Why should i use Drools?
- Drools is open source.
- It provides forward and backward chaining of rules execution
- It provides template base rule definitions
- It is implemented using JSR-94
- It has decision table facility
- Its rules are human readable
- It is integrated with Spring
I am no Drools expert. I have some familiarity with it though, by experimenting with it. I am unable to appreciate, why would i need it.
My Typical Applications are Business Web Applications. Yes they do have some amount of Rules. But those are implemented using Database Tables, SQL Queries, and a nice UI in for the Business-Users to modify the Rules. Rules are not arbitrary, they are carefully thought-thru before being put in Production.
My Business Users would never ever use a (Drools)Scripting Language to modify Anything. Let Alone Modify Rules. They are perfectly happy using UI Screens to modify Rules. Plus they can make a zillion syntax mistakes in a Drools files, if i let them anywhere near it.
- Why should i use Drools in this scenario?
- There are Drools fanatics i have met who insist i should change all my code to make use of Drools.
So, is Drools useful? I am not sure.
Rule Engines can offer a lot of value, in certain instances.
First, many rule engines work in a more declarative way. A very crude example would be AWK, where you can assign regexes to blocks of code. When the regex is seen by the file scanner, the block of code is executed.
You can see that in this case, if you had, say, a large AWK file and you wanted to add Yet Another "rule", you can readily go to the bottom of the file, add you regex and logic, and be done with it. Specifically, for many applications, you're not particularly concerned with what the other rules are doing, and the rules don't really interoperate with each other.
Thus the AWK file becomes more like a "rule soup". This "rule soup" nature lets folks focus very tightly on their domain with little concern for all of the other rules that may be in the system.
For example, Frank is interested in orders with a total of more than $1000, so he puts in to the rule system that he's interested. "IF order.total > 1000 THEN email Frank".
Meanwhile, Sally wants all orders from the west coast: "IF order.source == 'WEST_COAST' THEN email Sally".
So, you can see in this trivial, contrived case, that an order can satisfy both rules, yet both rules are independent of each other. A $1200 order from the West Coast notifies both Frank and Sally. When Frank is no longer concerned, he'll simply yank his rule out from the soup.
For many situations, this flexibility can be very powerful. It can also, like this case, be exposed to end users for simple rules. Using high level expressions and perhaps lightweight scripting.
Now, clearly, in a complicated system there are all sorts of interrelationships that can happen, an this is why the entire system is not "Done with rules". Someone, somewhere is going to be in charge of the rules not getting out of hand. But that doesn't necessarily lessen the value such a system can provide.
Mind this doesn't even go in to things like expert systems, where rules fire on data that rules can create, but a simpler rules system.
Anyway, I hope this example shows how a rules system can help augment a larger application.
Rules Engine - pros and cons
Most rule engines that I have seen are viewed as a black box by system code. If I were to build a domain model, I would probably want certain business rules to be intrinsic to the domain model, e.g. business rules that tell me when an object has invalid values. This allows multiple systems to share the domain model without duplicating business logic. I could have each system use the same rule service to validate my domain model, but this appears to weaken my domain model (as was pointed out in the question). Why? Because instead of consistently enforcing my business rules across all systems at all times, I am relying on system programmers to determine when the business rules should be enforced (by calling the rule service). This may not be a problem if the domain model comes to you completely populated, but can be problematic if you're dealing with a user interface or system that changes values in the domain model over its lifetime.
There is another class of business rules: decision making. For example, an insurance company may need to classify the risk of underwriting an applicant and arrive at a premium. You could place these types of business rules in your domain model, but a centralized decision for scenarios like this are usually desirable and, actually, fit quite well into a service-oriented architecture. This does beg the question of why a rule engine and not system code. The place where a rule engine may be a better choice is where business rules responsible for the decision change over time (as some other answers have pointed out).
Rule engines usually allow you to change rules without restarting your system or deploying new executable code (regardless of what promises you receive from a vendor, do make sure you test your changes in a non-production environment because, even if the rule engine is flawless, humans are still changing the rules). If you're thinking, "I can do that by using a database to store values that change", you're right. A rule engine is not a magical box that does something new . It is intended to be a tool that provides a higher level of abstraction so you can focus less on reinventing the wheel. Many vendors take this a step further by letting you create templates so that business users can fill in the blanks instead of learning a rule language.
One parting caution about templates: templates can never take less time than writing a rule without a template because the template must, at the bare minimum, describe the rule. Plan for a higher initial cost (the same as if you were to build a system that used a database to store values that change vs. writing the rules in directly in system code) - the ROI is because you save on future maintenance of system code.