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-<!doctype debiandoc  PUBLIC  "-//DebianDoc//DTD DebianDoc//EN">
-<debiandoc>
-  <book>
-    <titlepag>
-      <title> The APT project design document</title>
-      <author>
-	<name>Manoj Srivastava</name>
-	<email>srivasta@debian.org</email>
-      </author>
-      <version>$Id: design.sgml,v 1.4 2003/02/12 15:05:45 doogie Exp $</version>
-      <abstract>
-	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.
-      </abstract>
-      <copyright>
-	<copyrightsummary>Copyright &copy;1997 Manoj Srivastava
-	</copyrightsummary>
-	<p>
-	  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.</p>
-	<p>
-	  This is distributed in the hope that it will be useful, but
-	  <em>without any warranty</em>; without even the implied
-	  warranty of merchantability or fitness for a particular
-	  purpose.  See the GNU General Public License for more
-	  details.</p>
-
-	<p>
-	  You should have received a copy of the GNU General Public
-	  License with your Debian system, in
-	  <tt>/usr/share/common-licenses/GPL</tt>, or with the
-	  <prgn/debiandoc-sgml/ source package as the file
-	  <tt>COPYING</tt>.  If not, write to the Free Software
-	  Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139,
-	  USA.</p>
-      </copyright>
-    </titlepag>
-    <chapt id="introduction">
-      <heading>Introduction</heading>
-      <p>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.</p>
-
-      <p> 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)</p>
-    </chapt>
-    <chapt>
-      <heading>Requirements</heading>
-      <p>
-	<enumlist compact="compact">
-	  <item>
-	    <p>
-	      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.</p>
-	  </item>
-	  <item>
-	    <p>
-	      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.</p>
-	  </item>
-	  <item>
-	    <p>
-	      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.
-	    </p>
-	  </item>
-	  <item>
-	    <p>
-	      This would allow APT to handle <em>standard
-		installations</em>, 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.</p>
-	  </item>
-	  <item>
-	    <p>
-	      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)
-	    </p>
-	  </item>
-	  <item>
-	    <p>
-	      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,
-	      <em>configure immediately</em> requests mentioned below
-	      should be handled.</p>
-	  </item>
-	  <item>
-	    <p>
-	      Handle replacement of a package providing a virtual
-	      package with another (for example, it has been very
-	      difficult replacing <prgn>sendmail</prgn> with
-	      <prgn>smail</prgn>, or vice versa), making sure that the
-	      dependencies are still satisfied. </p>
-	  </item>
-	  <item>
-	    <p>
-	      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. </p>
-	  </item>
-	  <item>
-	    <p>
-	      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.</p>
-	  </item>
-	  <item>
-	    <p>
-	      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.</p>
-	  </item>
-	  <item>
-	    <p>
-	      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).</p>
-	  </item>
-	  <item>
-	    <p>
-	      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)</p>
-	  </item>
-	</enumlist> 
-      </p>
-    </chapt>
-    <chapt>
-      <heading>Procedural description</heading>
-      <p><taglist>
-	  <tag>Set Options</tag>
-	  <item>
-	    <p>
-	      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.</p>
-	  </item>
-	  <tag>Updates</tag>
-	  <item>
-	    <p>
-	      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.</p>
-	  </item>
-	  <tag>Local status</tag>
-	  <item>
-	    <p>
-	      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.
-	    </p>
-	  </item>
-	  <tag>Relationship determination</tag>
-	  <item>
-	    <p>
-	      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.</p>
-	  </item>
-	  <tag>Selection</tag>
-	  <item>
-	    <p>
-	      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)
-	    </p>
-	  </item>
-	  <tag>Ordering of package installations and configuration </tag>
-	  <item>
-	    <p>
-	      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).</p>
-	  </item>
-	  <tag>Action</tag>
-	  <item>
-	    <p>
-	      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.</p>
-	  </item>
-
-	</taglist>
-      </p>
-    </chapt>
-    <chapt>
-      <heading>Modules and interfaces</heading>
-      <p><taglist>
-	  <tag>The user interface module</tag>
-	  <item>
-	    <p> Look at Behan Webster's documentation.</p> 
-	  </item>
-	  <tag>Widget set</tag>
-	  <item>
-	    <p>
-	      Related closely to above Could some one present design
-	      decisions of the widget set here?</p>
-	  </item>
-	  <tag>pdate Module</tag>
-	  <item>
-	    <p>	    
-	      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)</p>
-	    <p>
-	      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. </p>
-	    <p><enumlist>
-		<item>
-		  <p>FTP methods</p>
-		</item>
-		<item>
-		  <p>mount and file traversal module(s)?</p>
-		</item>
-		<item>
-		  <p>Other methods ???</p>
-		</item>
-	      </enumlist>
-	    </p>
-	  </item>
-	  <tag>Status file parser/generator</tag>
-	  <item>
-	    <p> 
-	      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 <tt>dpkg
-		-BORGiE</tt> on a file system.</p>
-	  </item>
-	  <tag>Package file parser/generator</tag>
-	  <item>
-	    <p>
-	      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. </p>
-	  </item>
-	  <tag>Dependency module</tag>
-	  <item>
-	    <p><list>
-		<item>
-		  <p>dependency/conflict determination and linking</p>
-		</item>
-		<item>
-		  <p>reverse dependency generator. Maybe merged with above</p>
-		</item>
-	      </list>
-	    </p>
-	  </item>
-	  <tag>Package ordering Module</tag>
-	  <item>
-	    <p>Create an ordering of the actions to be taken.</p>
-	  </item>
-	  <tag>Event generator</tag>
-	  <item>
-	    <p>module to interact with dpkg</p>
-	  </item>
-	</taglist>
-    </chapt>
-    <chapt>
-      <heading>Data flow and conversions analysis.</heading>
-      <p> 
-	<example>
-                                                          ____________
-                                                       __\|ftp modules|
-                                                      /  /|___________|
-                                    _ ____________   /     ________________
-                                    |    update   | /     |mount/local file|
-        |==========================>|   module    |/_____\|  traversals    |
-        |                           |_____________|      /|________________|
-        |                             ^        ^
-        |                             |        |               ______________
-  ______|_______    _ _____ ______    |   _____v________      \|            |
- |Configuration |   |configuration|   |   |Packages Files|  ===|Status file |
- |  module      |<=>|    data     |   |   |______________| /  /|____________|
- |______________|   |_____________|   |        ^          /
-         ^                            |        |         /
-         |                            | _______v_______|/_
-         |                            | |              |    ________________
-         |                            | |              |/_\|   Dependency  |
-         |                            | |backing store |\ /|     Module    |
-         |                            | |______________|  _|_______________|
-         |                             \       ^          /|       ^
-         |                              \      |         /         |
-         |                              _\|____v_______|/__    ____v_______
-         |_____________________________\| User interaction|    |    dpkg   |
-                                       /|_________________|<==>|  Invoker  |
-                                                               |___________|
-
-	</example>
-      <p> dpkg also interacts with status and available files.</p>
-
-
-      <p>
-	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.</p>
-
-      <p>The other focal point for APT is the user interface.</p> 
-    </chapt>
-  </book>
-</debiandoc>