Przemyslaw,

   There are no plans to change the behavior for 5.0. In 5.1 with a feature that we call "true modify" we may be able to improve the situation.

   Let me explain it in a better way: the "from" expression is a black box for the engine. You can call *any* code in that expression and a common case is to use the expression to pull data from a database on-demand. Since it is a black box, the engine assumes that every time the expression is re-evaluated you are bringing new data into the engine and as so it creates new fact handles for it. A modify in most Rete engines is a retract+assert and it is like this in drools 4-5 too. So, when you modify the fact that is written before the "from" in LHS, it will retract all facts previously created by "from", re-evaluate it, and create new fact handles for them.

   Since no-loop and lock-on-active use the fact handles to determine when a fact is equals to another, you get this side effects.

   A way to work around the problem is have your data in the working memory, reducing the usage of from. Another way to work around the problem is if you can write the patterns that use from before the patterns that are modified by your rules. E.g:

Rule X
when
    $a : A()
    $b : B() from somewhere
    $c : C()
    $d : D()
then
    // ...
end

   If $a is modified by any rule or by the application, the whole LHS of the above rule is re-evaluated. If $c is modified, only $c and $d are reevaluated, and so on. Placing the facts that you modify frequently "after" any pattern that uses "from" is a way to prevent this behavior. It is not always possible, I know, but it helps with this side-effect whenever possible.

   []s
   Edson



2009/2/13 Przemysław Różycki <P.Rozycki@amg.net.pl>
Edson, Michal,

thank you for your answers. If I understand well, Drools can't recognize if the $fact2 has been changed by the current rule or not.
Anyway, I found such a behaviur as a bit confusing, so my second question is, is it a desirable behaviour? And is it going to be changed or left as is?

Best regards
Przemyslaw

Edson Tirelli pisze:

   Michal is correct. Every time a "from" expression is executed, Drools does not know if a result for the expression is new or not and it creates a new fact handle, bypassing the no-loop.

   []s
   Edson

2009/2/13 Michal Bali <michalbali@gmail.com <mailto:michalbali@gmail.com>>


   This is what I think is going on:
   when you call update($fact1) it will first retract $fact1 which in
   turn retracts $fact2, the $fact1 is then inserted again which cause
   another 'from' evaluation and a NEW $fact2 is pulled -> no-loop
   doesn't work because Drools sees it as different data
   I am just guessing here ...


   On Fri, Feb 13, 2009 at 9:12 AM, Przemysław Różycki
   <P.Rozycki@amg.net.pl <mailto:P.Rozycki@amg.net.pl>> wrote:

       Hello,

       Doesn't really anyone have any logical explanation of why such a
       rule loops?

       To remind the rule:


       rule "Hello World"
              no-loop
              when
                      $fact1 : MyFactBean1(  )
                      $fact2 : MyFactBean2(  ) from $fact1
              then
                      System.out.println("Fired!!!");
                      update($fact1);
       end

       If I don't use 'from' everything is fine.
       I really didn't find in a documentation, that 'from' causes any
       special behaviour of the rule looping.


       Best regards,
       --        Przemysław Różycki
       AMG.net, A Bull Group Company
       ul. Ł±kowa 29
       90-554 ŁódĽ
       www.amg.net.pl <http://www.amg.net.pl>




       Przemysław Różycki pisze:

           Thanks for answer, Vikrant, but I don't think so. I have
           always used no-loop without any additions and it worked
           (when I didn't use from). BTW, if you comment a condition
           with from, then the rule does not loop. I also checked with
           "no-loop true" and result is the same.

           Best regards,
           Przemek

           Vikrant Yagnick pisze:

               I have to quickly recheck the documentation by I think
               you have to write "no-loop true". Just writing no-loop
               defaults to false I guess.

               -----Original Message-----
               From: rules-users-bounces@lists.jboss.org
               <mailto:rules-users-bounces@lists.jboss.org>
               [mailto:rules-users-bounces@lists.jboss.org
               <mailto:rules-users-bounces@lists.jboss.org>] On Behalf
               Of Przemyslaw Rózycki
               Sent: Tuesday, February 10, 2009 3:24 PM
               To: Rules Users List
               Subject: [rules-users] Rules with from always loops
               (Drools 4.0.7)

               Hello,

               why does such a rule always loop? Even if no-loop is used?

               rule "Hello World"
                      no-loop
                      when
                              $fact1 : MyFactBean1(  )
                              $fact2 : MyFactBean2(  ) from $fact1
                      then
                              System.out.println("Fired!!!");
                              update($fact1);
               end

               The full code of my test application and drl is attached.

               Best regards,
               --                Przemysław Różycki
               AMG.net, A Bull Group Company
               ul. Ł±kowa 29
               90-554 ŁódĽ
               www.amg.net.pl <http://www.amg.net.pl>



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--
 Edson Tirelli
 JBoss Drools Core Development
 JBoss, a division of Red Hat @ www.jboss.com <http://www.jboss.com>


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--
 Edson Tirelli
 JBoss Drools Core Development
 JBoss, a division of Red Hat @ www.jboss.com