I am old school drools 2.x and had created based on the HOUSE example created by own SMF DSL based on drools 2.x. Than had to take a sabbatical from that project. As of today, I have been asked to resurrect that project and not a whole lot of drools 2.x has made it to 4.x and of course would like to remain current. My implementation had an expression engine ( SableCC ) that I had fused in with a customized Semantic Module. I can get more information out for u , if there is a way to resuse some code. Any pointers as to how to go about doing this will be highly appreciated. http://blog.athico.com/2007/12/drools-extensible-process-definition.html seems to be an attempt to explain how the port can take place, but does not seem to have a lot of concreteness around it.