path: root/docs/dconf-overview.xml

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<refentry id="dconf-overview">

  <refentryinfo>
    <title>dconf Overview</title>
    <productname>dconf</productname>

    <authorgroup>
      <author>
        <contrib>Developer</contrib>
        <firstname>Ryan</firstname>
        <surname>Lortie</surname>
        <email>desrt@desrt.ca</email>
      </author>
    </authorgroup>
  </refentryinfo>

  <refmeta>
    <refentrytitle>dconf</refentrytitle>
    <manvolnum>7</manvolnum>
    <refmiscinfo class="manual">Conventions and miscellaneous</refmiscinfo>
  </refmeta>

  <refnamediv>
    <refname>dconf</refname>
    <refpurpose>A configuration systen</refpurpose>
  </refnamediv>

  <refsect1>
    <title>Description</title>

    <para>
      dconf is a simple key/value storage system that is heavily optimised for reading.  This makes it an ideal
      system for storing user preferences (which are read 1000s of times for each time the user changes one).
      It was created with this usecase in mind.
    </para>
    <para>
      All preferences are stored in a single large binary file.  Layering of preferences is possible using
      multiple files (ie: for site defaults).  Lock-down is also supported.  The binary file for the defaults
      can optionally be compiled from a set of plain text keyfiles.
    </para>
    <para>
      dconf has a partial client/server architecture.  It uses D-Bus.  The server is only involved in writes
      (and is not activated in the user session until the user modifies a preference).  The service is stateless
      and can exit freely at any time (and is therefore robust against crashes).  The list of paths that each
      process is watching is stored within the D-Bus daemon itself (as D-Bus signal match rules).
    </para>
    <para>
      Reads are performed by direct access (via mmap) to the on-disk database which is essentially a hashtable.
      For this reason, dconf reads typically involve zero system calls and are comparable to a hashtable lookup
      in terms of speed.  Practically speaking, in simple non-layered setups, dconf is less than 10 times slower
      than GHashTable.
    </para>
    <para>
      Writes are assumed only to happen in response to explicit user interaction (like clicking on a checkbox in
      a preferences dialog) and are therefore not optimised at all.  On some file systems, dconf-service will
      call fsync() for every write, which can introduce a latency of up to 100ms.  This latency is hidden by the
      client libraries through a clever "fast" mechanism that records the outstanding changes locally (so they
      can be read back immediately) until the service signals that a write has completed.
    </para>
    <para>
      The binary database format that dconf uses by default is not suitable for use on NFS, where mmap does not
      work well. To handle this common use case, dconf can be configured to place its binary database in
      <envar>XDG_RUNTIME_DIR</envar> (which is guaranteed to be local, but non-persistent) and synchronize it
      with a plain text keyfile in the users home directory.
    </para>
  </refsect1>

  <refsect1>
    <title>Profiles</title>

    <para>
      A profile is a list of configuration databases that dconf consults to find the value for a key. The user's personal
      database always takes the highest priority, followed by the system databases in the order prescribed by the profile.
    </para>

    <para>
      A profile file might look like the following:
      <screen>
user-db:user
system-db:local
system-db:site
      </screen>
    </para>

    <para>
      Each line in a profile specifies one dconf database. The first line indicates the database used to write changes, and the
      remaining lines indicate read-only databases. (The first line should specify a user-db or service-db, so that users can actually
      make configuration changes.)
    </para>

    <para>
      A "user-db" line specifies a user database. These databases are found in <filename><envar>$XDG_CONFIG_HOME</envar>/dconf/</filename>.
      The name of the file to open in that directory is exactly as it is written in the profile. This file is expected to be in the binary
      dconf database format. Note that <envar>$XDG_CONFIG_HOME</envar> cannot be set/modified per terminal or session, because then the writer
      and reader would be working on different DBs (the writer is started by DBus and cannot see that variable).
    </para>

    <para>
      A "service-db" line instructs dconf to place the binary database file for the user database in <envar>$XDG_RUNTIME_DIR</envar>.
      Since this location is not persistent, the rest of the line instructs dconf how to store the database persistently. A typical
      line is <literal>service-db:keyfile/user</literal>, which tells dconf to synchronize the binary database with a plain text
      keyfile in <filename><envar>$XDG_CONFIG_HOME</envar>/dconf/user.txt</filename>. The synchronization is bi-directional.
    </para>

    <para>
      A "system-db" line specifies a system database. These databases are found in <filename>/etc/dconf/db/</filename>. Again, the name of
      the file to open in that directory is exactly as it is written in the profile and the file is expected to be in the dconf database
      format.
    </para>

    <para>
      On startup, dconf consults checks for profiles in the following order:
    </para>

    <orderedlist numeration="lowerroman">
      <listitem>
        <para>
          <envar>$DCONF_PROFILE</envar> environment variable. If set, dconf will attempt to open the named profile.
          <envar>$DCONF_PROFILE</envar> can specify a relative path to a file in <filename>/etc/dconf/profile/</filename>,
          or an absolute path (such as in a user's home directory). The profile name can only use alphanumeric characters or '_'.
        </para>
      </listitem>
      <listitem>
        <para>
          If <envar>$DCONF_PROFILE</envar> is not set, dconf will attempt to open the profile named <filename>dconf.profile</filename>
          located in <envar>$XDG_RUNTIME_DIR</envar>, this location is reserverd for an optional dconf pam module that looks
          for profiles specific to this user, please refer to the <link linkend="pammodule">PAM Module section</link> for further info on its behaviour.
        </para>
      </listitem>
      <listitem>
        <para>
          A profile located in <filename>dconf/profile/user</filename> under <filename>/etc</filename> or any of
          the directories listed in <envar>$XDG_DATA_DIRS</envar>. Note that dconf defaults to
          <filename>/usr/local/share/:/usr/share/</filename> if not set <envar>$XDG_DATA_DIRS</envar> is not set
          and that it will honour the order in which directories are listed within the environment variable value.
        </para>
      </listitem>
      <listitem>
        <para>
          If dconf cannot find a profile through any of the locations described above then the effect is as if the profile was specified by this file:
          <screen>
    user-db:user
          </screen>
          That is, the user's personal database is consulted and there are no system settings.
        </para>
      </listitem>
    </orderedlist>
  </refsect1>

  <refsect1 id="pammodule">
    <title>PAM Module</title>

    <para>
      To allow sysadmins to be able to specify a profile specific to each user we have created a PAM module
      looks for a profile located in <filename>dconf/profile/<envar>$USERNAME</envar>.profile</filename> within
      <filename>/etc</filename> or any of the directories listed in <envar>$XDG_DATA_DIRS</envar> and then it
      creates a symbolic link located in <filename><envar>$XDG_RUNTIME_DIR</envar>/dconf.profile</filename>.
    </para>
    <para>
      System administrators and integrators should note that the <filename>pam_dconf.so</filename> module should
      be loaded after <filename>pam_systemd.so</filename> and <filename>pam_env.so</filename> modules. Those
      systems not using systemd should also make sure that whatever module that handles
      <envar>$XDG_RUNTIME_DIR</envar> or <envar>$XDG_DATA_DIRS</envar> should be loaded before
      <filename>pam_dconf.so</filename>.
    </para>
  </refsect1>

  <refsect1>
    <title>Key Files</title>

    <para>
      To facilitate system configuration with a text editor, dconf can populate databases from plain text keyfiles. For any given system
      database, keyfiles can be placed into the <filename>/etc/dconf/db/<replaceable>database</replaceable>.d/</filename> directory. The
      keyfiles contain groups of settings as follows:
     </para>
     <screen>
# Some useful default settings for our site

[system/proxy/http]
host='172.16.0.1'
enabled=true

[org/gnome/desktop/background]
picture-uri='file:///usr/local/rupert-corp/company-wallpaper.jpeg'
     </screen>
     <para>
      After changing keyfiles, the database needs to be updated with the
      <citerefentry><refentrytitle>dconf</refentrytitle><manvolnum>1</manvolnum></citerefentry> tool.
    </para>
  </refsect1>

  <refsect1>
    <title>Locks</title>

    <para>
      System databases can contain 'locks' for keys. If a lock for a particular key or subpath is installed into a database
      then no database listed above that one in the profile will be able to modify any of the affected settings. This can be
      used to enforce mandatory settings.
    </para>

    <para>
      To add locks to a database, place text files in the <filename>/etc/dconf/db/<replaceable>database</replaceable>.d/locks</filename>
      directory, where <replaceable>database</replaceable> is the name of a system database, as specified in the profile. The files
      contain list of keys to lock, on per line. Lines starting with a # are ignored. Here is an example:
    </para>
    <screen>
# prevent changes to the company wallpaper
/org/gnome/desktop/background/picture-uri
    </screen>
    <para>
      After changing locks, the database needs to be updated with the
      <citerefentry><refentrytitle>dconf</refentrytitle><manvolnum>1</manvolnum></citerefentry> tool.
    </para>
  </refsect1>

  <refsect1>
    <title>Portability</title>

    <para>
      dconf mostly targets Free Software operating systems.  It will theoretically run on Mac OS but there isn't
      much point to that (since Mac OS applications want to store preferences in plist files).  It is not
      possible to use dconf on Windows because of the inability to rename over a file that's still in use (which
      is what the dconf-service does on every write).
    </para>
  </refsect1>

  <refsect1>
    <title>API stability</title>

    <para>
      The dconf API is not particularly friendly, and is not guaranteed to be stable.  Because of this and the
      lack of portability, you almost certainly want to use some sort of wrapper API around it.  The wrapper API
      used by GTK+ and GNOME applications is
      <ulink url="http://developer.gnome.org/gio/stable/GSettings.html">GSettings</ulink>, which is included as
      part of GLib. GSettings has backends for Windows (using the registry) and Mac OS (using property lists) as
      well as its dconf backend and is the proper API to use for graphical applications.
    </para>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para>
      <citerefentry><refentrytitle>dconf-service</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>dconf-editor</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>dconf</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <ulink url="http://developer.gnome.org/gio/stable/GSettings.html">GSettings</ulink>
    </para>
  </refsect1>
</refentry>