ctcreel

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.

Esper is an incredibly powerful application ready to tackle the most mind bending problems right out of the box. My team tackled some extremely difficult problems using Esper where we had repeatedly failed with other rules/CEP engines.

In addition to being a fantastic code base, the open source project is run very well and the code is rock solid.