NAME

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

IMPLEMENTING OFFLINE SYSTEM UPDATES

       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 or /etc/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 or /etc/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 or
           /etc/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 or /etc/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 and /etc/system-update symlink is gone,
           the generator won't redirect default.target anymore and the system now boots into the
           default target again.

RECOMMENDATIONS

        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 and /etc/system-update symlinks 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

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

NOTES

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