When should you NOT use a Rules Engine?
I get very nervous when I see people using very large rule sets (e.g., on the order of thousands of rules in a single rule set). This often happens when the rules engine is a singleton sitting in the center of the enterprise in the hope that keeping rules DRY will make them accessible to many apps that require them. I would defy anyone to tell me that a Rete rules engine with that many rules is well-understood. I'm not aware of any tools that can check to ensure that conflicts don't exist.
I think partitioning rules sets to keep them small is a better option. Aspects can be a way to share a common rule set among many objects.
I prefer a simpler, more data driven approach wherever possible.
The one poit I've noticed to be "the double edged sword" is:
placing the logic in hands of non technical staff
I've seen this work great, when you have one or two multidisciplinary geniuses on the non technical side, but I've also seen the lack of technicity leading to bloat, more bugs, and in general 4x the development/maintenance cost.
Thus you need to consider your user-base seriously.
I will give 2 examples from personal experience where using a Rules Engine was a bad idea, maybe that will help:-
- On a past project, I noticed that the rules files (the project used Drools) contained a lot of java code, including loops, functions etc. They were essentially java files masquerading as rules file. When I asked the architect on his reasoning for the design I was told that the "Rules were never intended to be maintained by business users".
Lesson: They are called "Business Rules" for a reason, do not use rules when you cannot design a system that can be easily maintained/understood by Business users.
- Another case; The project used rules because requirements were poorly defined/understood and changed often. The development team's solution was to use rules extensively to avoid frequent code deploys.
Lesson: Requirements tend to change a lot during initial release changes and do not warrant usage of rules. Use rules when your business changes often (not requirements). Eg:- A software that does your taxes will change every year as taxation laws change and usage of rules is an excellent idea. Release 1.0 of an web app will change often as users identify new requirements but will stabilize over time. Do not use rules as an alternative to code deploy.