I need a simple rules engine, I think? [closed]

Question

I need some advice on the best approach to solving this problem.

I've researched DROOLS, Java Rule Engine and a few others. All of these are powerful, and have good things about them. I don't know which (if any) will be the best choice for me.

I have one business object. (simplified for demo)

Person
     firstName:String
     lastName:String
     departMent:String
     hireDate:Date

I need to write an editor in a web application that will allow for complex rules to be built around these fields. I need to support complex nested AND/OR logic. I just need basic operators, and the rule should simply evaluate to true or false. If the rule evaluates to true or false, one action will occur respectively.

For example,

firstName CONTAINS "value" AND (lastName EQUALS "input" OR department CONTAINS "input")

I had thought, maybe I should just write my own parser and evaluate the logic in my own code. I don't know what to do, any advice or links to something to read would be greatly appreciated. Is there a specific design pattern I could research?

How would you solve this problem? One reservations about the rules engines is that maybe they are too complex for just a simple problem?

Answer 1

I would take a look at some sample rules engine interfaces and see which ones I like. You can look at web based email rules interfaces to get some ideas. If you really need a simple rules engine, you just need to create a good interface, and then you can send the rules to the server with javascript.

Answer 2

Probably not. You need a decent domain model. Not one where your objects are just data placeholders. Are your users likely to be able to understand and use such a complex rules system, and wouldn't those that do prefer just programming in java, where they have proper encapsulation and refactoring support? Rules systems only work with simple rules on a restricted domain, where you can explain people not trained as programmers how to build them. And don't forget rules building is just programming, so you still need version control, tests, and don't want globals.

Answer 3

Wouldn't Jython be usefull?

Each expression/complex-rule could be the body of a function. So the user provides the body, and your code puts the function spec around it, and then execute it.

You can also put any Java objects / variables of your own into the jython context to be used in your script/ function body.

Then you have a standardized, extendable widely used language at your fingertips. But i think the Jython editor could be a challenge.

Answer 4

This is not an yes/no question, but I can probably share my experiences, and hope it helps. I have used DROOLS quite successfully in a few projects. Apart from some cases (another team had issues with DROOLS under heavy load,) DROOLS is quite an useful library.

I built an application which:
1. read input from a source
2. chose the next action based on the input from a set of available operations

As trivial as it looks, it needed to be very flexible:
1. The input was a variable set of name-value pairs, names not predetermined.
2. values, presence/absence of certain name/values (based on occurrence/absence of events), trigger different actions.
3. The business rules can change while the application is running.

Maybe there are better solutions, but for better or worse, I ended up using DROOLS. I developed a BPEL in which the decisions are made by the DROOLS component. The DROOLS component internally reads the decision making rules from a Microsoft Excel spreadsheet. It rebuilds the rules if there is a change in the file. Now the domain experts change this spreadsheet when required, and we do not go through painful deployments!

If you want a sophisticated UI, DROOLS Guvnor is a readily available web-application (with rich UI,) which would help your domain/subject-matter experts to build rules and store them in a database.

Answer 5

The Drools documentation talks about when to use a rules engine. http://downloads.jboss.com/drools/docs/5.1.1.34858.FINAL/drools-expert/html_single/index.html#d0e181

From the docs...

The shortest answer to this is "when there is no satisfactory traditional programming approach to solve the problem.". Given that short answer, some more explanation is required. The reason why there is no "traditional" approach is possibly one of the following:

-- The problem is just too fiddle for traditional code.

The problem may not be complex, but you can't see a non-fragile way of building a solution for it.

-- The problem is beyond any obvious algorithmic solution.

It is a complex problem to solve, there are no obvious traditional solutions, or basically the problem isn't fully understood.

-- The logic changes often

The logic itself may even be simple but the rules change quite often. In many organizations software releases are few and far between and pluggable rules can help provide the "agility" that is needed and expected in a reasonably safe way.

-- Domain experts (or business analysts) are readily available, but are nontechnical.

Domain experts often possess a wealth of knowledge about business rules and processes. They typically are nontechnical, but can be very logical. Rules can allow them to express the logic in their own terms. Of course, they still have to think critically and be capable of logical thinking. Many people in nontechnical positions do not have training in formal logic, so be careful and work with them, as by codifying business knowledge in rules, you will often expose holes in the way the business rules and processes are currently understood.

When not to use...

As rule engines are dynamic (dynamic in the sense that the rules can be stored and managed and updated as data), they are often looked at as a solution to the problem of deploying software. (Most IT departments seem to exist for the purpose of preventing software being rolled out.) If this is the reason you wish to use a rule engine, be aware that rule engines work best when you are able to write declarative rules. As an alternative, you can consider data-driven designs (lookup tables), or script processing engines where the scripts are managed in a database and are able to be updated on the fly.

Answer 6

Have you tried JBehave?

JBehave is a framework for Behaviour-Driven Development (BDD). BDD is an evolution of test-driven development (TDD) and acceptance-test driven design, and is intended to make these practices more accessible and intuitive to newcomers and experts alike. It shifts the vocabulary from being test-based to behaviour-based, and positions itself as a design philosophy.