man/systemd.offline-updates.xml

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<refentry id="systemd.offline-updates">
  <refentryinfo>
    <title>systemd.offline-updates</title>
    <productname>systemd</productname>

    <authorgroup>
      <author>
        <contrib>Developer</contrib>
        <firstname>Lennart</firstname>
        <surname>Poettering</surname>
        <email>lennart@poettering.net</email>
      </author>
    </authorgroup>
  </refentryinfo>

  <refmeta>
    <refentrytitle>systemd.offline-updates</refentrytitle>
    <manvolnum>7</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>systemd.offline-updates</refname>
    <refpurpose>Implementation of offline updates in systemd</refpurpose>
  </refnamediv>

  <refsect1>
    <title>Implementing Offline System Updates</title>

    <para>This man page describes how to implement "offline" system updates with systemd. By "offline"
    OS updates we mean package installations and updates that are run with the system booted into a
    special system update mode, in order to avoid problems related to conflicts of libraries and
    services that are currently running with those on disk. This document is inspired by this
    <ulink url="https://wiki.gnome.org/Design/OS/SoftwareUpdates">GNOME design whiteboard</ulink>.
    </para>

    <para>The logic:</para>

    <orderedlist>
      <listitem>
        <para>The package manager prepares system updates by downloading all (RPM or DEB or
        whatever) packages to update off-line in a special directory
        <filename noindex="true">/var/lib/system-update</filename> (or
        another directory of the package/upgrade manager's choice).</para>
      </listitem>

      <listitem>
        <para>When the user OK'ed the update, the symlink <filename>/system-update</filename> is
        created that points to <filename noindex="true">/var/lib/system-update</filename> (or
        wherever the directory with the upgrade files is located) and the system is rebooted. This
        symlink is in the root directory, since we need to check for it very early at boot, at a
        time where <filename>/var</filename> is not available yet.</para>
      </listitem>

      <listitem>
        <para>Very early in the new boot
        <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
        checks whether <filename>/system-update</filename> exists. If so, it (temporarily and for
        this boot only) redirects (i.e. symlinks) <filename>default.target</filename> to
        <filename>system-update.target</filename>, a special target that is pulls in the base system
        (i.e. <filename>sysinit.target</filename>, so that all file systems are mounted but little
        else) and the system update units.</para>
      </listitem>

      <listitem>
        <para>The system now continues to boot into <filename>default.target</filename>, and
        thus into <filename>system-update.target</filename>. This target pulls in all system
        update units. Only one service should perform an update (see the next point), and all
        the other ones should exit cleanly with a "success" return code and without doing
        anything. Update services should be ordered after <filename>sysinit.target</filename>
        so that the update starts after all file systems have been mounted.</para>
      </listitem>

      <listitem>
        <para>As the first step, an update service should check if the
        <filename>/system-update</filename> symlink points to the location used by that update
        service. In case it does not exist or points to a different location, the service must exit
        without error. It is possible for multiple update services to be installed, and for multiple
        update services to be launched in parallel, and only the one that corresponds to the tool
        that <emphasis>created</emphasis> the symlink before reboot should perform any actions. It
        is unsafe to run multiple updates in parallel.</para>
      </listitem>

      <listitem>
        <para>The update service should now do its job. If applicable and possible, it should
        create a file system snapshot, then install all packages.  After completion (regardless
        whether the update succeeded or failed) the machine must be rebooted, for example by
        calling <command>systemctl reboot</command>. In addition, on failure the script should
        revert to the old file system snapshot (without the symlink).</para>
      </listitem>

      <listitem>
        <para>The upgrade scripts should exit only after the update is finished. It is expected
        that the service which performs the upgrade will cause the machine to reboot after it
        is done. If the <filename>system-update.target</filename> is successfully reached, i.e.
        all update services have run, and the <filename>/system-update</filename> symlink still
        exists, it will be removed and the machine rebooted as a safety measure.</para>
      </listitem>

      <listitem>
        <para>After a reboot, now that the <filename>/system-update</filename> symlink is gone,
        the generator won't redirect <filename>default.target</filename> anymore and the system
        now boots into the default target again.</para>
      </listitem>
    </orderedlist>
  </refsect1>

  <refsect1>
    <title>Recommendations</title>

    <orderedlist>
      <listitem>
        <para>To make things a bit more robust we recommend hooking the update script into
        <filename>system-update.target</filename> via a <filename noindex='true'>.wants/</filename>
        symlink in the distribution package, rather than depending on <command>systemctl
        enable</command> in the postinst scriptlets of your package. More specifically, for your
        update script create a .service file, without [Install] section, and then add a symlink like
        <filename noindex='true'>/usr/lib/systemd/system-update.target.wants/foobar.service</filename>
        → <filename noindex='true'>../foobar.service</filename> to your package.</para>
      </listitem>

      <listitem>
        <para>Make sure to remove the <filename>/system-update</filename> symlink as early as
        possible in the update script to avoid reboot loops in case the update fails.</para>
      </listitem>

      <listitem>
        <para>Use <varname>FailureAction=reboot</varname> in the service file for your update script
        to ensure that a reboot is automatically triggered if the update fails.
        <varname>FailureAction=</varname> makes sure that the specified unit is activated if your
        script exits uncleanly (by non-zero error code, or signal/coredump). If your script succeeds
        you should trigger the reboot in your own code, for example by invoking logind's
        <command>Reboot()</command> call or calling <command>systemctl reboot</command>. See
        <ulink url="https://www.freedesktop.org/wiki/Software/systemd/logind">logind dbus API</ulink>
        for details.</para>
      </listitem>

      <listitem>
        <para>The update service should declare <varname>DefaultDependencies=false</varname>,
        <varname>Requires=sysinit.target</varname>, <varname>After=sysinit.target</varname>,
        and explicitly pull in any other services it requires.</para>
      </listitem>
    </orderedlist>
  </refsect1>

  <refsect1>
    <title>See also</title>

    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry project='mankier'><refentrytitle>dnf.plugin.system-upgrade</refentrytitle><manvolnum>8</manvolnum></citerefentry>
    </para>
  </refsect1>
</refentry>