1SYSTEMD.OFFLINE-UPDATES(7) systemd.offline-updates SYSTEMD.OFFLINE-UPDATES(7)
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6 systemd.offline-updates - Implementation of offline updates in systemd
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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].
15 The logic:
17 1. The package manager prepares system updates by downloading all
19 (.rpm 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).
22 2. When the user OK'ed the update, the symlink /system-update or
24 /etc/system-update is created that points to /var/lib/system-update
25 (or wherever the directory with the upgrade files is located) and
26 the system is rebooted. This symlink is in the root directory,
27 since we need to check for it very early at boot, at a time where
28 /var/ is not available yet.
29 3. Very early in the new boot systemd-system-update-generator(8)
31 checks whether /system-update or /etc/system-update exists. If so,
32 it (temporarily and for this boot only) redirects (i.e. symlinks)
33 default.target to system-update.target, a special target that pulls
34 in the base system (i.e. sysinit.target, so that all file systems
35 are mounted but little else) and the system update units.
36 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.
44 5. As the first step, an update service should check if the
46 /system-update or /etc/system-update symlink points to the location
47 used by that update service. In case it does not exist or points to
48 a different location, the service must exit without error. It is
49 possible for multiple update services to be installed, and for
50 multiple update services to be launched in parallel, and only the
51 one that corresponds to the tool that created the symlink before
52 reboot should perform any actions. It is unsafe to run multiple
53 updates in parallel.
54 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).
61 7. The update scripts should exit only after the update is finished.
63 It is expected that the service which performs the update 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 or /etc/system-update
67 symlink still exists, it will be removed and the machine rebooted
68 as a safety measure.
69 8. After a reboot, now that the /system-update and /etc/system-update
71 symlink is gone, the generator won't redirect default.target
72 anymore and the system now boots into the default target again.
73
75 1. To make things a bit more robust we recommend hooking the update
76 script into system-update.target via a .wants/ symlink in the
77 distribution package, rather than depending on systemctl enable in
78 the postinst scriptlets of your package. More specifically, for
79 your update script create a .service file, without [Install]
80 section, and then add a symlink like
81 /usr/lib/systemd/system/system-update.target.wants/foobar.service →
82 ../foobar.service to your package.
83 2. Make sure to remove the /system-update and /etc/system-update
85 symlinks as early as possible in the update script to avoid reboot
86 loops in case the update fails.
87 3. Use FailureAction=reboot in the service file for your update script
89 to ensure that a reboot is automatically triggered if the update
90 fails. FailureAction= makes sure that the specified unit is
91 activated if your script exits uncleanly (by non-zero error code,
92 or signal/coredump). If your script succeeds you should trigger the
93 reboot in your own code, for example by invoking logind's Reboot()
94 call or calling systemctl reboot. See org.freedesktop.login1(5) for
95 details about the logind D-Bus API.
96 4. The update service should declare DefaultDependencies=no,
98 Requires=sysinit.target, After=sysinit.target,
99 After=system-update-pre.target, Before=system-update.target and
100 explicitly pull in any other services it requires.
101 5. It may be desirable to always run an auxiliary unit when booting
103 into offline-updates mode, which itself does not install updates.
104 To do this create a .service file with
105 Wants=system-update-pre.target and Before=system-update-pre.target
106 and add a symlink to that file under
107 /usr/lib/systemd/system-update.target.wants .
108
110 systemd(1), systemd.generator(7), systemd-system-update-generator(8),
111 dnf.plugin.system-upgrade(8)
112
114 1. GNOME design whiteboard
115 https://wiki.gnome.org/Design/OS/SoftwareUpdates
116systemd 254 SYSTEMD.OFFLINE-UPDATES(7)