Introducing Drools

by Mario Fusco
Red Hat – Senior Software Engineer
mario.fusco@gmail.com
twitter: @mariofusco
Introducing

Drools Vision
Drools was born as a rule engine, but following the vision of
becoming a single platform for business modelling, it realized
it could achieve this goal only by leveraging 3 complementary
business modelling techniques:
• Business Rule Management (Drools Expert)
• Business Process Management (Drools Flow)
• Complex Event Processing (Drools Fusion)

Business Logic integration System
Drools 5 – Business Logic Integration Platform
Drools
Guvnor
Drools
Fusion
Drools
Flow
Drools
Expert
Drools
Planner

What a rule-based program is
• A rule-based program is made up of discrete rules, each of
which applies to some subset of the problem
• It is simpler, because you can concentrate on the rules for one
situation at a time
• It can be more flexible in the face of fragmentary or poorly
conditioned inputs
• Used for problems involving control, diagnosis, prediction,
classification, pattern recognition … in short, all problems
without clear algorithmic solutions
Declarative
(What to do)
Imperative
(How to do it)
Vs.

When should you use a Rule Engine?
• The problem is beyond any obvious algorithmic solution or
it isn't fully understood
• The logic changes often
• Domain experts (or business analysts) are readily available,
but are nontechnical
• You want to isolate the key parts of your business logic,
especially the really messy parts

Rule's anatomy
rule “<name>”
<attribute> <value>
when
<LHS>
then
<RHS>
end
Quotes on Rule names are optional
if the rule name has no spaces.
salience <int>
agenda-group <string>
no-loop <boolean>
auto-focus <boolean>
duration <long>
....
Pattern-matching
against objects in the
Working Memory
Code executed when
a match is found

Imperative vs Declarative
public void helloMark(Person person) {
if ( person.getName().equals( “mark” ) {
System.out.println( “Hello Mark” );
}
}
rule “Hello Mark”
when
Person( name == “mark” )
then
System.out.println( “Hello Mark” );
end
A method must be called directly Specific passing
of arguments
Rules can never be called directly Specific instances cannot be
passed but are automatically
selected with pattern-matching

What is a pattern
Person( name == “mark” )
Pattern
Object Type Field Constraint
Field Name Restriction

Rule's definition
// Java
public class Applicant {
private String name;
private int age;
private boolean valid;
// getter and setter here
}
rule "Is of valid age" when
$a : Applicant( age >= 18 )
then
modify( $a ) { valid = true };
end
// DRL
declare Applicant
name : String
age : int
valid : boolean
end

Building
KnowledgeBuilder kbuilder =
KnowledgeBuilderFactory.newKnowledgeBuilder();
kbuilder.add(
ResourceFactory.newClassPathResource("Rules.drl"),
ResourceType.DRL);
KnowledgeBuilderErrors errors = kbuilder.getErrors();
if (kbuilder.hasErrors()) {
System.err.println(kbuilder.getErrors().toString());
}
KnowledgeBase kbase =
KnowledgeBaseFactory.newKnowledgeBase();
kbase.addKnowledgePackages(kbuilder.getKnowledgePackages());

Executing
StatelessKnowledgeSession ksession =
kbase.newStatelessKnowledgeSession();
Applicant applicant = new Applicant( "Mr John Smith", 21 );
assertFalse( applicant.isValid() );
ksession.execute( applicant );
assertTrue( applicant.isValid() );

More Pattern Examples
Person( $age : age )
Person( age == ( $age + 1 ) )
Person(age > 30 && < 40 || hair in (“black”, “brown”) )
Person(pets contain $rover )
Person(pets[’rover’].type == “dog”)

Conditional Elements
not Bus( color = “red” )
exists Bus( color = “red” )
forall ( $bus : Bus( color == “red” ) )
$owner : Person( name == “mark” )
Pet( name == “rover” ) from $owner.pets
$zipCode : ZipCode()
Person( ) from $hbn.getNamedQuery(“Find People”)
.setParameters( [ “zipCode” : $zipCode ] )
.list()
Hibernate session
'from' can work
on any expression

Timers & Calendars
rule R1
timer 1m30s
when
$l : Light( status == “on” )
then
modify( $l ) { status = “off” };
When the light is on, and has been
on for 1m30s then turn it off
Send alert every
quarter of an hour
Execute now and after
1 hour duration only during weekday
rule R3
calendars "weekday"
timer (int:0 1h)
when
Alarm()
then
sendEmail( ”Alert!” )
rule R2
timer ( cron: 0 0/15 * * * * )
when
Alarm()
then
sendEmail( ”Alert!” )

Truth Maintenance System (1)
rule "Issue Adult Bus Pass" when
$p : Person( age >= 18 )
then
insert(new AdultBusPass( $p ) );
end
rule "Issue Child Bus Pass" when
$p : Person( age < 18 )
then
insert(new ChildBusPass( $p ) );
end
Coupled logic
What happens
when the child
becomes adult?

Truth Maintenance System (2)
rule "Who Is Child" when
$p : Person( age < 18 )
then
logicalInsert( new IsChild( $p ) )
end
rule "Issue Child Bus Pass" when
$p : Person( )
IsChild( person =$p )
then
logicalInsert(new ChildBusPass( $p ) );
end
De-couples the logic
Maintains the truth by
automatically retracting
Encapsulate
knowledge providing
semantic abstractions
for this encapsulation

Complex Event Processing
Event
A record of state change in the application domain at a
particular point in time
Complex Event
An abstraction of other events called its members
Complex Event Processing
Processing multiple events with the goal of identifying
the meaningful events within the event cloud

Drools Fusion
• Drools modules for Complex Event Processing
• Understand and handle events as a first class
platform citizen (actually special type of Fact)
• Select a set of interesting events in a cloud or
stream of events
• Detect the relevant relationship (patterns)
among these events
• Take appropriate actions based on the
patterns detected

Events as Facts in Time
rule
"Sound the alarm"
when
$f : FireDetected( )
not( SprinklerActivated( this after[0s,10s] $f ) )
then
// sound the alarm
end
Temporal
relationships
between events

Workflows as Business Processes
A workflow is a process that describes the
order in which a series of steps need to be
executed, using a flow chart.

Drools Flow (aka jBPM5)
• Allows to model business processes
• Eclipse-based editor to support workflows
graphical creation
• Pluggable persistence and transaction based
on JPA / JTA
• Based on BPMN 2.0 specification
• Can be used with Drools to model your
business logic as combination of processes,
events and rules

Drools Guvnor
• Centralised repository for Drools Knowledge
Bases
• Web based GUIs
• ACL for rules and other artifacts
• Version management
• Integrated testing

Drools Planner
• Works on all kinds of planning problems
– NP-complete
– Hard & soft constraints
– Huge search space
• Planning constraints can be weighted and are
written as declarative score rules
• The planner algorithm is configurable. It searches
through the solutions within a given amount of
time and returns the best solution found

Mario Fusco
Red Hat – Senior Software Engineer
mario.fusco@gmail.com
twitter: @mariofusco
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