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

SYSTEMD.OFFLINE-UPDATES(7) systemd.offline-updatesSYSTEMD.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 is 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         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-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         top

       Implementing Offline System Updates[3], 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

        2. logind dbus API
           https://www.freedesktop.org/wiki/Software/systemd/logind

        3. Implementing Offline System Updates
           https://www.freedesktop.org/wiki/Software/systemd/SystemUpdates/

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 2017-03-13.  If you dis‐
       cover 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 233                                       SYSTEMD.OFFLINE-UPDATES(7)