man/systemd.offline-updates.xml

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   6 <refentry id="systemd.offline-updates">
   7   <refentryinfo>
   8     <title>systemd.offline-updates</title>
   9     <productname>systemd</productname>
  10   </refentryinfo>
  11
  12   <refmeta>
  13     <refentrytitle>systemd.offline-updates</refentrytitle>
  14     <manvolnum>7</manvolnum>
  15   </refmeta>
  16
  17   <refnamediv>
  18     <refname>systemd.offline-updates</refname>
  19     <refpurpose>Implementation of offline updates in systemd</refpurpose>
  20   </refnamediv>
  21
  22   <refsect1>
  23     <title>Implementing Offline System Updates</title>
  24
  25     <para>This man page describes how to implement "offline" system updates with systemd. By "offline"
  26     OS updates we mean package installations and updates that are run with the system booted into a
  27     special system update mode, in order to avoid problems related to conflicts of libraries and
  28     services that are currently running with those on disk. This document is inspired by this
  29     <ulink url="https://wiki.gnome.org/Design/OS/SoftwareUpdates">GNOME design whiteboard</ulink>.
  30     </para>
  31
  32     <para>The logic:</para>
  33
  34     <orderedlist>
  35       <listitem>
  36         <para>The package manager prepares system updates by downloading all (.rpm or .deb or
  37         whatever) packages to update off-line in a special directory
  38         <filename index="false">/var/lib/system-update</filename> (or
  39         another directory of the package/upgrade manager's choice).</para>
  40       </listitem>
  41
  42       <listitem>
  43         <para>When the user OK'ed the update, the symlink <filename>/system-update</filename> is
  44         created that points to <filename index="false">/var/lib/system-update</filename> (or
  45         wherever the directory with the upgrade files is located) and the system is rebooted. This
  46         symlink is in the root directory, since we need to check for it very early at boot, at a
  47         time where <filename>/var</filename> is not available yet.</para>
  48       </listitem>
  49
  50       <listitem>
  51         <para>Very early in the new boot
  52         <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  53         checks whether <filename>/system-update</filename> exists. If so, it (temporarily and for
  54         this boot only) redirects (i.e. symlinks) <filename>default.target</filename> to
  55         <filename>system-update.target</filename>, a special target that pulls in the base system
  56         (i.e. <filename>sysinit.target</filename>, so that all file systems are mounted but little
  57         else) and the system update units.</para>
  58       </listitem>
  59
  60       <listitem>
  61         <para>The system now continues to boot into <filename>default.target</filename>, and
  62         thus into <filename>system-update.target</filename>. This target pulls in all system
  63         update units. Only one service should perform an update (see the next point), and all
  64         the other ones should exit cleanly with a "success" return code and without doing
  65         anything. Update services should be ordered after <filename>sysinit.target</filename>
  66         so that the update starts after all file systems have been mounted.</para>
  67       </listitem>
  68
  69       <listitem>
  70         <para>As the first step, an update service should check if the
  71         <filename>/system-update</filename> symlink points to the location used by that update
  72         service. In case it does not exist or points to a different location, the service must exit
  73         without error. It is possible for multiple update services to be installed, and for multiple
  74         update services to be launched in parallel, and only the one that corresponds to the tool
  75         that <emphasis>created</emphasis> the symlink before reboot should perform any actions. It
  76         is unsafe to run multiple updates in parallel.</para>
  77       </listitem>
  78
  79       <listitem>
  80         <para>The update service should now do its job. If applicable and possible, it should
  81         create a file system snapshot, then install all packages.  After completion (regardless
  82         whether the update succeeded or failed) the machine must be rebooted, for example by
  83         calling <command>systemctl reboot</command>. In addition, on failure the script should
  84         revert to the old file system snapshot (without the symlink).</para>
  85       </listitem>
  86
  87       <listitem>
  88         <para>The update scripts should exit only after the update is finished. It is expected
  89         that the service which performs the update will cause the machine to reboot after it
  90         is done. If the <filename>system-update.target</filename> is successfully reached, i.e.
  91         all update services have run, and the <filename>/system-update</filename> symlink still
  92         exists, it will be removed and the machine rebooted as a safety measure.</para>
  93       </listitem>
  94
  95       <listitem>
  96         <para>After a reboot, now that the <filename>/system-update</filename> symlink is gone,
  97         the generator won't redirect <filename>default.target</filename> anymore and the system
  98         now boots into the default target again.</para>
  99       </listitem>
 100     </orderedlist>
 101   </refsect1>
 102
 103   <refsect1>
 104     <title>Recommendations</title>
 105
 106     <orderedlist>
 107       <listitem>
 108         <para>To make things a bit more robust we recommend hooking the update script into
 109         <filename>system-update.target</filename> via a <filename index="false">.wants/</filename>
 110         symlink in the distribution package, rather than depending on <command>systemctl
 111         enable</command> in the postinst scriptlets of your package. More specifically, for your
 112         update script create a .service file, without [Install] section, and then add a symlink like
 113         <filename index="false">/usr/lib/systemd/system-update.target.wants/foobar.service</filename>
 114         → <filename index="false">../foobar.service</filename> to your package.</para>
 115       </listitem>
 116
 117       <listitem>
 118         <para>Make sure to remove the <filename>/system-update</filename> symlink as early as
 119         possible in the update script to avoid reboot loops in case the update fails.</para>
 120       </listitem>
 121
 122       <listitem>
 123         <para>Use <varname>FailureAction=reboot</varname> in the service file for your update script
 124         to ensure that a reboot is automatically triggered if the update fails.
 125         <varname>FailureAction=</varname> makes sure that the specified unit is activated if your
 126         script exits uncleanly (by non-zero error code, or signal/coredump). If your script succeeds
 127         you should trigger the reboot in your own code, for example by invoking logind's
 128         <command>Reboot()</command> call or calling <command>systemctl reboot</command>. See
 129         <ulink url="https://www.freedesktop.org/wiki/Software/systemd/logind">logind dbus API</ulink>
 130         for details.</para>
 131       </listitem>
 132
 133       <listitem>
 134         <para>The update service should declare <varname>DefaultDependencies=no</varname>,
 135         <varname>Requires=sysinit.target</varname>, <varname>After=sysinit.target</varname>,
 136         <varname>After=system-update-pre.target</varname>, <varname>Before=system-update.target</varname>
 137         and explicitly pull in any other services it requires.</para>
 138       </listitem>
 139
 140       <listitem>
 141         <para>It may be desirable to always run an auxiliary unit when booting
 142         into offline-updates mode, which itself does not install updates. To
 143         do this create a .service file with
 144         <varname>Wants=system-update-pre.target</varname> and
 145         <varname>Before=system-update-pre.target</varname> and add a symlink
 146         to that file under
 147         <filename index="false">/usr/lib/systemd/system-update.target.wants</filename>
 148         .</para>
 149       </listitem>
 150     </orderedlist>
 151   </refsect1>
 152
 153   <refsect1>
 154     <title>See also</title>
 155
 156     <para>
 157       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 158       <citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 159       <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 160       <citerefentry project='mankier'><refentrytitle>dnf.plugin.system-upgrade</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 161     </para>
 162   </refsect1>
 163 </refentry>