Prolog web programming
prolog, rule-engine
Solution
For SWI-Prolog, you could look to Thea2 which has support for SWRL in Prolog and can also be attached to external reasoners via JPL such as HermiT for OWL/SWRL reasoning, or Pellet, etc.
On a personal note, I have used JPL several times in the past to enable web-apps with a SWI-Prolog backend, which works just fine if you intend to program your web app using a language which is executable on a JVM, like Java, Groovy, or Scala, for example. Another alternative would be to hook SWI-Prolog into a C or C++ environment, which I haven't tried for a web-app.
If your web-app is using another development language that doesn't run on a JVM or in C/C++, then this mightn't be the right path for you as it seems to be a bit harder to connect a running SWI-Prolog environment to other language environments. However, that said, we have successfully implemented a SWI-Prolog-to-anything bridge using HTTP before, but this is less than ideal if performance is a necessity.
Problem
At work, there was a discussion of using prolog as the backend for a rules engine on a web-app. How would this get tied into existing systems? Are there available prolog libraries for other languages allowing the invocation of prolog modules?