systemd.offline-updates(7) — Linux manual page

NAME | IMPLEMENTING OFFLINE SYSTEM UPDATES | RECOMMENDATIONS | SEE ALSO | NOTES | COLOPHON

SYSTEMD.OFFLINE-UPDATES(7)ystemd.offline-updatesSTEMD.OFFLINE-UPDATES(7)

NAME         top

       systemd.offline-updates - Implementation of offline updates in
       systemd

IMPLEMENTING OFFLINE SYSTEM UPDATES         top

       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
       GNOME design whiteboard[1].

       The logic:

        1. The package manager prepares system updates by downloading
           all (.rpm or .deb or whatever) packages to update off-line in
           a special directory /var/lib/system-update (or another
           directory of the package/upgrade manager's choice).

        2. When the user OK'ed the update, the symlink /system-update is
           created that points to /var/lib/system-update (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 /var/ is not available yet.

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

        4. The system now continues to boot into default.target, and
           thus into system-update.target. 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
           sysinit.target so that the update starts after all file
           systems have been mounted.

        5. As the first step, an update service should check if the
           /system-update 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 created the
           symlink before reboot should perform any actions. It is
           unsafe to run multiple updates in parallel.

        6. 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 systemctl reboot. In addition, on
           failure the script should revert to the old file system
           snapshot (without the symlink).

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

        8. After a reboot, now that the /system-update symlink is gone,
           the generator won't redirect default.target anymore and the
           system now boots into the default target again.

RECOMMENDATIONS         top

        1. To make things a bit more robust we recommend hooking the
           update script into system-update.target via a .wants/ symlink
           in the distribution package, rather than depending on
           systemctl enable 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
           /usr/lib/systemd/system/system-update.target.wants/foobar.service
           → ../foobar.service to your package.

        2. Make sure to remove the /system-update symlink as early as
           possible in the update script to avoid reboot loops in case
           the update fails.

        3. Use FailureAction=reboot in the service file for your update
           script to ensure that a reboot is automatically triggered if
           the update fails.  FailureAction= 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 Reboot() call or calling
           systemctl reboot. See org.freedesktop.login1(5) for details
           about the logind D-Bus API.

        4. The update service should declare DefaultDependencies=no,
           Requires=sysinit.target, After=sysinit.target,
           After=system-update-pre.target, Before=system-update.target
           and explicitly pull in any other services it requires.

        5. It may be desirable to always run an auxiliary unit when
           booting into offline-updates mode, which itself does not
           install updates. To do this create a .service file with
           Wants=system-update-pre.target and
           Before=system-update-pre.target and add a symlink to that
           file under /usr/lib/systemd/system-update.target.wants .

SEE ALSO         top

       systemd(1), systemd.generator(7),
       systemd-system-update-generator(8), dnf.plugin.system-upgrade(8)

NOTES         top

        1. GNOME design whiteboard
           https://wiki.gnome.org/Design/OS/SoftwareUpdates

COLOPHON         top

       This page is part of the systemd (systemd system and service
       manager) project.  Information about the project can be found at
       ⟨http://www.freedesktop.org/wiki/Software/systemd⟩.  If you have
       a bug report for this manual page, see
       ⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
       This page was obtained from the project's upstream Git repository
       ⟨https://github.com/systemd/systemd.git⟩ on 2021-04-01.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2021-04-01.)  If you discover any rendering
       problems in this HTML version of the page, or you believe there
       is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to
       man-pages@man7.org

systemd 248                                   SYSTEMD.OFFLINE-UPDATES(7)

Pages that refer to this page: systemd.directives(7)systemd.generator(7)systemd.index(7)systemd.special(7)systemd-system-update-generator(8)