Has a system that incorporated a rule engine ever been TRULY successful? [closed]

Question

Our system (exotic commodity derivative trade capture and risk management) is being redeveloped shortly. One proposal that I have heard is that a rule engine will be incorporated to make it easier for the end-users (commodities traders, so fairly sophisticated) to make certain changes to the business logic.

I am a little skeptical of rules engines. The agilist in me wonders if they are just a technical solution to a process problem... ie. it takes too long for our developers to respond to the business's need for change. The solution to that problem should be a more collaborative approach to development, better test coverage, more agile practices all around.

Hearing about situations where a rule engine was truly a boon (especially in a trading environment) would be certainly helpful.

Answer 1

I've seen two applications that used the Blaze Rete engine from Fair Issac.

One application slammed thousands of rules into a single knowledge base, had terrible memory problems, has become a black box that few understand. I would not call that a success, but it is running in production.

Another application used decision trees to represent on the order of hundreds of questions on a medical form to disposition clients. It was done so elegantly that business people can update the rules as needed, without having to involve a developer. (Still has to be deployed by one, though.) I'd call that a great success.

So it depends on how well focused the problem is, the size of the rule set, the knowledge of the developers. My prejudice is that simply making a rules engine a single point of failure and dumping rules into it probably isn't a good approach. I'd start with a data-driven or table-driven approach and grow that until a rules engine was needed. I'd also strive to encapsulate the rules engine as part of the behavior of an object. I'd hide the rules engine from users and try to partition the rules space into the domain model.

Answer 2

I don't know if I'd say they're ever truly a boon, but I think they can certainly be valuable. I worked on a system for a few years in the insurance industry, where a rules engine was employed quite successfully to allow the business users to create rules that determined what policies were legal, depending on the state.

For instance, if you had to have a copay in certain states, or certain combinations of deductible and copay were not allowed, either because of product considerations, or because it was simply illegal due to state law.

The number of states that the company operated in, along with the constant change in rules (quarterly) would make this a dizzying coding practice. More importantly, it's not in the expertise of a programmer. It adds extra pointless communication where the end user is describing the rule to be put in effect to a programmer who is not an insurance industry expert like they are.

Designed correctly, a rules engine can still enable a workflow system that allows for good testing. In this case, the rules were stored in a database, and there were QA and PROD databases. So the BA's could test their rules in QA, and then promote them to PROD.

As with anything, its usually about the implementation, and not the actual technique.