[rules-dev] Declarative Agenda

[Mark Proctor]

I just got a first cut working for "declarative agenda". The idea here 
is rules can control which rule can or cannot fire. Because this is 
highly experimental it is off by default and users must explicitely 
enable it, it will stay this way until we are happy with the solution. 
My hope is that it will provide a more declarative approach to execution 
control; which will enable more readable and maintainable rules compared 
to using magic salience values and in some circumstances control objects..

         KnowledgeBaseConfiguration kconf = 
KnowledgeBaseFactory.newKnowledgeBaseConfiguration();
         kconf.setOption( DeclarativeAgendaOption.ENABLED );
         KnowledgeBase kbase = KnowledgeBaseFactory.newKnowledgeBase( 
kconf );

The basic idea is:
-All matched rule's Activations are inserted into WorkingMemory as 
facts. So you can now match against an Activation the rules metadata and 
declarations are available as -fields on the Activation object.
-You can use the kcontext.block( $a ) for the current rule to block the 
selected activation. Only when that rule becomes false will the 
activation be elegible for firing. If it is already elebible for firing 
and is later blocked, it will be removed from the agenda until it is 
unblocked.
-An activation may have multiple blockers, all blockers must became 
false, so they are removed to enable the activation to fire
-kcontext.unblockAll( $a ) is an over-ride rule that will remove all 
blockers regardless
- at activationListener('direct') allows a rule to fire as soon as it's 
matched, this is to be used for rules that block/unblock activations, it 
is not desirable for these rules to have side effects that impact else 
where. The name may change later, this is actually part of the pluggable 
terminal node handlers I made, which is an "internal" feature for the 
moment.

I should be committing this later today, and will send a follow up email 
once it hits HEAD, but here is a unit test. It uses a control role to 
stop all rules with metadata declaring the rules to be in the "sales" 
department. Only when that control rule becomes false can they fire.

package org.domain.test
import org.drools.runtime.rule.Activation
global java.util.List list
dialect 'mvel'

rule rule1 @department('sales')
when
      $s : String( this == 'go1' )
then
     list.add( kcontext.rule.name + ':' + $s );
end
rule rule2 @department('sales')
when
      $s : String( this == 'go1' )
then
     list.add( kcontext.rule.name + ':' + $s );
end
rule rule3 @department('sales')
when
      $s : String( this == 'go1' )
then
     list.add( kcontext.rule.name + ':' + $s );
end
rule blockerAllSalesRules @activationListener('direct')
when
      $s : String( this == 'go2' )
      $i : Activation( department == 'sales' )
then
     list.add( $i.rule.name + ':' + $s  );
     kcontext.block( $i );
end

KnowledgeBaseConfiguration kconf = 
KnowledgeBaseFactory.newKnowledgeBaseConfiguration();
kconf.setOption( DeclarativeAgendaOption.ENABLED );
KnowledgeBase kbase = KnowledgeBaseFactory.newKnowledgeBase( kconf );
kbase.addKnowledgePackages( kbuilder.getKnowledgePackages() );
StatefulKnowledgeSession ksession = kbase.newStatefulKnowledgeSession();
List list = new ArrayList();
ksession.setGlobal( "list", list);
ksession.insert(  "go1" );
FactHandle go2 = ksession.insert(  "go2" );
ksession.fireAllRules();
assertEquals( 3, list.size() ); // none of the rules 1-3 fire, as they 
are blocked.
assertTrue( list.contains( "rule1:go2" ));
assertTrue( list.contains( "rule2:go2" ));
assertTrue( list.contains( "rule3:go2" ));

list.clear();

ksession.retract( go2 ); // the blocker rule is nolonger true, so rules 
1-3 can now fire.
ksession.fireAllRules();

assertEquals( 3, list.size() );
assertTrue( list.contains( "rule1:go1" ));
assertTrue( list.contains( "rule2:go1" ));
assertTrue( list.contains( "rule3:go1" ));

ksession.dispose();