I was wondering if you know of any way to use prolog for stream processing, that is, some kind of reactive programming, or at least to let a query run on a knowledge base that is continuously updated (effectively a stream), and continuously output the output of the reasoning?
Anything implemented in the popular "prologs", such as SWI-prolog?
You can use Logtalk's support for event-driven programming to define monitors that watch for knowledge base update events and react accordingly. You can run Logtalk using most Prolog systems as the backed compiler, including SWI-Prolog.
The event-driven features are described e.g. in the user manual: http://logtalk.org/manuals/userman/events.html
The current distribution contains some examples of using events and monitors. An interesting one considering your question is the bricks example:
https://github.com/LogtalkDotOrg/logtalk3/tree/master/examples/bricks
Running this example first and then looking at its code should give you as good idea of what you can do with system wide events and monitors.