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