Merge pull request #2849 from keszybz/offline-updates

Offline updates man page

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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-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 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>systemct 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-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>dnf.plugin.system-upgrade</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+    </para>
+  </refsect1>
+</refentry>