Accumulate functions are completely pluggable in Drools. Drools ships with a few out of the box (min, max, sum, etc), but there is no perf difference between the out of the box and the user defined ones, as the out of the box functions are implemented in the exact same way and you can even use them as examples to create your own functions. Check drools expert documentation on the "accumulate" section. Also, this blog shows how to implement your own: http://blog.athico.com/2009/06/how-to-implement-accumulate-functions.html []s Edson 2010/1/11 Ahmed Hafsi <hafsi(a)fzi.de&gt; -- Edson Tirelli JBoss Drools Core Development JBoss by Red Hat @ www.jboss.com