Rules Engine - pros and cons

Question

I\'m auditing a project that uses what is called a Rules Engine. In short, it\'s a way to externalize business logic from application code.

This concept is entirely

Answer 1

The biggest complexity from my experience in Rule Engines is that:

1. from OOP POV it's a real pain to refactor and test rules written in a declarative language while you are refactoring code that affects them.
2. Often we should always think about the execution order of rules which turns into a mess when there are lots of them.
3. Some minor changes may trigger incorrect behaviour of rules leading to production bugs. In practice it's not always possible to cover all cases with tests up front.
4. Rules mutating objects used in other ones also increase complexity causing developers to break them into stages.