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SYSTEMD.OFFLINE-UPDATES(7)  systemd.offline-updates SYSTEMD.OFFLINE-UPDATES(7)
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 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 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
           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
        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-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 logind dbus API[2] for
           details.
        4. The update service should declare DefaultDependencies=false,
           Requires=sysinit.target, After=sysinit.target,
           After=system-update-pre.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
        2. logind dbus API
           https://www.freedesktop.org/wiki/Software/systemd/logind
systemd 239                                         SYSTEMD.OFFLINE-UPDATES(7)