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