The best thing to do, if you can, is simply to write the rules so they
don't leave any extra facts behind; i.e., if one rule asserts some
fact, then some other rule matches it and deletes it. That's going to
be the easiest and most efficient solution in the long run.
Cloning a Rete object is always going to be slow, because it's not one
object -- it's a network of potentially thousands of interconnected
objects.
If, though, you absolutely can't get the rules to clean up after
themselves, I've got a nifty piece of code written by a user that
implements "unwind/protect." Unfortunately I can't give credit because
I don't remember where it came from anymore -- if the author is
listening, speak up, please. Anyway, here are the functions. There are
different versions for Jess 5 and Jess 6 (-5 and -6.) I haven't used
this in a while so it's possible it might need tweaking.
(deffunction protect ()
(bind ?p (assert (protect-to-here)))
(store PROTECT ?p))
(deffunction unwind-6 ()
(bind ?p (call (fetch PROTECT) getFactId))
(bind ?v (new java.util.Vector))
(bind ?facts (call (engine) listFacts))
(while (call ?facts hasMoreElements)
(bind ?fact (call ?facts nextElement))
(if (< ?p (call ?fact getFactId)) then
(?v addElement ?fact)))
(bind ?facts (call ?v elements))
(while (call ?facts hasMoreElements)
(bind ?fact (call ?facts nextElement))
(retract ?fact)))
(deffunction unwind-5 ()
(bind ?p (fetch PROTECT))
(bind ?v (new java.util.Vector))
(bind ?facts (call (engine) listFacts))
(while (call ?facts hasMoreElements)
(bind ?fact (call ?facts nextElement))
(if (< ?p (call ?fact getFactId)) then
(?v addElement ?fact)))
(bind ?facts (call ?v elements))
(while (call ?facts hasMoreElements)
(bind ?fact (call ?facts nextElement))
(retract (call ?fact getFactId))))
;; Example of using this
(assert (foo 1) (foo 2) (foo 3))
(protect)
;; This displays only the foo facts
(facts)
(assert (boo 1) (boo 2) (boo 3))
;; see six facts here
(facts)
(unwind-6)
;; see only the foo facts again.
(facts)
I think Joe Kopena wrote:
>
> I'm curious if anyone has any suggestions on handling the following
> situation:
>
> I have a knowledge base in my program, the Jess Rete object. The program
> receives a message and feeds it into the knowledge base so it can be
> interpreted properly. The program then looks for certain facts in the
> knowledge base (ie: a query defined by the message) and acts
> accordingly. Once the message is processed, I want to rollback the rete
> to the state it was before the message came in so the rete won't grow
> too big as I continue to receive messages. Complicating things, messages
> might be coming in concurrently. I understand that the rete's methods
> and parts are synchronized and thread safe (correct?). However, it does
> mean that I can't do something simple like count the number of facts
> before the message, count the number afterwards and subtract the
> difference from the tail of the facts list. I was thinking of cloning
> the rete for the message handler so it can't touch the original. This
> sounds potentially slow, but not nearly as slow as reading in all the
> facts to initialize the kb. It does sound difficult though. It's not
> clear to me just what that would entail, how many links the rete has and
> so forth. Would it be not worth the effort and easier to just assert a
> ton (couple hundred) facts into a new Rete everytime a message comes in?
>
>
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---------------------------------------------------------
Ernest Friedman-Hill
Distributed Systems Research Phone: (925) 294-2154
Sandia National Labs FAX: (925) 294-2234
Org. 8920, MS 9012 [EMAIL PROTECTED]
PO Box 969 http://herzberg.ca.sandia.gov
Livermore, CA 94550
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