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Re: [PRD] Default conflict resolution strategy (ISSUE-64) (Was; Re: [Fwd: Re: PRD Review])

From: Mark Proctor <mproctor@redhat.com>
Date: Wed, 30 Jul 2008 14:31:14 +0100
Message-ID: <48906D22.6090209@redhat.com>
To: Gary Hallmark <gary.hallmark@oracle.com>
CC: Christian de Sainte Marie <csma@ilog.fr>, RIF WG <public-rif-wg@w3.org>

Gary Hallmark wrote:
> I don't think Jess no-loop is easily simulated because I don't think 
> it is easy to express in the PRD semantic framework.  I'm not overly 
> concerned about no-loop because I think the Jess no-loop declaration 
> is somewhat ill-conceived. I am more concerned that one of the reasons 
> that no-loop seems hard to express is that the agenda and working 
> memory are updated after a picked sequence of actions is run to 
> completion, rather than the more usual semantics that the agenda and 
> working memory are updated after each action.  This could be 
> problematic if we have actions that internally access the agenda or WM 
> (e.g. a print-agenda builtin or a counter increment builtin)
I thought that the Jess agenda was updated as soon as the actions are 
executed, just that it was not evaluated until the end of the consequence?
> Mark Proctor wrote:
>> (deftemplate person (slot age) )
>> (defrule rule1 (declare (no-loop TRUE)) (person (age ?a) ) => (assert 
>> (person (age (+ ?a 1) ) ) ) (printout t fire " " rule1) )
>> (assert (person (age 200) ) )
>> (run)
>> I made the above example to test our Jess. What do we do about clips? 
>> I guess no-loop can be simulated with objects acting as semaphores on 
>> the LHS pattern constraints.
>> Mark
>> Gary Hallmark wrote:
>>> Christian de Sainte Marie wrote:
>>>> Gary Hallmark wrote:
>>>>>> Mark Proctor wrote:
>>>>>>> Refaction in JRules is the same as jess/clips/drools no-loop. 
>>>>> I'd be surprised if it is exactly the same.   What do you expect 
>>>>> from the following ruleset:
>>>>> p(1)
>>>>> q(1)
>>>>> q(2)
>>>>> (* rule1[loop->false] *)
>>>>> Do(external(print("rule1 fired, ?x=" ?x))) :- q(?x)
>>>>> (* rule2[loop->false] *)
>>>>> Do(Retract(p(1)) p(1) external(print("rule2 fired, ?x=" ?x))) :- 
>>>>> And(p(1) q(?x))
>>>> I guess you mean:
>>>> rule1: if q(?x) then print("rule1 fired, ?x=" ?x)
>>>> rule2: if p(1) and q(?x) then Retract(p(1)), Assert(p(1)), print 
>>>> ("rule2 fired, ?x=" ?x)
>>>> Hey, Gary, this is not in the spec, you can write them rules in a 
>>>> way that makes sense :-)
>>> I think we'll need to speak somewhat formally here to be sure we 
>>> understand each other.
>>>>> With Oracle/Jess, you get
>>>>> rule2 fired, ?x=2
>>>>> rule1 fired, ?x=2
>>>>> rule1 fired, ?x=1
>>>> With JRules, you bind only objects, so, the example does not really 
>>>> work.
>>>> But if you could bind primitive data types, (I have to check, but I 
>>>> think) 
>>> I think you'd better check.
>>>> that you would fire rule2 with ?x=1 as well. 
>>> Do you mean rule2 fires twice, once with ?x=1 and once with ?x=2?  
>>> That demonstrates a difference in semantics.  As I said, with Oracle 
>>> and Jess, rule2 fires once with ?x=2 (because ?x=2 is the more 
>>> recent binding)
>>>> And that is certainly what you would get with the semantics that 
>>>> was described in PRD.
>>> Please clarify whether you mean rule2 fires once or twice, and then 
>>> please "prove" it using the PRD semantics.  I don't see how PRD can 
>>> give the Oracle/Jess semantics without some substantial changes.  
>>> PRD assumes that mergeInstances is not called between actions of the 
>>> same rule; yet in Oracle/Jess (and most engines, I think) the agenda 
>>> is modified by individual actions and this is why when the (actions 
>>> of the) first instance of rule2 is executed, the second instance is 
>>> removed.  But when the first instance of rule1 is executed, nothing 
>>> happens to the second instance of rule1 and so rule1 is executed twice.
>>>>> The semantics of Oracle/Jess is that a rule's actions cannot 
>>>>> re-activate that rule.  Initially, because of the input data, 
>>>>> rule1 and rule2 each have 2 activations.  When rule2 fires, the 
>>>>> retraction removes the other rule2 activation.  The assert action 
>>>>> cannot reactivate rule2 because of the no-loop property.
>>>> I do not understand that. I mean, I do not understand the intended 
>>>> behaviour: do you mean that p(1) after rule2 has been fired is 
>>>> distinct from p(1) before, and that, somehow, the engine recognizes 
>>>> the difference? Does this have to do with the assertion being 
>>>> logical (and the engine tracing the origin of it)? I mean, as in a 
>>>> TMS: Jess has a TMS facility, right? I have to check how we handle 
>>>> refraction (or equivalent) wrt logical statements in JRules...
>>> No, it has to do with the fact that even though the initial and 
>>> final states of the sequence of actions Do(Retract(p(1)) p(1)) are 
>>> the same (assuming p(1) exists in the initial state), this sequence 
>>> of actions is different from Do().  And this difference is not 
>>> accounted for in the semantics.  This difference is needed to 
>>> explain the Oracle/Jess no-loop semantics.
>>>>> We need to carefully specify these strategies to be sure we agree 
>>>>> what they are, and then we need to see what the intersection is, 
>>>>> and if it is  >= 1, give it an explicit name(s) (and probably not 
>>>>> in the metadata as my example does).
>>>> Absolutely agree.
>>> BTW, Oracle/Jess does not support all-at-once rule firing, other 
>>> than by aggregation.  Nor do they support random firing. I think 
>>> these are rare features, and do not merit standardization. 
>> I'm ok with that. These are mostly hacks to get over the overhead of 
>> a single agenda, I think more research needs to be done here that 
>> more strongly supports parallelisation techniques for PR systems.

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Received on Wednesday, 30 July 2008 13:33:42 UTC

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