Is using Rule Engine to implement chain of rules [complex business logic] overkill?

Question

Recently, I am reading about the rule engines in JBOSS Drools Manual [ref - 2.2.5. Strong and Loose Coupling]. Below is the excerpt from it 'If your rules are all strongly coupled, the chances are that the rules will have future inflexibility, and more significantly, that perhaps a rule engine is overkill (as the logic is a clear chain of rules - and can be hard coded. [A Decision Tree may be in order]). This is not to say that strong or weak coupling is inherently bad, but it is a point to keep in mind when considering a rule engine and in how you capture the rules. "Loosely" coupled rules should result in a system that allows rules to be changed, removed and added without requiring changes to other rules that are unrelated.'

Does that mean, the rule engine is not suitable option to implement complex business logic [tightly coupled rules or chain of rules].

In my current project, we have chain of rules i.e. outcome of 1 rule decides the outcome of another rule and so on. The application has many internal variables to chain the rules. I thought rules engine might help to handle the complexity with the added advantage of declarative rules and dynamic business logic.

Discussion in this regard will be helpful ...

Answer 1

Some logic just isn't all that easy to get right even if you exactly know the order and nature of the tests you need to perform, and the actions that should result. Examples are Corporate Audits, Means testing for Assistance programs, Insurance regulations, etc.

Most Rules Engines today are beginning to include a Decision Table feature, which in all reality introduces some "limited strong coupling" (I don't know if that is really a term, but it is how I understand the effect in systems such as ILog, Drools, etc.). This is helpful because some tests are just related to other tests and decision tables are far better for structuring these tests than IF THEN ELSE structures.

Corticon (a proprietary Rules Engine) and DTRules (an open source Rules Engine) just toss the whole loosely coupled rules approach, and just build decision tables. The idea is that giving a nice structure for the construction of your decisions (which amount to decision trees underneath everything) is easier for many applications.

Answer 2

"Avoiding strong coupling" has nothing to do with "avoiding complexity".

What the documentation advocates is that you should not call one rule from another, instead, each rule should produce an outcome, and the outcome itself should trigger the next rule in chain. This way rules do not concern with what happens next, and instead they deal with facts (aha!). And if - instead of writing rules for facts - you focus on writing an ordinary flow of procedural logic, you do not really need the added complexity of a rules engine.

The difference is subtle, but not more subtle than rules like "don't put your business logic in view" or "don't put your database access code in your business logic".

Answer 3

In IBM ILOG Jrules there are many ways you can represent your rules.

1. In plain english like language , if else kind of syntax.
2. decision table - for a set of value
3. decision tree - multiple outcomes , branching out.

so you can decide on which way you can use the above three ways which fits to crack your complex rules into a more simpler form.

you can use rule flow orchestration with conditional flow to tackle " outcome of 1 rule will be the input of another rule "

Answer 4

I think it all depends on UI, not on actual logic that the rule(s) contain(s). Remember that most of today's engines are decision-table-mentality-based. And all that "Strong and Loose Coupling" is nothing more than buzz words used as an excuse for a bad UI or lack of it. Normal engine can handle execution of a rule of any complexity. Note that this is my personal opinion, lots of folks out there would completely disagree. So, please don't rush to downgrade my answer, I'm just trying to help :)

The typical notion is that the rule with complex logic would look really "messed up" or even impossible to implement in a decision table. Normally, guys try to combat that by dividing complex rules into smaller simple rules and stack them up into rule sets based on execution priorities.

There are new engines that have decision-table-less UIs by implementing parentheses instead of priorities. Parentheses also help with complexity.