man/systemd.offline-updates.xml

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