Updated Sept 19 2008: more links for Ruleby

Not a major category among the many rule engines implemented in Java and other languages, but some of the implementations are solid and interesting.

In fact, Ruby rule engines are particularly interesting because my hosting service ( Hostmonster ) provides them as installable Ruby gems. The Radiant CMS is also available as an application platform to connect the rule engine to the outside world.

Ruleby is probably the most popular and mature at this time, in part because it is well-implemented and written in pure Ruby. It is also a clean implementation of the forward-chaining RETE algorithm. No backchaining.

Yet another reason to take notice of Ruleby is that founder Joe Kutner did a really excellent presentation on rule engines at the Ruby Hoedown 2008. It's about a half-hour long and one could spend many hours slogging though web sites and learn less about rule engines. Highly recommended.

Ruby Rools is not based on RETE, but has a rule scheduling mechanism.

There are also some small Ruby workflow engines, such as the OpenWFEru workflow engine, that may be interesting.