systemd.offline-updates — Implementation of offline updates in systemd
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 GNOME design whiteboard.
The package manager prepares system updates by downloading all (RPM or DEB or
whatever) packages to update off-line in a special directory
another directory of the package/upgrade manager's choice).
When the user OK'ed the update, the symlink
created that points to
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
/var is not available yet.
Very early in the new boot
/system-update exists. If so, it (temporarily and for
this boot only) redirects (i.e. symlinks)
system-update.target, a special target that pulls in the base system
sysinit.target, so that all file systems are mounted but little
else) and the system update units.
The system now continues to boot into
system-update.target. This target pulls in all system
update units. Only one service should perform an update (see the next point), and all
the other ones should exit cleanly with a "success" return code and without doing
anything. Update services should be ordered after
so that the update starts after all file systems have been mounted.
As the first step, an update service should check if the
/system-update symlink points to the location used by that update
service. In case it does not exist or points to a different location, the service must exit
without error. It is possible for multiple update services to be installed, and for multiple
update services to be launched in parallel, and only the one that corresponds to the tool
that created the symlink before reboot should perform any actions. It
is unsafe to run multiple updates in parallel.
The update service 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 systemctl reboot. In addition, on failure the script should revert to the old file system snapshot (without the symlink).
The upgrade scripts should exit only after the update is finished. It is expected
that the service which performs the upgrade will cause the machine to reboot after it
is done. If the
system-update.target is successfully reached, i.e.
all update services have run, and the
/system-update symlink still
exists, it will be removed and the machine rebooted as a safety measure.
After a reboot, now that the
/system-update symlink is gone,
the generator won't redirect
default.target anymore and the system
now boots into the default target again.
To make things a bit more robust we recommend hooking the update script into
system-update.target via a
symlink in the distribution package, rather than depending on systemctl
enable 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
../foobar.service to your package.
Make sure to remove the
/system-update symlink as early as
possible in the update script to avoid reboot loops in case the update fails.
FailureAction=reboot in the service file for your update script
to ensure that a reboot is automatically triggered if the update fails.
FailureAction= 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
Reboot() call or calling systemctl reboot. See
logind dbus API
The update service should declare
and explicitly pull in any other services it requires.