Systemd/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 is implemented starting with systemd 183.</para>

    <para>Here are some guidelines how to implement "offline" OS 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 upgrade package directory is called) 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 a systemd generator 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 new target that is intended to pull 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 the OS update script,
        which is executed after all file systems are mounted.</para>
      </listitem>

      <listitem>
        <para>The system update script now creates a btrfs snapshot (if possible), then installs all
        RPMs. After completion (regardless whether the update succeeded or failed) the
        /system-update symlink is removed. In addition, on failure it reverts to the old btrfs state
        (modulo the aforementioned symlink), on success it leaves the newly made changes in
        place.</para>
      </listitem>

      <listitem>
        <para>The system is rebooted. Since the <filename>/system-update</filename> symlink is gone,
        the generator won't redirect <filename>default.target</filename> after reboot 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 early in the
        update script to avoid reboot loops in case the update fails.</para>
      </listitem>

      <listitem>
        <para>Use <varname>OnFailure=reboot.target</varname> in the service file for your update
        script to ensure that a reboot is automatically triggered if the update fails.
        <varname>OnFailure=</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. See
        <ulink url="http://www.freedesktop.org/wiki/Software/systemd/logind">logind dbus API</ulink>
        for details.</para>
      </listitem>
    </orderedlist>
  </refsect1>

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

    <para>
      <ulink url="http://www.freedesktop.org/wiki/Software/systemd/SystemUpdates/">Implementing Offline System Updates</ulink>,
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>
    </para>
  </refsect1>
</refentry>