man/systemd.offline-updates.xml

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