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
           /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, and explicitly pull in any other services it requires.

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