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1 | <?xml version='1.0'?> <!--*-nxml-*--> |
2 | <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" | |
3 | "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> | |
4 | ||
5 | <!-- | |
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6 | SPDX-License-Identifier: LGPL-2.1+ |
7 | ||
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8 | This file is part of systemd. |
9 | ||
10 | Copyright 2013 Lennart Poettering | |
45f0c64e | 11 | Copyright 2016 Zbigniew Jędrzejewski-Szmek |
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12 | |
13 | systemd is free software; you can redistribute it and/or modify it | |
14 | under the terms of the GNU Lesser General Public License as published by | |
15 | the Free Software Foundation; either version 2.1 of the License, or | |
16 | (at your option) any later version. | |
17 | ||
18 | systemd is distributed in the hope that it will be useful, but | |
19 | WITHOUT ANY WARRANTY; without even the implied warranty of | |
20 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU | |
21 | Lesser General Public License for more details. | |
22 | ||
23 | You should have received a copy of the GNU Lesser General Public License | |
24 | along with systemd; If not, see <http://www.gnu.org/licenses/>. | |
25 | --> | |
26 | ||
27 | <refentry id="systemd.offline-updates"> | |
28 | <refentryinfo> | |
29 | <title>systemd.offline-updates</title> | |
30 | <productname>systemd</productname> | |
31 | ||
32 | <authorgroup> | |
33 | <author> | |
34 | <contrib>Developer</contrib> | |
35 | <firstname>Lennart</firstname> | |
36 | <surname>Poettering</surname> | |
37 | <email>lennart@poettering.net</email> | |
38 | </author> | |
39 | </authorgroup> | |
40 | </refentryinfo> | |
41 | ||
42 | <refmeta> | |
43 | <refentrytitle>systemd.offline-updates</refentrytitle> | |
44 | <manvolnum>7</manvolnum> | |
45 | </refmeta> | |
46 | ||
47 | <refnamediv> | |
48 | <refname>systemd.offline-updates</refname> | |
49 | <refpurpose>Implementation of offline updates in systemd</refpurpose> | |
50 | </refnamediv> | |
51 | ||
52 | <refsect1> | |
53 | <title>Implementing Offline System Updates</title> | |
54 | ||
45f0c64e | 55 | <para>This man page describes how to implement "offline" system updates with systemd. By "offline" |
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56 | OS updates we mean package installations and updates that are run with the system booted into a |
57 | special system update mode, in order to avoid problems related to conflicts of libraries and | |
58 | services that are currently running with those on disk. This document is inspired by this | |
59 | <ulink url="https://wiki.gnome.org/Design/OS/SoftwareUpdates">GNOME design whiteboard</ulink>. | |
60 | </para> | |
61 | ||
62 | <para>The logic:</para> | |
63 | ||
64 | <orderedlist> | |
65 | <listitem> | |
66 | <para>The package manager prepares system updates by downloading all (RPM or DEB or | |
67 | whatever) packages to update off-line in a special directory | |
68 | <filename noindex="true">/var/lib/system-update</filename> (or | |
69 | another directory of the package/upgrade manager's choice).</para> | |
70 | </listitem> | |
71 | ||
72 | <listitem> | |
73 | <para>When the user OK'ed the update, the symlink <filename>/system-update</filename> is | |
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74 | created that points to <filename noindex="true">/var/lib/system-update</filename> (or |
75 | wherever the directory with the upgrade files is located) and the system is rebooted. This | |
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76 | symlink is in the root directory, since we need to check for it very early at boot, at a |
77 | time where <filename>/var</filename> is not available yet.</para> | |
78 | </listitem> | |
79 | ||
80 | <listitem> | |
45f0c64e | 81 | <para>Very early in the new boot |
2ac5fe95 | 82 | <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry> |
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83 | checks whether <filename>/system-update</filename> exists. If so, it (temporarily and for |
84 | this boot only) redirects (i.e. symlinks) <filename>default.target</filename> to | |
fec0ccea | 85 | <filename>system-update.target</filename>, a special target that pulls in the base system |
45f0c64e ZJS |
86 | (i.e. <filename>sysinit.target</filename>, so that all file systems are mounted but little |
87 | else) and the system update units.</para> | |
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88 | </listitem> |
89 | ||
90 | <listitem> | |
2b656050 ZJS |
91 | <para>The system now continues to boot into <filename>default.target</filename>, and |
92 | thus into <filename>system-update.target</filename>. This target pulls in all system | |
93 | update units. Only one service should perform an update (see the next point), and all | |
94 | the other ones should exit cleanly with a "success" return code and without doing | |
95 | anything. Update services should be ordered after <filename>sysinit.target</filename> | |
301a21a8 | 96 | so that the update starts after all file systems have been mounted.</para> |
45f0c64e ZJS |
97 | </listitem> |
98 | ||
99 | <listitem> | |
2b656050 | 100 | <para>As the first step, an update service should check if the |
61233823 | 101 | <filename>/system-update</filename> symlink points to the location used by that update |
2b656050 | 102 | service. In case it does not exist or points to a different location, the service must exit |
45f0c64e | 103 | without error. It is possible for multiple update services to be installed, and for multiple |
2b656050 | 104 | update services to be launched in parallel, and only the one that corresponds to the tool |
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105 | that <emphasis>created</emphasis> the symlink before reboot should perform any actions. It |
106 | is unsafe to run multiple updates in parallel.</para> | |
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107 | </listitem> |
108 | ||
109 | <listitem> | |
2b656050 ZJS |
110 | <para>The update service should now do its job. If applicable and possible, it should |
111 | create a file system snapshot, then install all packages. After completion (regardless | |
112 | whether the update succeeded or failed) the machine must be rebooted, for example by | |
113 | calling <command>systemctl reboot</command>. In addition, on failure the script should | |
114 | revert to the old file system snapshot (without the symlink).</para> | |
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115 | </listitem> |
116 | ||
117 | <listitem> | |
2b656050 ZJS |
118 | <para>The upgrade scripts should exit only after the update is finished. It is expected |
119 | that the service which performs the upgrade will cause the machine to reboot after it | |
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120 | is done. If the <filename>system-update.target</filename> is successfully reached, i.e. |
121 | all update services have run, and the <filename>/system-update</filename> symlink still | |
122 | exists, it will be removed and the machine rebooted as a safety measure.</para> | |
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123 | </listitem> |
124 | ||
125 | <listitem> | |
126 | <para>After a reboot, now that the <filename>/system-update</filename> symlink is gone, | |
127 | the generator won't redirect <filename>default.target</filename> anymore and the system | |
128 | now boots into the default target again.</para> | |
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129 | </listitem> |
130 | </orderedlist> | |
131 | </refsect1> | |
132 | ||
133 | <refsect1> | |
134 | <title>Recommendations</title> | |
135 | ||
136 | <orderedlist> | |
137 | <listitem> | |
138 | <para>To make things a bit more robust we recommend hooking the update script into | |
139 | <filename>system-update.target</filename> via a <filename noindex='true'>.wants/</filename> | |
140 | symlink in the distribution package, rather than depending on <command>systemctl | |
141 | enable</command> in the postinst scriptlets of your package. More specifically, for your | |
142 | update script create a .service file, without [Install] section, and then add a symlink like | |
143 | <filename noindex='true'>/usr/lib/systemd/system-update.target.wants/foobar.service</filename> | |
144 | → <filename noindex='true'>../foobar.service</filename> to your package.</para> | |
145 | </listitem> | |
146 | ||
147 | <listitem> | |
45f0c64e ZJS |
148 | <para>Make sure to remove the <filename>/system-update</filename> symlink as early as |
149 | possible in the update script to avoid reboot loops in case the update fails.</para> | |
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150 | </listitem> |
151 | ||
152 | <listitem> | |
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153 | <para>Use <varname>FailureAction=reboot</varname> in the service file for your update script |
154 | to ensure that a reboot is automatically triggered if the update fails. | |
155 | <varname>FailureAction=</varname> makes sure that the specified unit is activated if your | |
156 | script exits uncleanly (by non-zero error code, or signal/coredump). If your script succeeds | |
157 | you should trigger the reboot in your own code, for example by invoking logind's | |
b17649ee | 158 | <command>Reboot()</command> call or calling <command>systemctl reboot</command>. See |
28a0ad81 | 159 | <ulink url="https://www.freedesktop.org/wiki/Software/systemd/logind">logind dbus API</ulink> |
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160 | for details.</para> |
161 | </listitem> | |
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162 | |
163 | <listitem> | |
164 | <para>The update service should declare <varname>DefaultDependencies=false</varname>, | |
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165 | <varname>Requires=sysinit.target</varname>, <varname>After=sysinit.target</varname>, |
166 | and explicitly pull in any other services it requires.</para> | |
45f0c64e | 167 | </listitem> |
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168 | </orderedlist> |
169 | </refsect1> | |
170 | ||
171 | <refsect1> | |
172 | <title>See also</title> | |
173 | ||
174 | <para> | |
c5915c63 | 175 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, |
45f0c64e | 176 | <citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>, |
2ac5fe95 | 177 | <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
0a07667d | 178 | <citerefentry project='mankier'><refentrytitle>dnf.plugin.system-upgrade</refentrytitle><manvolnum>8</manvolnum></citerefentry> |
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179 | </para> |
180 | </refsect1> | |
181 | </refentry> |