You need no-loop on the rules because of the call to “modify(s)” . Modify changes the  fact in working memory and the engine is automatically notified of the changes at the end of the modify block. After modify the same rule could hit again as the conditions are still true.

The first rule doesn’t need the no-loop it because of the check “recommendPurchase == null”

Joe

From: rules-users-bounces@lists.jboss.org [mailto:rules-users-bounces@lists.jboss.org] On Behalf Of Bryan Hansen
Sent: Sunday, November 02, 2008 8:21 PM
To: rules-users@lists.jboss.org
Subject: [rules-users] no-loop usage question

I have been reading through the documentation and trying some of the tutorials and have a question about the no-loop attribute.

Not really relevant to the post, but I figured I would mention it, I have been following this post and updating it to use the current version of Drools. http://www.onjava.com/pub/a/onjava/2005/08/03/drools.html?page=7

I have written my rules as such:

package org.drools.examples

import org.drools.examples.StockOffer;

rule PriceBelow100
    dialect "java"
    when
        s: StockOffer( stockPrice < 100 && recommendPurchase == null )
    then
        modify( s ) {
            setRecommendPurchase(StockOffer.YES)
        }
end

rule NegativeStockPrice
    dialect "java"
    no-loop true
    when
        s: StockOffer( stockPrice < 0 )
    then
        modify( s ) {
            setRecommendPurchase(StockOffer.NO)
        }
end       

rule DontBuyXYZStock
    dialect "java"
    no-loop true
    when
        s: StockOffer(stockName == "XYZ" && stockPrice < 10 )
    then
        modify( s ) {
            setRecommendPurchase(StockOffer.YES)
        }
end       

rule BuyXYZStock
    dialect "java"
    no-loop true
    when
        s: StockOffer(stockName == "XYZ" && stockPrice > 10 )
    then
        modify( s ) {
            setRecommendPurchase(StockOffer.NO)
        }
end       



My question is, why do I have to put no-loop on every rule (except for the first). After the first time through the rules, it shouldn't be modifying to where the rules would fire it again. Am I doing something wrong with how my rules are written? My sample data is this:

package org.drools.examples;

import org.drools.examples.BusinessLayer;
import org.drools.examples.StockOffer;

import junit.framework.TestCase;

/*
 * JUnit test for the business rules in the
 * application.
 *
 * This also acts a 'simulator' for the business
 * rules - allowing us to specify the inputs,
 * examine the outputs and see if they match our
 * expectations before letting the code loose in 
 * the real world.
 */
public class BusinessRuleTest extends TestCase {
    /**
     * Tests the purchase of a stock
     */
    public void testStockBuy() throws Exception {

        // Create a Stock with simulated values
        StockOffer testOffer = new StockOffer();
        testOffer.setStockName("MEGACORP");
        testOffer.setStockPrice(22);
        testOffer.setStockQuantity(1000);

        // Run the rules on it
        BusinessLayer.evaluateStockPurchase(testOffer);

        // Is it what we expected?
        assertTrue(testOffer.getRecommendPurchase() != null);

        assertTrue("YES".equals(testOffer.getRecommendPurchase()));
    }

    /**
     * Tests the purchase of a stock makes sure the system will not accept
     * negative numbers.
     */
    public void testNegativeStockBuy() throws Exception {

        // Create a Stock with our simulated values
        StockOffer testOffer = new StockOffer();
        testOffer.setStockName("MEGACORP");
        testOffer.setStockPrice(-22);
        testOffer.setStockQuantity(1000);

        // Run the rules on it
        BusinessLayer.evaluateStockPurchase(testOffer);

        // Is it what we expected?
        assertTrue("NO".equals(testOffer.getRecommendPurchase()));
    }

    /**
     * Makes sure the system will buy stocks of XYZ corp only if it really cheap
     */
    public void testXYZStockBuy() throws Exception {

        // Create a Stock with our simulated values
        StockOffer testOfferLow = new StockOffer();
        StockOffer testOfferHigh = new StockOffer();

        testOfferLow.setStockName("XYZ");
        testOfferLow.setStockPrice(9);
        testOfferLow.setStockQuantity(1000);

        testOfferHigh.setStockName("XYZ");
        testOfferHigh.setStockPrice(11);
        testOfferHigh.setStockQuantity(1000);

        // Run the rules on it and test
        BusinessLayer.evaluateStockPurchase(testOfferLow);
        assertTrue("YES".equals(testOfferLow.getRecommendPurchase()));

        BusinessLayer.evaluateStockPurchase(testOfferHigh);
        assertTrue("NO".equals(testOfferHigh.getRecommendPurchase()));
    }
}


I can post up all the code if this isn't enough to answer the question.

Thanks!