[rules-users] not sure if this is a bug of drools or my bad usage...

[Simon Chen]

What I had is a very simplified version of how calculating transitive
closure could go wrong...

Let's say we have two rules:
rule 1
when
  link(a,b)
then
  insertLogical(new reachable(a,b))

rule 2
when
  link(a,b) reachable(b,c)
then
  insertLogical(new reachable(a,c))

Let's say, I have link(a,b), link(b,c), link(b,a), link(c,b). So,
we'll have reachable(a,b), reachable(b,c), reachable(a,c), etc. But,
after I retract link(a,b) and link(b,a), guess what, reachable(c,a)
still exists! This doesn't sound right to me.

But in Drools, this is possible, because we have:
reachable(c,a) <- link(c,b), reachable(b,a)
reachable(b,a) <- link(b,c), reachable(c,a)

The problem here is that we actually inserted reachable(b,a) multiple
times: first supported by link(b,a) and rule 1, and secondly by
link(b,c) and reachable(c,a) and rule 2. When reachable(b,a) was
inserted the second time, link(b,c) and reachable(c,a) become the
additional supporting condition - maintained by the truth maintenance
system. So, even if link(b,a) is retracted, reachable(b,a) still
exists further supporting reachable(c,a).

Is it clearer?

Thanks.
-Simon

2011/3/7 Edson Tirelli <ed.tirelli at gmail.com>:
>
>    Simon,
>    The behavior seems correct to me as B is justified by either A or C (or
> both). Of course, from the initial state, A is required for C to first
> exist, but once it starts to exist, your rules say that B and C justify each
> other and so both remain in memory.
>    This is design as intended, but do you think that is wrong?
>    Edson
>
> 2011/3/7 Simon Chen <simonchennj at gmail.com>
>>
>> Hi all,
>>
>> An interesting finding:
>>
>> I have three simple rules:
>> rule "A2B"
>>        when
>>                A()
>>        then
>>                insertLogical(new B());
>> end
>> rule "B2C"
>>        when
>>                B()
>>        then
>>                insertLogical(new C());
>> end
>> rule "C2B"
>>        when
>>                C()
>>        then
>>                insertLogical(new B());
>> end
>>
>> Basically, once we have an A(), we'll logically insert a B(). Once we
>> have a B(), we'll logically insert a C(). Once we have a C(), we'll
>> logically insert a B().
>>
>> So, I first insert an A(), print all the objects. Retract A(), and
>> print all the objects. Here's what I got:
>> com.sample.B at 42
>> com.sample.C at 43
>> com.sample.A at 548997d1
>> after retract!
>> com.sample.B at 42
>> com.sample.C at 43
>>
>> So, B() and C(), which should be logically depend on A(), somehow are
>> not retracted. The problem I see is the truth maintenance system allow
>> B() and C() to depend on each other, thus not affected by losing A().
>>
>> Is this a bug or my bad usage?
>>
>> Thanks.
>> -Simon
>> _______________________________________________
>> rules-users mailing list
>> rules-users at lists.jboss.org
>> https://lists.jboss.org/mailman/listinfo/rules-users
>
>
>
> --
>   Edson Tirelli
>   JBoss Drools Core Development
>   JBoss by Red Hat @ www.jboss.com
>
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