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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 | <!-- | |
572eb058 | 6 | SPDX-License-Identifier: LGPL-2.1+ |
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7 | --> |
8 | ||
9 | <refentry id="systemd.offline-updates"> | |
10 | <refentryinfo> | |
11 | <title>systemd.offline-updates</title> | |
12 | <productname>systemd</productname> | |
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13 | </refentryinfo> |
14 | ||
15 | <refmeta> | |
16 | <refentrytitle>systemd.offline-updates</refentrytitle> | |
17 | <manvolnum>7</manvolnum> | |
18 | </refmeta> | |
19 | ||
20 | <refnamediv> | |
21 | <refname>systemd.offline-updates</refname> | |
22 | <refpurpose>Implementation of offline updates in systemd</refpurpose> | |
23 | </refnamediv> | |
24 | ||
25 | <refsect1> | |
26 | <title>Implementing Offline System Updates</title> | |
27 | ||
45f0c64e | 28 | <para>This man page describes how to implement "offline" system updates with systemd. By "offline" |
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29 | OS updates we mean package installations and updates that are run with the system booted into a |
30 | special system update mode, in order to avoid problems related to conflicts of libraries and | |
31 | services that are currently running with those on disk. This document is inspired by this | |
32 | <ulink url="https://wiki.gnome.org/Design/OS/SoftwareUpdates">GNOME design whiteboard</ulink>. | |
33 | </para> | |
34 | ||
35 | <para>The logic:</para> | |
36 | ||
37 | <orderedlist> | |
38 | <listitem> | |
39 | <para>The package manager prepares system updates by downloading all (RPM or DEB or | |
40 | whatever) packages to update off-line in a special directory | |
41 | <filename noindex="true">/var/lib/system-update</filename> (or | |
42 | another directory of the package/upgrade manager's choice).</para> | |
43 | </listitem> | |
44 | ||
45 | <listitem> | |
46 | <para>When the user OK'ed the update, the symlink <filename>/system-update</filename> is | |
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47 | created that points to <filename noindex="true">/var/lib/system-update</filename> (or |
48 | wherever the directory with the upgrade files is located) and the system is rebooted. This | |
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49 | symlink is in the root directory, since we need to check for it very early at boot, at a |
50 | time where <filename>/var</filename> is not available yet.</para> | |
51 | </listitem> | |
52 | ||
53 | <listitem> | |
45f0c64e | 54 | <para>Very early in the new boot |
2ac5fe95 | 55 | <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry> |
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56 | checks whether <filename>/system-update</filename> exists. If so, it (temporarily and for |
57 | this boot only) redirects (i.e. symlinks) <filename>default.target</filename> to | |
fec0ccea | 58 | <filename>system-update.target</filename>, a special target that pulls in the base system |
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59 | (i.e. <filename>sysinit.target</filename>, so that all file systems are mounted but little |
60 | else) and the system update units.</para> | |
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61 | </listitem> |
62 | ||
63 | <listitem> | |
2b656050 ZJS |
64 | <para>The system now continues to boot into <filename>default.target</filename>, and |
65 | thus into <filename>system-update.target</filename>. This target pulls in all system | |
66 | update units. Only one service should perform an update (see the next point), and all | |
67 | the other ones should exit cleanly with a "success" return code and without doing | |
68 | anything. Update services should be ordered after <filename>sysinit.target</filename> | |
301a21a8 | 69 | so that the update starts after all file systems have been mounted.</para> |
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70 | </listitem> |
71 | ||
72 | <listitem> | |
2b656050 | 73 | <para>As the first step, an update service should check if the |
61233823 | 74 | <filename>/system-update</filename> symlink points to the location used by that update |
2b656050 | 75 | service. In case it does not exist or points to a different location, the service must exit |
45f0c64e | 76 | without error. It is possible for multiple update services to be installed, and for multiple |
2b656050 | 77 | update services to be launched in parallel, and only the one that corresponds to the tool |
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78 | that <emphasis>created</emphasis> the symlink before reboot should perform any actions. It |
79 | is unsafe to run multiple updates in parallel.</para> | |
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80 | </listitem> |
81 | ||
82 | <listitem> | |
2b656050 ZJS |
83 | <para>The update service should now do its job. If applicable and possible, it should |
84 | create a file system snapshot, then install all packages. After completion (regardless | |
85 | whether the update succeeded or failed) the machine must be rebooted, for example by | |
86 | calling <command>systemctl reboot</command>. In addition, on failure the script should | |
87 | revert to the old file system snapshot (without the symlink).</para> | |
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88 | </listitem> |
89 | ||
90 | <listitem> | |
2b656050 ZJS |
91 | <para>The upgrade scripts should exit only after the update is finished. It is expected |
92 | that the service which performs the upgrade will cause the machine to reboot after it | |
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93 | is done. If the <filename>system-update.target</filename> is successfully reached, i.e. |
94 | all update services have run, and the <filename>/system-update</filename> symlink still | |
95 | exists, it will be removed and the machine rebooted as a safety measure.</para> | |
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96 | </listitem> |
97 | ||
98 | <listitem> | |
99 | <para>After a reboot, now that the <filename>/system-update</filename> symlink is gone, | |
100 | the generator won't redirect <filename>default.target</filename> anymore and the system | |
101 | now boots into the default target again.</para> | |
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102 | </listitem> |
103 | </orderedlist> | |
104 | </refsect1> | |
105 | ||
106 | <refsect1> | |
107 | <title>Recommendations</title> | |
108 | ||
109 | <orderedlist> | |
110 | <listitem> | |
111 | <para>To make things a bit more robust we recommend hooking the update script into | |
112 | <filename>system-update.target</filename> via a <filename noindex='true'>.wants/</filename> | |
113 | symlink in the distribution package, rather than depending on <command>systemctl | |
114 | enable</command> in the postinst scriptlets of your package. More specifically, for your | |
115 | update script create a .service file, without [Install] section, and then add a symlink like | |
116 | <filename noindex='true'>/usr/lib/systemd/system-update.target.wants/foobar.service</filename> | |
117 | → <filename noindex='true'>../foobar.service</filename> to your package.</para> | |
118 | </listitem> | |
119 | ||
120 | <listitem> | |
45f0c64e ZJS |
121 | <para>Make sure to remove the <filename>/system-update</filename> symlink as early as |
122 | possible in the update script to avoid reboot loops in case the update fails.</para> | |
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123 | </listitem> |
124 | ||
125 | <listitem> | |
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126 | <para>Use <varname>FailureAction=reboot</varname> in the service file for your update script |
127 | to ensure that a reboot is automatically triggered if the update fails. | |
128 | <varname>FailureAction=</varname> makes sure that the specified unit is activated if your | |
129 | script exits uncleanly (by non-zero error code, or signal/coredump). If your script succeeds | |
130 | you should trigger the reboot in your own code, for example by invoking logind's | |
b17649ee | 131 | <command>Reboot()</command> call or calling <command>systemctl reboot</command>. See |
28a0ad81 | 132 | <ulink url="https://www.freedesktop.org/wiki/Software/systemd/logind">logind dbus API</ulink> |
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133 | for details.</para> |
134 | </listitem> | |
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135 | |
136 | <listitem> | |
964c4eda | 137 | <para>The update service should declare <varname>DefaultDependencies=no</varname>, |
2b656050 | 138 | <varname>Requires=sysinit.target</varname>, <varname>After=sysinit.target</varname>, |
520741d0 | 139 | <varname>After=system-update-pre.target</varname> |
2b656050 | 140 | and explicitly pull in any other services it requires.</para> |
45f0c64e | 141 | </listitem> |
bc3c5e16 HG |
142 | |
143 | <listitem> | |
144 | <para>It may be desirable to always run an auxiliary unit when booting | |
145 | into offline-updates mode, which itself does not install updates. To | |
146 | do this create a .service file with | |
147 | <varname>Wants=system-update-pre.target</varname> and | |
148 | <varname>Before=system-update-pre.target</varname> and add a symlink | |
149 | to that file under | |
150 | <filename noindex='true'>/usr/lib/systemd/system-update.target.wants</filename> | |
151 | .</para> | |
152 | </listitem> | |
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153 | </orderedlist> |
154 | </refsect1> | |
155 | ||
156 | <refsect1> | |
157 | <title>See also</title> | |
158 | ||
159 | <para> | |
c5915c63 | 160 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, |
45f0c64e | 161 | <citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>, |
2ac5fe95 | 162 | <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
0a07667d | 163 | <citerefentry project='mankier'><refentrytitle>dnf.plugin.system-upgrade</refentrytitle><manvolnum>8</manvolnum></citerefentry> |
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164 | </para> |
165 | </refsect1> | |
166 | </refentry> |