Drools

Sofia Jonssonsofia.jonsson@callista.se

CADEC2006, Drools, Slide 2Copyright 2006, Callista Enterprise AB

Agenda

• Rule Engines - History/Background.• Problem Definition• Online Loan Application – an Example.

– Writing Rules.– Drools API.

• Summary & Conclusion

CADEC2006, Drools, Slide 3Copyright 2006, Callista Enterprise AB

Rule Engines - Basics

• Evaluates and executes business rules.

• Separates the business logic from the application code.– Compare to a database

• A sophisticated interpreter of if-then statements.– Declarative programming.

CADEC2006, Drools, Slide 4Copyright 2006, Callista Enterprise AB

Rule Engines – Short History

• Expert systems invented during AI research in the 70th and 80th.

• RETE algorithm invented 1979.• Interest has grown in later years.

– JSR 94 – Java Rule Engine API– Open source implementations– Drools part of JEMS (JBoss

Enterprise Middleware System)

CADEC2006, Drools, Slide 5Copyright 2006, Callista Enterprise AB

The Problem

• Increasing demand for agile and flexible applications. – Rapid moving markets– Different countries– Standard applications

• Developers aren’t business domain experts.

• Complex business rules.– Complexity tend to grow over time.

CADEC2006, Drools, Slide 6Copyright 2006, Callista Enterprise AB

Domain Specific Languages

• Used for a very specific problem domain.

• Made possible by Drools’ pluggable semantics framework.

• By specifying your own XML Schema you get rules validation.– Non programmers can edit the

rules!• Combine with default semantics.

CADEC2006, Drools, Slide 7Copyright 2006, Callista Enterprise AB

Domain Specific Languages - Ex

<drools:rule name="Main applicant must be living in Sweden"><condition>

<loan-application identifier="loanApp"> <main-applicant>

<address><country>

<not-equals>Sverige</not-equals>

</country> </address>

</main-applicant> </loan-application>

</condition><consequence>

<reject identifier="loanApp"> <reason>

Main applicant must be living in Sweden. </reason> </reject>

</consequence></drools:rule>

CADEC2006, Drools, Slide 8Copyright 2006, Callista Enterprise AB

Summary & Conclusion

• Consider using a rule engine:– If the application must be flexible.– If domain experts should update the

business rules.– If the application logic is very complex.

• Using a rule engine does not come without a cost.

• Consider development/test process.• Alternative approaches:

– Configurable threshold values– Scripting languages, e.g. Groovy

Questions?