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2<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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4<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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 index="false">/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 index="false">/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 update scripts should exit only after the update is finished. It is expected
89        that the service which performs the update 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 index="false">.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 index="false">/usr/lib/systemd/system/system-update.target.wants/foobar.service</filename>
114        → <filename index="false">../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        <citerefentry><refentrytitle>org.freedesktop.login1</refentrytitle><manvolnum>5</manvolnum></citerefentry>
130        for details about the logind D-Bus API.</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>, <varname>Before=system-update.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 index="false">/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>
164