man/systemd.offline-updates.xml

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   8   This file is part of systemd.
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  10   Copyright 2013 Lennart Poettering
  11   Copyright 2016 Zbigniew Jędrzejewski-Szmek
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  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
  55     <para>This man page describes how to implement "offline" system updates with systemd. By "offline"
  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
  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
  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>
  81         <para>Very early in the new boot
  82         <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  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
  85         <filename>system-update.target</filename>, a special target that pulls in the base system
  86         (i.e. <filename>sysinit.target</filename>, so that all file systems are mounted but little
  87         else) and the system update units.</para>
  88       </listitem>
  89
  90       <listitem>
  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>
  96         so that the update starts after all file systems have been mounted.</para>
  97       </listitem>
  98
  99       <listitem>
 100         <para>As the first step, an update service should check if the
 101         <filename>/system-update</filename> symlink points to the location used by that update
 102         service. In case it does not exist or points to a different location, the service must exit
 103         without error. It is possible for multiple update services to be installed, and for multiple
 104         update services to be launched in parallel, and only the one that corresponds to the tool
 105         that <emphasis>created</emphasis> the symlink before reboot should perform any actions. It
 106         is unsafe to run multiple updates in parallel.</para>
 107       </listitem>
 108
 109       <listitem>
 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>
 115       </listitem>
 116
 117       <listitem>
 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
 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>
 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>
 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>
 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>
 150       </listitem>
 151
 152       <listitem>
 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
 158         <command>Reboot()</command> call or calling <command>systemctl reboot</command>. See
 159         <ulink url="https://www.freedesktop.org/wiki/Software/systemd/logind">logind dbus API</ulink>
 160         for details.</para>
 161       </listitem>
 162
 163       <listitem>
 164         <para>The update service should declare <varname>DefaultDependencies=false</varname>,
 165         <varname>Requires=sysinit.target</varname>, <varname>After=sysinit.target</varname>,
 166         and explicitly pull in any other services it requires.</para>
 167       </listitem>
 168     </orderedlist>
 169   </refsect1>
 170
 171   <refsect1>
 172     <title>See also</title>
 173
 174     <para>
 175       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 176       <citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 177       <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 178       <citerefentry project='mankier'><refentrytitle>dnf.plugin.system-upgrade</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 179     </para>
 180   </refsect1>
 181 </refentry>