debian_debian-policy/debconf_spec/debconf_specification.xml

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<!ENTITY statuscodes_table SYSTEM "include/statuscodes.xml">
<!ENTITY command_list SYSTEM "include/commands.xml">
<!ENTITY priority_table SYSTEM "include/priorities.xml">
<!ENTITY type_table SYSTEM "include/types.xml">
<!ENTITY % versiondata SYSTEM "include/version.xml"> %versiondata;
]>
<article>

  <articleinfo>
    <title>Configuration management</title>
    <subtitle>Protocol version 2.1</subtitle>
    <releaseinfo>Revision 7.1, Debian Policy &version;, &date;</releaseinfo>
    <author>
      <firstname>Wichert</firstname>
      <surname>Akkerman</surname>
      <affiliation>
        <orgname>The Debian Project</orgname>
	<address><email>wakkerma@debian.org</email></address>
      </affiliation>
    </author>
    <author>
      <firstname>Joey</firstname>
      <surname>Hess</surname>
      <affiliation>
        <orgname>The Debian Project</orgname>
	<address><email>joeyh@debian.org</email></address>
      </affiliation>
    </author>
    <copyright>
      <year>1998</year>
      <year>1999</year>
      <year>2000</year>
      <holder>Wichert Akkerman and Joey Hess</holder>
    </copyright>
    <legalnotice>
      <para>
        This text is copyright by the authors under the terms of the
	BSD license, sans advertising clause.
      </para>
    </legalnotice>
  </articleinfo>

  <sect1>
    <title>
      Introduction
    </title>
    <para>
      Configuration management is quickly becoming a very important issue.
      Having programs which do cool stuff is great, but we need to store
      their configuration as well. We see more and more different
      configuration systems being introduced all the time, which is not very
      practical. This text introduces a general configuration management
      system which flexible enough to be used for all kinds of applications.
    </para>
  </sect1>

  <sect1>
    <title>
      Configuration Data
    </title>
    <sect2>
      <title>
        The configuration space
      </title>
      <para>
        All configuration information is stored in what I call the
        configuration space. This is a database with a special design
        which resembles the method we look at configuration information.
	This is done by defining a hierarchy of information. Each package
	receives its own space in the hierarchy.  Each package is free to
	use a flat space, or divide its space further into
	sub-hierarchies.  If multiple packages share a common purpose they
	may use a shared toplevel hierarchy, preferably with the same name
	as a shared (virtual) package name (for example, both
	<application>mutt</application> and <application>elm</application>
	can use <literal>mail-reader</literal>,
	<application>strn</application> and <application>nn</application>
	could use <literal>news-reader</literal>).  This
	shared tree can also be used as a default, ie a variable
	<literal>news-reader/nntpserver</literal> can be used by
	<application>strn</application> if <literal>strn/nntpserver</literal>
	does not exist.
      </para>
      <para>
        Each variable in the configuration space has some information
	associated with it. Most importantly, it has a value. It also may
	have a set of flags and a set of substitution data.
      </para>
    </sect2>
  </sect1>

  <sect1>
    <title>
      Templates
    </title>
    <para>
      Each variable in the configuration space is associated with some
      meta-data. The minimum meta-data associated with a variable is:
      long and short description, type, and default value. The meta-data
      is essentially static; the protocol described below does not allow it
      to be changed.
    </para>
    <para>
      The meta-data exists in a space with similar naming
      properties to the configuration space described above, and typically
      one variable in the configuration space will have associated with it
      metadata with the same name in the meta-data space. However, this need
      not be the case; many different variables can all be associated with
      the same meta-data. In effect the meta-data serves as a template
      for the configuration variable.
    </para>
    <sect2>
      <title>
        Template information
      </title>
      <para>
	So, what do we need to store in a variable template? Of course we
	need a name to identify the template. Template names are made up of
	components separated by the character `/' (slash).
	Each component is limited to alphanumerics and `+' `-' `.' `_'
	(plus, minus, full stop, underscore).
      </para>
      <para>
        A type is also needed so data can be verified. Here is a table
	of common types; implementations are free to make up more.
	&type_table;
      </para>
      <para>
	Of course a default value is useful as well, and
	finally we need a description of the variable. We actually use two
	descriptions: a short one (limited to 50 characters or so) and an
	extended one.
      </para>
      <para>
        The extended description may be word-wrapped by the
	FrontEnd. To make separate paragraphs in it, use <literal>.</literal>
	on a line by itself to separate them. Text in the extended
	description that is prefaced by additional whitespace will not be
	wordwrapped. Both the description and extended
        description may have substitutions embedded in them. Ie,
	<literal>${foo}</literal>. These will be expanded when the
	descriptions are displayed.
      </para>
      <para>
        This information is stored in a template file that consists of
	stanzas in a rfc-822 compliant format, separated by blank lines.
	Here is an example:
	<programlisting>
Template: hostname
Type: string
Default: debian
Description: unqualified hostname for this computer
 This is the name by which this computer will be known on the network. It
 has to be a unique name in your domain.

Template: domain
Type: string
Description: domain for this computer
  This is the domain your computer is a member of. Typically it is
  something like "mycompany.com" or "myuniversity.edu".
	</programlisting>
      </para>
      <para>
	For localization, the description field (and also the choices
	field of a select or multiselect type question, and the
	default field of a string or password type question) can be
	supplemented with versions for other languages. These are
	named <emphasis>Description-ll</emphasis>,
	<emphasis>Description-ll_LL</emphasis>,
	<emphasis>Description-ll_LL.encoding</emphasis> and so on.
      </para>
    </sect2>
  </sect1>
  <sect1>
    <title>
      Configuration frontends
    </title>
    <para>
      Of course applications can use the database and meta-database directly.
      But there should be a simple system to interact with the user that is
      simple and modular enough to be used with systems ranging from
      shell-scripts to Fortran programs. To do this we define a general
      frontend that can be driven using the simplest and most common form of
      communication: stdin and stdout.
    </para>
    <para>
      Using this simple form of communication gives us a great advantage: it
      becomes easy to change the frontend. That means the user can switch
      between a console, a graphical or even a web-interface at will.
    </para>
    <para>
      Besides being able to switch between types of frontends there is
      another important aspect of a good user interface: user friendliness.
      We have to account for the fact that some users know more then others
      and change the information we show or ask from the user. We do this by
      giving everything a priority and giving the user control over what
      kind of questions he wants to see. Experts can request to see
      everything, while novices get the option of only seeing only important
      questions. Finally there is an option to simply skip all questions, so
      it becomes possible to do automatic configuration using default values
      or values that are downloaded into the database from a remote
      location. This makes it simple for example to install and manage
      clusters or lab rooms or do installs for dummies.
    </para>
  </sect1>
  <sect1>
    <title>
      Communication with the frontend
    </title>
    <para>
      This communication between the frontend and the application should be
      as simple as possible. Since most IO implementations default to
      line-buffered IO, so we use a simple language where each command is
      exactly one line.
    </para>
    <para>
      After sending each command to stdout, the client
      should read one line from stdin. This is the response to the command,
      and it will be in the form of a number followed by whitespace and an
      optional string of text. The number is the status code, while the
      text provides additional information.
      &statuscodes_table;
    </para>
    <para>
      Here are the currently supported commands.
    </para>
    <itemizedlist>
        &command_list;
    </itemizedlist>
  </sect1>
  <sect1>
    <title>
      Debian install-time configuration
    </title>
    <para>
      Debian has had an excellent packaging system for a long time now. There is
      one thing missing though: a system to handle the configuration of
      packages so we don't have to stop the installation every time a package
      needs some data from the user or wants to show some information.
    </para>
    <para>
      We want to make a package which does not break older dpkg's, and we
      want to be able to get the configuration information before the package
      is unpacked. To do this we add two new files, config and templates, to
      the control.tar.gz of a .deb package. Since all installation-software
      (apt, dselect, dpkg) download the package before installing it, we can
      extract this before the package is unpacked.
    </para>
    <para>
      The templates file lists the templates for variables that this package
      uses. This is done using the format as used in the example in the
      section on templates.
    </para>
    <para>
      The config-file contains a new element, which I call the
      configmodule.  This is a program that will determine the
      configuration before the package is unpacked.  This means it is
      usually run <emphasis>before</emphasis> the preinst, and before
      the package is unpacked!
      <note>
	<simpara>Please see debconf-devel(7) for details.</simpara>
      </note>
      This is done to make sure that we can
      use the desired configuration in the preinst if necessary.
    </para>
    <para>
      How does the configmodule get its information?  The configmodule
      needs a way to retrieve information from the configuration space, ask
      the user for information if necessary, etc. But we don't want to
      implement a user interface for each package. To solve this we use a
      separate frontend as specified in the section on frontends.
    </para>
  </sect1>
</article>