1SYSTEMD.OFFLINE-UPDATES(7)  systemd.offline-updates SYSTEMD.OFFLINE-UPDATES(7)
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NAME

6       systemd.offline-updates - Implementation of offline updates in systemd
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IMPLEMENTING OFFLINE SYSTEM UPDATES

9       This man page describes how to implement "offline" system updates with
10       systemd. By "offline" OS updates we mean package installations and
11       updates that are run with the system booted into a special system
12       update mode, in order to avoid problems related to conflicts of
13       libraries and services that are currently running with those on disk.
14       This document is inspired by this GNOME design whiteboard[1].
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16       The logic:
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18        1. The package manager prepares system updates by downloading all (RPM
19           or DEB or whatever) packages to update off-line in a special
20           directory /var/lib/system-update (or another directory of the
21           package/upgrade manager's choice).
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23        2. When the user OK'ed the update, the symlink /system-update is
24           created that points to /var/lib/system-update (or wherever the
25           directory with the upgrade files is located) and the system is
26           rebooted. This symlink is in the root directory, since we need to
27           check for it very early at boot, at a time where /var is not
28           available yet.
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30        3. Very early in the new boot systemd-system-update-generator(8)
31           checks whether /system-update exists. If so, it (temporarily and
32           for this boot only) redirects (i.e. symlinks) default.target to
33           system-update.target, a special target that pulls in the base
34           system (i.e.  sysinit.target, so that all file systems are mounted
35           but little else) and the system update units.
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37        4. The system now continues to boot into default.target, and thus into
38           system-update.target. This target pulls in all system update units.
39           Only one service should perform an update (see the next point), and
40           all the other ones should exit cleanly with a "success" return code
41           and without doing anything. Update services should be ordered after
42           sysinit.target so that the update starts after all file systems
43           have been mounted.
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45        5. As the first step, an update service should check if the
46           /system-update symlink points to the location used by that update
47           service. In case it does not exist or points to a different
48           location, the service must exit without error. It is possible for
49           multiple update services to be installed, and for multiple update
50           services to be launched in parallel, and only the one that
51           corresponds to the tool that created the symlink before reboot
52           should perform any actions. It is unsafe to run multiple updates in
53           parallel.
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55        6. The update service should now do its job. If applicable and
56           possible, it should create a file system snapshot, then install all
57           packages. After completion (regardless whether the update succeeded
58           or failed) the machine must be rebooted, for example by calling
59           systemctl reboot. In addition, on failure the script should revert
60           to the old file system snapshot (without the symlink).
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62        7. The upgrade scripts should exit only after the update is finished.
63           It is expected that the service which performs the upgrade will
64           cause the machine to reboot after it is done. If the
65           system-update.target is successfully reached, i.e. all update
66           services have run, and the /system-update symlink still exists, it
67           will be removed and the machine rebooted as a safety measure.
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69        8. After a reboot, now that the /system-update symlink is gone, the
70           generator won't redirect default.target anymore and the system now
71           boots into the default target again.
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RECOMMENDATIONS

74        1. To make things a bit more robust we recommend hooking the update
75           script into system-update.target via a .wants/ symlink in the
76           distribution package, rather than depending on systemctl enable in
77           the postinst scriptlets of your package. More specifically, for
78           your update script create a .service file, without [Install]
79           section, and then add a symlink like
80           /usr/lib/systemd/system-update.target.wants/foobar.service →
81           ../foobar.service to your package.
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83        2. Make sure to remove the /system-update symlink as early as possible
84           in the update script to avoid reboot loops in case the update
85           fails.
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87        3. Use FailureAction=reboot in the service file for your update script
88           to ensure that a reboot is automatically triggered if the update
89           fails.  FailureAction= makes sure that the specified unit is
90           activated if your script exits uncleanly (by non-zero error code,
91           or signal/coredump). If your script succeeds you should trigger the
92           reboot in your own code, for example by invoking logind's Reboot()
93           call or calling systemctl reboot. See logind dbus API[2] for
94           details.
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96        4. The update service should declare DefaultDependencies=false,
97           Requires=sysinit.target, After=sysinit.target,
98           After=system-update-pre.target and explicitly pull in any other
99           services it requires.
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101        5. It may be desirable to always run an auxiliary unit when booting
102           into offline-updates mode, which itself does not install updates.
103           To do this create a .service file with
104           Wants=system-update-pre.target and Before=system-update-pre.target
105           and add a symlink to that file under
106           /usr/lib/systemd/system-update.target.wants .
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SEE ALSO

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

113        1. GNOME design whiteboard
114           https://wiki.gnome.org/Design/OS/SoftwareUpdates
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116        2. logind dbus API
117           https://www.freedesktop.org/wiki/Software/systemd/logind
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121systemd 239                                         SYSTEMD.OFFLINE-UPDATES(7)
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