A transactional update (also known as atomic upgrade) is an update that
is atomic:
The update does not influence the running system.
The machine can be powered off at any time. When powered on again either the unmodified old state or the new state is active. It is not possible to have a running system in an intermediate state.
can be rolled back:
If the upgrade fails or if a newer software version turns out to not be compatible with your infrastructure, the system can quickly be restored to a previous state.