Single Thread execution in DROOLS?

Question

My application handles multiple request but the rules in my knowledge session are executed by single thread only. For eg: Thread 1 and Thread 2 enters the knowledge session with the gap of 2millisec But Thread 1 executes its own rules and even the rules for Thread 2 is executed by Thread 1. Imagine if there are 1000 request which means the rules for each requests will be executed by only a 1 Thread?

Is there any way where in DROOLS where we can prevent this and ensure that the rules are executed by multiple Threads?

Below is a small sample test I tried:

Java Class:

        import java.math.BigDecimal;

        import org.drools.KnowledgeBase;
        import org.drools.KnowledgeBaseFactory;
        import org.drools.builder.KnowledgeBuilder;
        import org.drools.builder.KnowledgeBuilderError;
        import org.drools.builder.KnowledgeBuilderErrors;
        import org.drools.builder.KnowledgeBuilderFactory;
        import org.drools.builder.ResourceType;
        import org.drools.io.ResourceFactory;
        import org.drools.runtime.StatefulKnowledgeSession;
import org.drools.runtime.rule.WorkingMemoryEntryPoint;
import org.springframework.context.support.ClassPathXmlApplicationContext;



        public class DJ_Test {

            public static void main(String[] args) {
                try {
                    System.out.println("In main");
                    // load up the knowledge base
                    KnowledgeBase kbase = readKnowledgeBase();
                    final StatefulKnowledgeSession ksession = kbase.newStatefulKnowledgeSession();
                    final WorkingMemoryEntryPoint entry =ksession.getWorkingMemoryEntryPoint("RequestStream");

                    final Object  obj_1= new Object();
                    Thread t1 = new Thread(){
                        public void run(){System.out.println(Thread.currentThread().getName() + " is running");
                        entry.insert(obj_1);
                        ksession.fireAllRules();
                        System.out.println(Thread.currentThread().getName() + " is terminated");
                        }
                        };

                   final Object  obj_2= new Object();
                   Thread t2 = new Thread(){
                    public void run(){
                    try{
                        Thread.sleep(8000); 
                    }catch(Exception e){

                    }
                    System.out.println(Thread.currentThread().getName() + " is running");
                    entry.insert(obj_2);
                    ksession.fireAllRules();
                    System.out.println(Thread.currentThread().getName() + " is terminated");
                    }
                    };
                  t1.start();
                  t2.start();





                } catch (Throwable t) {
                    t.printStackTrace();
                }
            }

            private static KnowledgeBase readKnowledgeBase() throws Exception {
               /* KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();
                kbuilder.add(ResourceFactory.newClassPathResource("rulesFlow.bpmn"), ResourceType.BPMN2);
                kbuilder.add(ResourceFactory.newClassPathResource("KansasSalesTax.drl"), ResourceType.DRL);
                kbuilder.add(ResourceFactory.newClassPathResource("MissouriSalesTax.drl"), ResourceType.DRL);
                kbuilder.add(ResourceFactory.newClassPathResource("SalesTax.drl"), ResourceType.DRL);
                KnowledgeBuilderErrors errors = kbuilder.getErrors();
                if (errors.size() > 0) {
                    for (KnowledgeBuilderError error: errors) {
                        System.err.println(error);
                    }
                    throw new IllegalArgumentException("Could not parse knowledge.");
                }
                KnowledgeBase kbase = KnowledgeBaseFactory.newKnowledgeBase();
                kbase.addKnowledgePackages(kbuilder.getKnowledgePackages());
                return kbase;*/
                ClassPathXmlApplicationContext serviceContext = new ClassPathXmlApplicationContext( "droolsContext.xml" );
                return (KnowledgeBase) serviceContext.getBean("kbase1");
            }

            public static class DJ_Message {

                public static final int thread_1 = 1;
                public static final int thread_2 = 2;

                private String message;

                private int status;

                public String getMessage() {
                    return this.message;
                }

                public void setMessage(String message) {
                    this.message = message;
                }

                public int getStatus() {
                    return this.status;
                }

                public void setStatus(int status) {
                    this.status = status;
                }

            }

        }

DRL file

package com.sample

import com.sample.DroolsTest.Message;

//global CepService cepService;
declare Object
    @role( event )
end


rule "rule_1"
salience 100
    when
        $o : Object() from entry-point RequestStream
    then
        System.out.println( "Rule 1 fired by " + Thread.currentThread().getName() );
        Thread.sleep(5000);

end


rule "rule_2"
salience 80
    when
        $o : Object() from entry-point RequestStream
    then
        System.out.println( "Rule 2 fired by " + Thread.currentThread().getName() );
        Thread.sleep(5000);

end

rule "rule_3"
salience 60
    when
        $o : Object() from entry-point RequestStream
    then
        System.out.println( "Rule 3 fired by " + Thread.currentThread().getName() );
        //cepService.executingThread1();
end

rule "4"
    when
        Message( status == Message.GOODBYE, myMessage : message )
    then
        System.out.println( myMessage );
        //cepService.executingThread2();
end

Answer 1

StatefulKnowledgeSessions are not thread-safe according to the API docs. If you absolutely need to execute rules on multiple threads, reformulate your problem using StatelessKnowledgeSessions instead.

Answer 2

You can use Stateful knowledge session in multi threaded environment. Before your app starts you have to serialize "KnowledgeBase" into a file/db". Later each thread will not create its own copy of "KnowledgeBase" but will de serialize "KnowledgeBase" from the "file/db".

If we don't serialize/deserialize "KnowledgeBase", each thread when required will try to load rules and create its own "KnowledgeBase", finally at one point if threads increases your application might end up throwing "java.lan.OutOfMemory permgen" space error. As each thread will try to create its own copy of knowledgebase by loading classes again and again into the memory.