[rule-engine] When should you NOT use a Rules Engine?

I have a pretty decent list of the advantages of using a Rules Engine, as well as some reasons to use them, what I need is a list of the reasons why you should NOT use a Rules Engine

The best I have so far is this:

Rules engines are not really intended to handle workflow or process executions nor are workflow engines or process management tools designed to do rules.

Any other big reasons why you should not use them?

I'm a big fan of Business Rules Engines, since it can help you make your life much easier as a programmer. One of the first experiences I've had while working on a Data Warehouse project was to find Stored Procedures containing complicated CASE structures stretching over entire pages. It was a nightmare to debug, since it was very difficult to understand the logic applied in such long CASE structures, and to determine if you have an overlapping between a rule at page 1 of the code and another from page 5. Overall, we had more than 300 such rules embedded in the code.

When we've received a new development requirement, for something called Accounting Destination, which was involving treating more than 3000 rules, i knew something had to change. Back then I've been working on a prototype which later on become the parent of what now is a Custom Business Rule engine, capable of handling all SQL standard operators. Initially we've been using Excel as an authoring tool and , later on, we've created an ASP.net application which will allow the Business Users to define their own business rules, without the need of writing code. Now the system works fine, with very few bugs, and contains over 7000 rules for calculating this Accounting Destination. I don't think such scenario would have been possible by just hard-coding. And the users are pretty happy that they can define their own rules without IT becoming their bottleneck.

Still, there are limits to such approach:

• You need to have capable business users which have an excellent understanding of the company business.
• There is a significant workload on searching the entire system (in our case a Data Warehouse), in order to determine all hard-coded conditions which make sense to translate into rules to be handled by a Business Rule Engine. We've also had to take good care that these initial templates to be fully understandable by Business Users.
• You need to have an application used for rules authoring, in which algorithms for detection of overlapping business rules is implemented. Otherwise you'll end up with a big mess, where no one understands anymore the results they get. When you have a bug in a generic component like a Custom Business Rule Engine, it can be very difficult to debug and involve extensive tests to make sure that things that worked before also work now.

More details on this topic can be found on a post I've written: http://dwhbp.com/post/2011/10/30/Implementing-a-Business-Rule-Engine.aspx

Overall, the biggest advantage of using a Business Rule Engines is that it allows the users to take back control over the Business Rule definitions and authoring, without the need of going to the IT department each time they need to modify something. It also the reduces the workload over IT development teams, which can now focus on building stuff with more added value.

Cheers,

Nicolae

I will give 2 examples from personal experience where using a Rules Engine was a bad idea, maybe that will help:-

1. 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.

2. 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. ?

In my experience, rules engines work best when the following are true:

1. Well-defined doctrine for your problem domain
2. High quality (preferably automated) data to help drive most of your inputs
3. Access to subject matter experts
4. Software developers with experience creating expert systems

If any of these four traits are missing, you still might find a rules engine works for you, but every time I've tried it with even 1 missing, I've run into trouble.

I don't really understand some points such as :
a) business people needs to understand business very well, or;
b) disagreement on business people don't need to know the rule.

For me, as a people just touching BRE, the benefit of BRE is so called to let system adapt to business change, hence it's focused on adaptive of change.
Does it matter if the rule set up at time x is different from the rule set up at time y because of:
a) business people don't understand business, or;
b) business people don't understand rules?

GREAT article on when not to use a rules Engine...(as well as when to use one)....

http://www.jessrules.com/guidelines.shtml

Another option is if you have a linear set of rules that only apply once in any order to get an outcome is to create a groovy interface and have developers write and deploy these new rules. The advantage is that it is wickedly fast because normally you would pass the hibernate session OR jdbc session as well as any parameters so you have access to all your apps data but in an efficient manner. With a fact list, there can be alot of looping/matching that really can slow the system down.....It's another way to avoid a rules engine and be able to be deployed dynamically(yes, our groovy rules were deployed in a database and we had no recursion....it either met the rule or it didn't). It is just another option.....oh and one more benefit is not learning rules syntax for incoming developers. They have to learn some groovy but that is very close to java so the learning curve is much better.

It really depends on your context. Rules engine have their place and the above is just another option if you have rules on a project that you may want to deploy dynamically for very simplified situations that don't require a rules engine.

BASICALLY do NOT use a rules engine if you have a simple ruleset and can have a groovy interface instead.....just as dynamically deployable and new developers joining your team can learn it faster than the drools language.(but that's my opinion)

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 would strongly recommend business rules engines like Drools as open source or Commercial Rules Engine such as LiveRules.

• When you have a lot of business policies which are volatile in nature, it is very hard to maintain that part of the core technology code.
• The rules engine provides a great flexibility of the framework and easy to change and deploy.
• Rules engines are not to be used everywhere but need to used when you have lot of policies where changes are inevitable on a regular basis.

That's certainly a good start. The other thing with rules engines is that some things are well-understood, deterministic, and straight-forward. Payroll withholding is (or use to be) like that. You could express it as rules that would be resolved by a rules engine, but you could express the same rules as a fairly simple table of values.

So, workflow engines are good when you're expressing a longer-term process that will have persistent data. Rules engines can do a similar thing, but you have to do a lot of added complexity.

Rules engines are good when you have complicated knowledge bases and need search. Rules engines can resolve complicated issues, and can be adapted quickly to changing situations, but impose a lot of complexity on the base implementation.

Many decision algorithms are simple enough to express as a simple table-driven program without the complexity implied by a real rules engine.