An Introduction to
Business Rules with
JBoss Drools
John Paulett
October 12, 2009

Goals
Introduction to “business rules”
High level overview of JBoss Drools
Discussion of benefits and shortcomings
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Business Rules
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Problem Domain
Basic problem:
how to model “business logic”
In this context:
“business logic” ≠ code between UI and DB
“business logic” = policies of the business
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Business Logic Examples
Rate calculations at insurance company
● Depends on laws, age, sex, location, previous
history, desired coverage, etc.
Critical lab values at hospital
● Depends on age, previous values, other lab
tests, diagnose, mandates, etc.
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Business Rule Engine
Type of Expert System
Two types of rules:
● “Production (or Inference) Rules”
– If x, then y
– Often stateless
● “Reaction Rules” (Complex Event Processing)
– Wait for set of events
– Stateful
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Forward Chaining
1) Iterate through antecedents (if …)
2) Each time an antecedent is matched,
add knowledge of consequent (then …)
3) Go to #1, until goal reached
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Rete Algorithm
Naïve forward chaining does not scale
well as number of rules increases
Efficient implementation using directed
acyclic graph representation of rules
Higher memory consumption for
increased speed
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JBoss Drools
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JBoss Drools Suite
Guvnor – Business Rule Management
System
Expert – Production Rules
Flow – Workflow
Fusion – Complex Event Processing
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Drools Expert
Core Drools product
Forward chaining rules with Rete
Eclipse authoring plugin
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Ways to Write Rules
Technical Rule Language
Custom Domain Specific Languages
Guided Editor
Decision Table
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Technical Rule Language
rule "Hello World"
when
m : Message( status == Message.HELLO, myMessage : message )
then
System.out.println( myMessage );
m.setMessage( "Goodbye cruel world" );
m.setStatus( Message.GOODBYE );
update( m );
end
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Custom DSLs
Map “natural language” into Technical
Rule Language
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Guided Editor
From http://www.jboss.org/drools/drools-expert.html
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Decision Table
Excel XLS based
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Drools Guvnor
BRMS – Business Rule Management
System
Centralized knowledgebase
Web-based rule authoring
Versioning of rules
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Drools Guvnor
Manage rules through web console
From http://www.jboss.org/drools/drools-guvnor.html
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Drools Guvnor
Edit rules through web console
From http://www.jboss.org/drools/drools-guvnor.html
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Drools Flow
Workflow management
From http://www.jboss.org/drools/drools-guvnor.html
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Drools Fusion
Extension to Drools Expert for temporal
reasoning
Detects relevant patterns in “cloud or
stream of events”
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Discussion
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When To Use a Rules Engine
Large set of rules, with complex
branching
Rules likely to change over time, likely out
of sync with code changes
Involve “business users” in creation &
maintenance of rules
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Benefits
Separates “business logic” from
application code
“Hot” changes of rules (no recompile)
Improved performance for large rule sets
Communicates rules more clearly than
Java code
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Downsides
New tool
● Learning curve
● Extra point of failure
Overhead
● Not practical for small rule sets
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Alternatives
Java Rule Engine API - JSR 94
Closed Source:
● IBM's ILOG JRules, Fair Isaac's Blaze Advisor
Open Source:
● Jess, OpenRules, Zilonis, take
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References
http://www.jboss.org/drools/
http://java.sun.com/developer/technicalArticles/J2SE/JavaRule.html
http://java.sys-con.com/node/45082
http://herzberg.ca.sandia.gov/guidelines.shtml
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