lino vázquez – pjas at cern gs-ais-hr. declarative programming rules rules engines rete...

Introduction to Rules Engines& Drools

Lino Vázquez – PJAS at CERNGS-AIS-HR

Declarative programmingRulesRules EnginesRete algorithmWhy use rules enginesDrools

Agenda

Declarative programming

Decribe the “what” instead of the “how”

Define the problem, give some conditions and let the system do the rest

Declarative programming

In imperative programming we provide the logic and the control in an algorithm

Implicit sequence

In declarative programming we provide logic and we leave the control to the system

Sequence is “irrelevant”

Declarative programming

Imperative programming

1. Check the battery level of your cellphone

2. If battery is low, go to the bedroom and get the charger from the desk

Declarative programming

Declarative programming

◦ If low level of batteryGet a charger

◦ If need a chargerGet it from the desk

◦ If need something from the deskGo to the bedroom

Declarative programming

What is a rule?

Drools 6.0 user’s manual:

A Production Rule is a two-part structure to infer conclusions which result in actions.

What is a rule?

A rule is an If – Then where the “then” part is an action over knowledge

What is a rule?

The condition is usually simple (Properties of a POJO)

The consequence is a modification of the knowledge, adding more knowledge or ereasing knowledge.

Facts fire rules and consequences work on them

What is a rule?

What is a rule engine?

Pattern matcherSystem that matches facts against rules

What is a rule engine?

The matching is performed by the “Inference engine” using an algothim (Rete)

What is a rule engine?

Forward & Backward chaining

Forward chainning – Data driven

Backward chainning – Goal driven

Forward & Backward chaining

Rules enginesArchitecture

Rule base

Inference engine

Working memory

Architecture

ProductionMemory

(Rules)

WorkingMemory

(Facts)

Inference Engine

PatternMatcher

Agenda

Architecture

Rule engine flow

Exit

Agenda evaluation

Select rule

Fire rule

Add

Delete

Edit

Activate possible rules

No rules

Inject facts

WorkingMemory

(Facts)

Inference Engine(Rete / Leaps)

PatternMatcher

ProductionMemory

(Rules)

Agenda

Agenda

Rule 1

Rule 2

Rule 3

Rule 4

Rule 5

Rule 6

Rule 7

Rule 1

Rule 2

Rule 3

Rule 4

Rule 5

Rule 6

Rule 7

Rule 1

Rule 2

Rule 3

Rule 4

Rule 5

Rule 6

Rule 7

Rule 1

Rule 2

Rule 3

Rule 4

Rule 5

Rule 6

Rule 7

FactsFacts

*

Agenda

Rule 1

Rule 2

Rule 3

Rule 4

Rule 5

Rule 6

Rule 7

Rule 1

Rule 2

Rule 3

Rule 4

Rule 5

Rule 6

Rule 7

Rule 1

Rule 2

Rule 3

Rule 5

Rule 6

Rule 7

Rule 1

Rule 2

Rule 3

Rule 4

Rule 5

Rule 6

Rule 7

FactsFacts

*Rule 4

Agenda

Rule 1

Rule 2

Rule 3

Rule 4

Rule 5

Rule 6

Rule 7

Rule 1

Rule 2

Rule 3

Rule 4

Rule 5

Rule 6

Rule 7

Rule 1

Rule 2

Rule 3

Rule 5

Rule 6

Rule 7

Rule 1

Rule 2

Rule 3

Rule 4

Rule 5

Rule 6

Rule 7

Facts Rule 4

Agenda

WorkingMemory

(Facts)

Inference Engine(Rete / Leaps)

PatternMatcher

ProductionMemory

(Rules)

Agenda

Pattern matcher

If a person is 18 years old or older and less than 65, is an adult

If a person is an adult, we issue an adult ID card

Person (age >= 18, age < 65)

Person (isAdult == true)

Pattern matcher

Person (age >= 18, age < 65)

Pattern matcher

Pattern

Type Constrain

Field Restriction

Evaluator Value

Rete algorithm

Since 1975

Independent of the number or rules

Heavy memory use

Rete algorithm

Rule 1: If a new costumer spends more than 100€ and is his birthday, gets a 20% discount

Rule 2: If a new costumer spends more than 100€, gets a 10% discount

Rete algorithm

New Costumer

More than 100

Birthday

Rule 1*

Rule 2

Rule 1

New Costumer

More than 100

New Costumer

More than 100

Birthday

Rule 1*

Rule 2

Rule 1

New Costumer

More than 100

Birthday

Rule 1*

Rule 2

Rule 1

Why use a rule engine?

When to use a rule engine?

Your logic involves significant decision-making You can write decision rules for your logic Your logic is complex (3 or more nested if-

statements) Your logic needs to be flexible

◦ During development◦ After development

Execution speed is not your main goal You want domain experts to review/maintain the

logic

When to use a rule engine?

Born in 2001

Rete algorithm

Bought by Red Hat in 2005

Last stable version 6.1

Drools

If a person is 18 years old or older and less than 65, is an adult

If a person is an adult, we issue an adult ID card

Person (age >= 18, age < 65)

Person (isAdult == true)

Drools syntax

when $p : Person (age >= 18, < 65)then modify ($p){

setIsAdult(true) }end

If a person is 18 years old or older and less than 65, is an adult

Drools syntax

Features

when $p : Person (isAdult == true)then logicalInsert(new IdCard($p,‘Adult’); end

If a person is an adult, we issue an adult ID card

| Features

Truth Maintenanc

e

Conditional Elements

◦ not Person( age >=18)

◦ exists Person( age >=18)

◦ forall (Person( age >=18))

| Features

Keyword ‘from’

when $p : Person( zipcode== “1205”) Car( color == “red” ) from $p.cars

when $z : ZipCode()

Person() from $hbn.getQuery(“FindPeople”).setParameters([“zipCode” :

$z]).list()

| Features

Keyword ‘collect’

when $list : List( intValue > 100)

from collect (Person(age >= 18))then

print “People“ + $list;end

| Features

Keyword ‘accumulate’

when $sum : Number( intValue > 100)

from accumulate (Person(age >= 18, $s : savings ) sum

($s))then

print “Total savings “ + $sum;end

| Features

Timers & Calendars◦ Delays and Intervals◦ Cron

Time operators◦ before, after, coincides, overlaps, during…

Time windows+aggregations (Keyword ‘over’)

| Features

rule “High number of orders” when   Number( nbOrders : intValue > 100 )

from accumulate(           e: PlaceOrderEvent(session.storeId == “00001”)                          over window:size(12h),                    count(e) )then   System.err.println( "WARNING: Number of orders in

the last 12 hours > 100: " + nbOrders );end

| Features

Drools at CERN

Article II 1.01Retirement AgeThe retirement age shall be as follows:

a) For members who joined the Fund on or before 31December 2011, 65 years;

b) For members who joined the Fund on or after 1 January 2012, 67 years.

Drools at CERN

Rule “Retirement age” when

$m : Member()JoinedFund(member == $m,

date <= “31 –Dec-2011”)then

insert(new RetirementAge($m, 65)

Drools at CERN

Thank you