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