JBoss Drools

Drools first started as a RETE implementation in Java to take business rules and make it easier for developers and end users to define, modify and deploy ever-changing business logic. With time it evolved into a Business Logic Integrated Platform where not only rules are first class citizens but processes and events too.

Drools 5 support flows whom in concert with business rules makes a very expressive and natural definition of business processes.

The latest addition is Complex Event Processing (CEP), it handles large volumes of events and by using temporal operators you can infer, detect and act on temporal and/or sequential behavior such as fraud detection, intelligent automated trading, etc.

If you haven't tried to yet, you're missing much!!

Drools is a solid rules engine that is good for very specific problems. Recently the project added additional functionality in Fusion and Flow to handlle temporal and event based problems. Unfortunately these add ons are like AMC Harley's - broken on the show room floor and leaking oil no matter what you do. Redhat suspended training on Drools because the technology couldn't even handle the examples, a painful lesson my team learned first hand. This is not a technology for serious enterprise problems.