Truth Maintenance System

Truth Maintenance System (TMS) supports logical chaining in Drools.

TMS includes conditional elements, such as and/or/not/exists/test, logical concepts or operations, such as Assertion (Insert), Retraction (Retract), Modification (Update and Modify) and more�
Drools supports conditional element and as well as &&.

Drools supports conditional element or as well as ||.

Conditional Element exists checks for the existence of data in the Working Memory

Assertion
� Expert systems typically use the term assert or assertion to refer to facts made available to the system.
� However, due to "assert" being a keyword in most languages, DRools uses the insert keyword;
� These two terms are used interchangeably.
� When an Object is inserted it returns a FactHandle. This FactHandle is the token used to represent your inserted object within the WorkingMemory.
� It is also used for interactions with the WorkingMemory when you wish to retract or modify an object.


Person person = new Person("Bob");

FactHandle personHandle = ksession.insert( person );

Retraction
� Retraction is the removal of a fact from Working Memory.
� Any rules that are activated and dependent on that fact will be cancelled.
� Note that it is possible to have rules that depend on the nonexistence of a fact, in which case retracting a fact may cause a rule to activate. (See the not and exists keywords.)
� Retraction may be done using the FactHandle that was returned by the insert call.
� On the right hand side of a rule the retract statement is used, which works with a simple object reference.


Person person = new Person("Bob");

FactHandle personHandle = ksession.insert( person );

....

ksession.retract( personHandle );

Modification: Update & Modify
� The Rule Engine must be notified of modified facts, so that they can be reprocessed.
� You must use the update() method to notify the WorkingMemory of changed objects.
� Update() always takes the modified object as a second parameter, which allows you to specify new instances for immutable objects.
� Modify() statement is recommended on the right hand side of a rule the , as it makes the changes and notifies the engine in a single statement.

Example of modify with logical conditions:


when

   exists (application.criminalRecord) or application.person.age < 18

then

   modify(application.rejected = true);

end

Two rules with MODIFY and LOCK-ON-ACTIVE


rule "Assign people in North Carolina (NC) to sales region 1"

ruleflow-group "test"

lock-on-active true

when

    $p : Person( ) 

    $a : Address( state == "NC") from $p.address 

then

    modify ($p) {} // Assign person to sales region 1 in a modify block

end

----------------------------------------------------------------------------------------

rule "Apply a discount to people in the city of Raleigh"

ruleflow-group "test"

lock-on-active true

when

    $p : Person( ) 

    $a : Address( city == "Raleigh") from $p.address 

then

    modify ($p) {} // Apply discount to person in a modify block

end

Was it clear so far?

In the example, persons in Raleigh, NC should be assigned to sales region 1 and receive a discount;
i.e., you would expect both rules to activate and fire.
Instead you will find that only the second rule fires.
Why? Because of the lock-on-active true attribute.

Drools provide direct access to List & Map objects
� Directly access a List value by index
� Directly access a Map value by key
� Can use the contains keyword similar as in Java

Examples:


listOfProducts[0] = �redHat�;

mapOfProducts.put(�redHat�, aRedHatProductObject);



when

     mapOfProducts contains �redHat�

then

    modify($offer.add(mapOfProducts[�redHat�]) );

end

Drools function allows developers creating a method (function) in a rule source file
� Starts with the keyword �function� and makes sense when can be reused
� Very similar to a Java method, might be useful when a developer does not want to touch Java code


function void printTime() {

    logger.info((new Date()).toString());

}

Drools can declare new Type, which is very similar to introducing a new class in Java


declare Product

  productName : String

  productPrice : double

  productAvailability : java.util.Date

end

Declaration as well as Java classes might include metadata, for example @author Jeff Zhuk
Assignments:
1. Answer QnAs
2. Create at least one QnA related to the content and email to jeff.zhuk@javaschool.com

rule "Assign people in North Carolina (NC) to sales region 1" ruleflow-group "test" lock-on-active true when $p : Person( ) $a : Address( state == "NC") from $p.address then modify ($p) {} // Assign person to sales region 1 in a modify block end ---------------------------------------------------------------------------------------- rule "Apply a discount to people in the city of Raleigh" ruleflow-group "test" lock-on-active true when $p : Person( ) $a : Address( city == "Raleigh") from $p.address then modify ($p) {} // Apply discount to person in a modify block end

listOfProducts[0] = �redHat�; mapOfProducts.put(�redHat�, aRedHatProductObject); when mapOfProducts contains �redHat� then modify($offer.add(mapOfProducts[�redHat�]) ); end