlist>
</chapter>

<chapter id="ch3"><title>Procedural description</title>
<variablelist>
<varlistentry>
<term>Set Options</term>
<listitem>
<para>
This process handles setting of user or site options, and configuration of all
aspects of APT. It allows the user to set the location and order of package
sources, allowing them to set up source list details, like ftp site locations,
passwords, etc. Display options may also be set.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Updates</term>
<listitem>
<para>
Build a list of available packages, using source lists or a base location and
trawling for Packages files (needs to be aware of architecture). This may
involve finding and retrieving Packages files, storing them locally for
efficiency, and parsing the data for later use. This would entail contacting
various underlying access modules (ftp, cdrom mounts, etc) Use a backing store
for speed. This may also require downloading the actual package files locally
for speed.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Local status</term>
<listitem>
<para>
Build up a list of packages already installed. This requires reading and
writing the local??  status file. For remote installation, this should
probably use similar mechanisms as the Packages file retrieval does. Use
the backing store for speed. One should consider multiple backing stores,
one for each machine.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Relationship determination</term>
<listitem>
<para>
Determine forward and reverse dependencies. All known dependency fields should
be acted upon, since it is fairly cheap to do so. Update the backing store
with this information.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Selection</term>
<listitem>
<para>
Present the data to the user. Look at Behan Webster's documentation for the
user interface procedures. (Note: In the authors opinion deletions and reverse
dependencies should also be presented to the user, in a strictly symmetric
fashion; this may make it easier to prevent a package being removed that breaks
dependencies)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Ordering of package installations and configuration</term>
<listitem>
<para>
Build a list of events. Simple topological sorting gives order of packages
in dependency order. At certain points in this ordering,
predependencies/immediate configure directives cause an break in normal
ordering. We need to insert the uninstall/purge directive in the stream
(default: as early as possible).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Action</term>
<listitem>
<para>
Take the order of installations and removals and build up a stream of events
to send to the packaging system (dpkg). Execute the list of events if
successful. Do not partially install packages and leave system in broken
state. Go to The Selection step as needed.
</para>
</listitem>
</varlistentry>
</variablelist>
</chapter>

<chapter id="ch4"><title>Modules and interfaces</title>
<variablelist>
<varlistentry>
<term>The user interface module</term>
<listitem>
<para>
Look at Behan Webster's documentation.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Widget set</term>
<listitem>
<para>
Related closely to above Could some one present design decisions of the widget
set here?
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>pdate Module</term>
<listitem>
<para>
Distinct versions of the same package are recorded separately, but if multiple
Packages files contain the same version of a package, then only the first one
is recorded. For this reason, the least expensive update source should be
listed first (local file system is better than a remote ftp site)
</para>
<para>
This module should interact with the user interface module to set and change
configuration parameters for the modules listed below. It needs to record that
information in an on disk data file, to be read on future invocations.
</para>
<orderedlist numeration="arabic">
<listitem>
<para>
FTP methods
</para>
</listitem>
<listitem>
<para>
mount and file traversal module(s)?
</para>
</listitem>
<listitem>
<para>
Other methods ???
</para>
</listitem>
</orderedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>Status file parser/generator</term>
<listitem>
<para>
The status file records the current state of the system, listing the packages
installed, etc. The status file is also one method of communicating with dpkg,
since it is perfectly permissible for the user to use APT to request packages
be updated, put others on hold, mark other for removal, etc, and then run
<literal>dpkg -BORGiE</literal> on a file system.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Package file parser/generator</term>
<listitem>
<para>
Related to above. Handle multiple Packages files, from different
sources. Each package contains a link back to the packages file structure
that contains details about the origin of the data.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Dependency module</term>
<listitem>
<itemizedlist>
<listitem>
<para>
dependency/conflict determination and linking
</para>
</listitem>
<listitem>
<para>
reverse dependency generator. Maybe merged with above
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>Package ordering Module</term>
<listitem>
<para>
Create an ordering of the actions to be taken.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Event generator</term>
<listitem>
<para>
module to interact with dpkg
</para>
</listitem>
</varlistentry>
</variablelist>
</chapter>

<chapter id="ch5"><title>Data flow and conversions analysis.</title>
<screen>
                                                          ____________
                                                       __\|ftp modules|
                                                      /  /|___________|
                                    _ ____________   /     ________________
                                    |    update   | /     |mount/local file|
        |==========================&gt;|   module    |/_____\|  traversals    |
        |                           |_____________|      /|________________|
        |                             ^        ^
        |                             |        |               ______________
  ______|_______    _ _____ ______    |   _____v________      \|            |
 |Configuration |   |configuration|   |   |Packages Files|  ===|Status file |
 |  module      |&lt;=&gt;|    data     |   |   |______________| /  /|____________|
 |______________|   |_____________|   |        ^          /
         ^                            |        |         /
         |                            | _______v_______|/_
         |                            | |              |    ________________
         |                            | |              |/_\|   Dependency  |
         |                            | |backing store |\ /|     Module    |
         |                            | |______________|  _|_______________|
         |                             \       ^          /|       ^
         |                              \      |         /         |
         |                              _\|____v_______|/__    ____v_______
         |_____________________________\| User interaction|    |    dpkg   |
                                       /|_________________|&lt;==&gt;  Invoker  |
                                                               |___________|
</screen>
<para>
dpkg also interacts with status and available files.
</para>
<para>
The backing store and the associated data structures are the core of APT. All
modules essentially revolve around the backing store, feeding it data, adding
and manipulating links and relationships between data in the backing store,
allowing the user to interact with and modify the data in the backing store,
and finally writing it out as the status file and possibly issuing directives
to dpkg.
</para>
<para>
The other focal point for APT is the user interface.
</para>
</chapter>

</book>