man/systemd.offline-updates.xml

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   5
   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> or
  44         <filename>/etc/system-update</filename> is created that points to
  45         <filename index="false">/var/lib/system-update</filename> (or wherever the directory with
  46         the upgrade files is located) and the system is rebooted. This symlink is in the root
  47         directory, since we need to check for it very early at boot, at a time where
  48         <filename>/var/</filename> is not available yet.</para>
  49       </listitem>
  50
  51       <listitem>
  52         <para>Very early in the new boot
  53         <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  54         checks whether <filename>/system-update</filename> or
  55         <filename>/etc/system-update</filename> exists. If so, it (temporarily and for this boot
  56         only) redirects (i.e. symlinks) <filename>default.target</filename> to
  57         <filename>system-update.target</filename>, a special target that pulls in the base system
  58         (i.e. <filename>sysinit.target</filename>, so that all file systems are mounted but little
  59         else) and the system update units.</para>
  60       </listitem>
  61
  62       <listitem>
  63         <para>The system now continues to boot into <filename>default.target</filename>, and
  64         thus into <filename>system-update.target</filename>. This target pulls in all system
  65         update units. Only one service should perform an update (see the next point), and all
  66         the other ones should exit cleanly with a "success" return code and without doing
  67         anything. Update services should be ordered after <filename>sysinit.target</filename>
  68         so that the update starts after all file systems have been mounted.</para>
  69       </listitem>
  70
  71       <listitem>
  72         <para>As the first step, an update service should check if the
  73         <filename>/system-update</filename> or <filename>/etc/system-update</filename> symlink
  74         points to the location used by that update service. In case it does not exist or points to a
  75         different location, the service must exit without error. It is possible for multiple update
  76         services to be installed, and for multiple update services to be launched in parallel, and
  77         only the one that corresponds to the tool that <emphasis>created</emphasis> the symlink
  78         before reboot should perform any actions. It is unsafe to run multiple updates in
  79         parallel.</para>
  80       </listitem>
  81
  82       <listitem>
  83         <para>The update service should now do its job. If applicable and possible, it should
  84         create a file system snapshot, then install all packages.  After completion (regardless
  85         whether the update succeeded or failed) the machine must be rebooted, for example by
  86         calling <command>systemctl reboot</command>. In addition, on failure the script should
  87         revert to the old file system snapshot (without the symlink).</para>
  88       </listitem>
  89
  90       <listitem>
  91         <para>The update scripts should exit only after the update is finished. It is expected
  92         that the service which performs the update will cause the machine to reboot after it
  93         is done. If the <filename>system-update.target</filename> is successfully reached, i.e.
  94         all update services have run, and the <filename>/system-update</filename> or
  95         <filename>/etc/system-update</filename> symlink still exists, it will be removed and
  96         the machine rebooted as a safety measure.</para>
  97       </listitem>
  98
  99       <listitem>
 100         <para>After a reboot, now that the <filename>/system-update</filename> and
 101         <filename>/etc/system-update</filename> symlink is gone, the generator won't redirect
 102         <filename>default.target</filename> anymore and the system now boots into the default
 103         target again.</para>
 104       </listitem>
 105     </orderedlist>
 106   </refsect1>
 107
 108   <refsect1>
 109     <title>Recommendations</title>
 110
 111     <orderedlist>
 112       <listitem>
 113         <para>To make things a bit more robust we recommend hooking the update script into
 114         <filename>system-update.target</filename> via a <filename index="false">.wants/</filename>
 115         symlink in the distribution package, rather than depending on <command>systemctl
 116         enable</command> in the postinst scriptlets of your package. More specifically, for your
 117         update script create a .service file, without [Install] section, and then add a symlink like
 118         <filename index="false">/usr/lib/systemd/system/system-update.target.wants/foobar.service</filename>
 119         → <filename index="false">../foobar.service</filename> to your package.</para>
 120       </listitem>
 121
 122       <listitem>
 123         <para>Make sure to remove the <filename>/system-update</filename> and
 124         <filename>/etc/system-update</filename> symlinks as early as possible in the update
 125         script to avoid reboot loops in case the update fails.</para>
 126       </listitem>
 127
 128       <listitem>
 129         <para>Use <varname>FailureAction=reboot</varname> in the service file for your update script
 130         to ensure that a reboot is automatically triggered if the update fails.
 131         <varname>FailureAction=</varname> makes sure that the specified unit is activated if your
 132         script exits uncleanly (by non-zero error code, or signal/coredump). If your script succeeds
 133         you should trigger the reboot in your own code, for example by invoking logind's
 134         <command>Reboot()</command> call or calling <command>systemctl reboot</command>. See
 135         <citerefentry><refentrytitle>org.freedesktop.login1</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 136         for details about the logind D-Bus API.</para>
 137       </listitem>
 138
 139       <listitem>
 140         <para>The update service should declare <varname>DefaultDependencies=no</varname>,
 141         <varname>Requires=sysinit.target</varname>, <varname>After=sysinit.target</varname>,
 142         <varname>After=system-update-pre.target</varname>, <varname>Before=system-update.target</varname>
 143         and explicitly pull in any other services it requires.</para>
 144       </listitem>
 145
 146       <listitem>
 147         <para>It may be desirable to always run an auxiliary unit when booting
 148         into offline-updates mode, which itself does not install updates. To
 149         do this create a .service file with
 150         <varname>Wants=system-update-pre.target</varname> and
 151         <varname>Before=system-update-pre.target</varname> and add a symlink
 152         to that file under
 153         <filename index="false">/usr/lib/systemd/system-update.target.wants</filename>
 154         .</para>
 155       </listitem>
 156     </orderedlist>
 157   </refsect1>
 158
 159   <refsect1>
 160     <title>See Also</title>
 161
 162     <para><simplelist type="inline">
 163       <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
 164       <member><citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
 165       <member><citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
 166       <member><citerefentry project='mankier'><refentrytitle>dnf.plugin.system-upgrade</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
 167     </simplelist></para>
 168   </refsect1>
 169 </refentry>