A backend is just a compiled
.so object that is
loaded at run-time and provides an interface to the underlying package
A backend converts an asynchronous request into either a new thread
in the same process, or executes external "glue" files that
can be written in any language.
Backends do not have to support all of the commands supported by PackageKit. Each backend advertises the commands and options it supports, and the frontend will only show UI elements for those commands.