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<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!--
This file is part of systemd.
Copyright 2013 Lennart Poettering
systemd is free software; you can redistribute it and/or modify it
under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or
(at your option) any later version.
systemd is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License
along with systemd; If not, see <http://www.gnu.org/licenses/>.
-->
<refentry id="systemd.offline-updates">
<refentryinfo>
<title>systemd.offline-updates</title>
<productname>systemd</productname>
<authorgroup>
<author>
<contrib>Developer</contrib>
<firstname>Lennart</firstname>
<surname>Poettering</surname>
<email>lennart@poettering.net</email>
</author>
</authorgroup>
</refentryinfo>
<refmeta>
<refentrytitle>systemd.offline-updates</refentrytitle>
<manvolnum>7</manvolnum>
</refmeta>
<refnamediv>
<refname>systemd.offline-updates</refname>
<refpurpose>Implementation of offline updates in systemd</refpurpose>
</refnamediv>
<refsect1>
<title>Implementing Offline System Updates</title>
<para>This is implemented starting with systemd 183.</para>
<para>Here are some guidelines how to implement "offline" OS updates with systemd. By "offline"
OS updates we mean package installations and updates that are run with the system booted into a
special system update mode, in order to avoid problems related to conflicts of libraries and
services that are currently running with those on disk. This document is inspired by this
<ulink url="https://wiki.gnome.org/Design/OS/SoftwareUpdates">GNOME design whiteboard</ulink>.
</para>
<para>The logic:</para>
<orderedlist>
<listitem>
<para>The package manager prepares system updates by downloading all (RPM or DEB or
whatever) packages to update off-line in a special directory
<filename noindex="true">/var/lib/system-update</filename> (or
another directory of the package/upgrade manager's choice).</para>
</listitem>
<listitem>
<para>When the user OK'ed the update, the symlink <filename>/system-update</filename> is
created that points to <filename noindex="true">/var/lib/system-update</filename>
(or wherever the upgrade package directory is called) and the system is rebooted. This
symlink is in the root directory, since we need to check for it very early at boot, at a
time where <filename>/var</filename> is not available yet.</para>
</listitem>
<listitem>
<para>Very early in the new boot a systemd generator checks whether
<filename>/system-update</filename> exists. If so, it (temporarily and for this boot only)
redirects (i.e. symlinks) <filename>default.target</filename> to
<filename>system-update.target</filename>, a new target that is intended to pull in the base
system (i.e. <filename>sysinit.target</filename>, so that all file systems are mounted but
little else) and the system update units.</para>
</listitem>
<listitem>
<para>The system now continues to boot into <filename>default.target</filename>, and thus
into <filename>system-update.target</filename>. This target pulls in the OS update script,
which is executed after all file systems are mounted.</para>
</listitem>
<listitem>
<para>The system update script now creates a btrfs snapshot (if possible), then installs all
RPMs. After completion (regardless whether the update succeeded or failed) the
/system-update symlink is removed. In addition, on failure it reverts to the old btrfs state
(modulo the aforementioned symlink), on success it leaves the newly made changes in
place.</para>
</listitem>
<listitem>
<para>The system is rebooted. Since the <filename>/system-update</filename> symlink is gone,
the generator won't redirect <filename>default.target</filename> after reboot and the
system now boots into the default target again.</para>
</listitem>
</orderedlist>
</refsect1>
<refsect1>
<title>Recommendations</title>
<orderedlist>
<listitem>
<para>To make things a bit more robust we recommend hooking the update script into
<filename>system-update.target</filename> via a <filename noindex='true'>.wants/</filename>
symlink in the distribution package, rather than depending on <command>systemctl
enable</command> in the postinst scriptlets of your package. More specifically, for your
update script create a .service file, without [Install] section, and then add a symlink like
<filename noindex='true'>/usr/lib/systemd/system-update.target.wants/foobar.service</filename>
→ <filename noindex='true'>../foobar.service</filename> to your package.</para>
</listitem>
<listitem>
<para>Make sure to remove the <filename>/system-update</filename> symlink early in the
update script to avoid reboot loops in case the update fails.</para>
</listitem>
<listitem>
<para>Use <varname>OnFailure=reboot.target</varname> in the service file for your update
script to ensure that a reboot is automatically triggered if the update fails.
<varname>OnFailure=</varname> makes sure that the specified unit is activated if your script
exits uncleanly (by non-zero error code, or signal/coredump). If your script succeeds you
should trigger the reboot in your own code, for example by invoking logind's
<command>Reboot()</command> call. See
<ulink url="http://www.freedesktop.org/wiki/Software/systemd/logind">logind dbus API</ulink>
for details.</para>
</listitem>
</orderedlist>
</refsect1>
<refsect1>
<title>See also</title>
<para>
<ulink url="http://www.freedesktop.org/wiki/Software/systemd/SystemUpdates/">Implementing Offline System Updates</ulink>,
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
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