man/systemd.offline-updates.xml

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