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authorLuke Shumaker <lukeshu@sbcglobal.net>2016-12-17 03:04:41 -0500
committerLuke Shumaker <lukeshu@sbcglobal.net>2016-12-17 03:04:41 -0500
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tree6cdd53846655b04b178b4b8057c915c61a9cb525 /man/systemd.offline-updates.xml
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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
- Copyright 2016 Zbigniew Jędrzejewski-Szmek
-
- 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 man page describes how to implement "offline" system 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 directory with the upgrade files is located) 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
- <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
- 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 special target that is pulls 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 system update unit,
- which starts the system update script after all file systems have been mounted.</para>
- </listitem>
-
- <listitem>
- <para>As the first step, the update script should check if the
- <filename>/system-update</filename> symlink points to the location used by that update
- script. In case it does not exists or points to a different location, the script must exit
- without error. It is possible for multiple update services to be installed, and for multiple
- update scripts to be launched in parallel, and only the one that corresponds to the tool
- that <emphasis>created</emphasis> the symlink before reboot should perform any actions. It
- is unsafe to run multiple updates in parallel.</para>
- </listitem>
-
- <listitem>
- <para>The update script should now do its job. If applicable and possible, it should
- create a file system snapshot, then install all packages.
- After completion (regardless whether the update succeeded or failed) the machine
- must be rebooted, for example by calling <command>systemctl reboot</command>.
- In addition, on failure the script should revert to the old file system snapshot
- (without the symlink).</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 as early as
- possible in the update script to avoid reboot loops in case the update fails.</para>
- </listitem>
-
- <listitem>
- <para>Use <varname>FailureAction=reboot</varname> in the service file for your update script
- to ensure that a reboot is automatically triggered if the update fails.
- <varname>FailureAction=</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 or calling <command>systemctl reboot</command>. See
- <ulink url="http://www.freedesktop.org/wiki/Software/systemd/logind">logind dbus API</ulink>
- for details.</para>
- </listitem>
-
- <listitem>
- <para>The update service should declare <varname>DefaultDependencies=false</varname>,
- and pull in any services it requires explicitly.</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>,
- <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
- <citerefentry project='mankier'><refentrytitle>dnf.plugin.system-upgrade</refentrytitle><manvolnum>8</manvolnum></citerefentry>
- </para>
- </refsect1>
-</refentry>