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1 | <?xml version='1.0'?> <!--*-nxml-*--> | |
2 | <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" | |
3 | "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> | |
4 | <!-- SPDX-License-Identifier: LGPL-2.1+ --> | |
5 | ||
6 | <refentry id="systemd.offline-updates"> | |
7 | <refentryinfo> | |
8 | <title>systemd.offline-updates</title> | |
9 | <productname>systemd</productname> | |
10 | </refentryinfo> | |
11 | ||
12 | <refmeta> | |
13 | <refentrytitle>systemd.offline-updates</refentrytitle> | |
14 | <manvolnum>7</manvolnum> | |
15 | </refmeta> | |
16 | ||
17 | <refnamediv> | |
18 | <refname>systemd.offline-updates</refname> | |
19 | <refpurpose>Implementation of offline updates in systemd</refpurpose> | |
20 | </refnamediv> | |
21 | ||
22 | <refsect1> | |
23 | <title>Implementing Offline System Updates</title> | |
24 | ||
25 | <para>This man page describes how to implement "offline" system updates with systemd. By "offline" | |
26 | OS updates we mean package installations and updates that are run with the system booted into a | |
27 | special system update mode, in order to avoid problems related to conflicts of libraries and | |
28 | services that are currently running with those on disk. This document is inspired by this | |
29 | <ulink url="https://wiki.gnome.org/Design/OS/SoftwareUpdates">GNOME design whiteboard</ulink>. | |
30 | </para> | |
31 | ||
32 | <para>The logic:</para> | |
33 | ||
34 | <orderedlist> | |
35 | <listitem> | |
36 | <para>The package manager prepares system updates by downloading all (RPM or DEB or | |
37 | whatever) packages to update off-line in a special directory | |
38 | <filename noindex="true">/var/lib/system-update</filename> (or | |
39 | another directory of the package/upgrade manager's choice).</para> | |
40 | </listitem> | |
41 | ||
42 | <listitem> | |
43 | <para>When the user OK'ed the update, the symlink <filename>/system-update</filename> is | |
44 | created that points to <filename noindex="true">/var/lib/system-update</filename> (or | |
45 | wherever the directory with the upgrade files is located) and the system is rebooted. This | |
46 | symlink is in the root directory, since we need to check for it very early at boot, at a | |
47 | time where <filename>/var</filename> is not available yet.</para> | |
48 | </listitem> | |
49 | ||
50 | <listitem> | |
51 | <para>Very early in the new boot | |
52 | <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry> | |
53 | checks whether <filename>/system-update</filename> exists. If so, it (temporarily and for | |
54 | this boot only) redirects (i.e. symlinks) <filename>default.target</filename> to | |
55 | <filename>system-update.target</filename>, a special target that pulls in the base system | |
56 | (i.e. <filename>sysinit.target</filename>, so that all file systems are mounted but little | |
57 | else) and the system update units.</para> | |
58 | </listitem> | |
59 | ||
60 | <listitem> | |
61 | <para>The system now continues to boot into <filename>default.target</filename>, and | |
62 | thus into <filename>system-update.target</filename>. This target pulls in all system | |
63 | update units. Only one service should perform an update (see the next point), and all | |
64 | the other ones should exit cleanly with a "success" return code and without doing | |
65 | anything. Update services should be ordered after <filename>sysinit.target</filename> | |
66 | so that the update starts after all file systems have been mounted.</para> | |
67 | </listitem> | |
68 | ||
69 | <listitem> | |
70 | <para>As the first step, an update service should check if the | |
71 | <filename>/system-update</filename> symlink points to the location used by that update | |
72 | service. In case it does not exist or points to a different location, the service must exit | |
73 | without error. It is possible for multiple update services to be installed, and for multiple | |
74 | update services to be launched in parallel, and only the one that corresponds to the tool | |
75 | that <emphasis>created</emphasis> the symlink before reboot should perform any actions. It | |
76 | is unsafe to run multiple updates in parallel.</para> | |
77 | </listitem> | |
78 | ||
79 | <listitem> | |
80 | <para>The update service should now do its job. If applicable and possible, it should | |
81 | create a file system snapshot, then install all packages. After completion (regardless | |
82 | whether the update succeeded or failed) the machine must be rebooted, for example by | |
83 | calling <command>systemctl reboot</command>. In addition, on failure the script should | |
84 | revert to the old file system snapshot (without the symlink).</para> | |
85 | </listitem> | |
86 | ||
87 | <listitem> | |
88 | <para>The upgrade scripts should exit only after the update is finished. It is expected | |
89 | that the service which performs the upgrade will cause the machine to reboot after it | |
90 | is done. If the <filename>system-update.target</filename> is successfully reached, i.e. | |
91 | all update services have run, and the <filename>/system-update</filename> symlink still | |
92 | exists, it will be removed and the machine rebooted as a safety measure.</para> | |
93 | </listitem> | |
94 | ||
95 | <listitem> | |
96 | <para>After a reboot, now that the <filename>/system-update</filename> symlink is gone, | |
97 | the generator won't redirect <filename>default.target</filename> anymore and the system | |
98 | now boots into the default target again.</para> | |
99 | </listitem> | |
100 | </orderedlist> | |
101 | </refsect1> | |
102 | ||
103 | <refsect1> | |
104 | <title>Recommendations</title> | |
105 | ||
106 | <orderedlist> | |
107 | <listitem> | |
108 | <para>To make things a bit more robust we recommend hooking the update script into | |
109 | <filename>system-update.target</filename> via a <filename noindex='true'>.wants/</filename> | |
110 | symlink in the distribution package, rather than depending on <command>systemctl | |
111 | enable</command> in the postinst scriptlets of your package. More specifically, for your | |
112 | update script create a .service file, without [Install] section, and then add a symlink like | |
113 | <filename noindex='true'>/usr/lib/systemd/system-update.target.wants/foobar.service</filename> | |
114 | → <filename noindex='true'>../foobar.service</filename> to your package.</para> | |
115 | </listitem> | |
116 | ||
117 | <listitem> | |
118 | <para>Make sure to remove the <filename>/system-update</filename> symlink as early as | |
119 | possible in the update script to avoid reboot loops in case the update fails.</para> | |
120 | </listitem> | |
121 | ||
122 | <listitem> | |
123 | <para>Use <varname>FailureAction=reboot</varname> in the service file for your update script | |
124 | to ensure that a reboot is automatically triggered if the update fails. | |
125 | <varname>FailureAction=</varname> makes sure that the specified unit is activated if your | |
126 | script exits uncleanly (by non-zero error code, or signal/coredump). If your script succeeds | |
127 | you should trigger the reboot in your own code, for example by invoking logind's | |
128 | <command>Reboot()</command> call or calling <command>systemctl reboot</command>. See | |
129 | <ulink url="https://www.freedesktop.org/wiki/Software/systemd/logind">logind dbus API</ulink> | |
130 | for details.</para> | |
131 | </listitem> | |
132 | ||
133 | <listitem> | |
134 | <para>The update service should declare <varname>DefaultDependencies=no</varname>, | |
135 | <varname>Requires=sysinit.target</varname>, <varname>After=sysinit.target</varname>, | |
136 | <varname>After=system-update-pre.target</varname> | |
137 | and explicitly pull in any other services it requires.</para> | |
138 | </listitem> | |
139 | ||
140 | <listitem> | |
141 | <para>It may be desirable to always run an auxiliary unit when booting | |
142 | into offline-updates mode, which itself does not install updates. To | |
143 | do this create a .service file with | |
144 | <varname>Wants=system-update-pre.target</varname> and | |
145 | <varname>Before=system-update-pre.target</varname> and add a symlink | |
146 | to that file under | |
147 | <filename noindex='true'>/usr/lib/systemd/system-update.target.wants</filename> | |
148 | .</para> | |
149 | </listitem> | |
150 | </orderedlist> | |
151 | </refsect1> | |
152 | ||
153 | <refsect1> | |
154 | <title>See also</title> | |
155 | ||
156 | <para> | |
157 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
158 | <citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>, | |
159 | <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, | |
160 | <citerefentry project='mankier'><refentrytitle>dnf.plugin.system-upgrade</refentrytitle><manvolnum>8</manvolnum></citerefentry> | |
161 | </para> | |
162 | </refsect1> | |
163 | </refentry> |