MF (Martin Fowler) Bliki: RulesEngine:
"Even so, there's still value in a BusinessReadableDSL, and indeed this is an area where I do see value in this computational model. But here too lie dragons. The biggest one is that while it can make sense to cast your eyes down a list of rules and see that each one makes sense, the interaction of rules can often be quite complex - particularly with chaining - where the actions of rules changes the state on which other rules' conditions are based. So I often hear that it was easy to set up a rules system, but very hard to maintain it because nobody can understand this implicit program flow. This is the dark side of leaving the imperative computational model. For all the faults of imperative code, it's relatively easy to understand how it works. With a production rule system, it seems easy to get to a point where a simple change in one place causes lots unintended consequences, which rarely work out well."