apt/doc/design.sgml

<!doctype debiandoc  PUBLIC  "-//DebianDoc//DTD DebianDoc//EN">
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  <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 GNU/Linux 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> Diety/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>