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Re: isa and hasa in UML

From: Francis McCabe <fgm@fla.fujitsu.com>
Date: Fri, 30 May 2003 13:25:40 -0700
Cc: www-ws-arch@w3.org
To: Walden Mathews <waldenm@optonline.net>
Message-Id: <E1AD3D5B-92DC-11D7-BA7E-000393A3327C@fla.fujitsu.com>

On Friday, May 30, 2003, at 12:05  PM, Walden Mathews wrote:

> This is interesting, and a little bit shocking.  I can't tell from this
> what the goal of the action item is.  If I knew that, I might make a
> helpful suggestion.
> IN GENERAL, although UML might not be the formal modeling
> venue of choice, there is a reality check which UML can be
> used to perform:  See if UML is capable of capturing the essence
> of the WSA, without becoming too "AR" in the process ("R" stands
> for "retentive").  If it is not, consider the thing being modeled as
> a prime candidate for the bit bucket.  KISS and all that...

I think that it is possible to use UML. However, don't expect to use 
unchanged the `built-in' relations of is-a and association. UML allows 
you to define your own relations, and even to use OCL to formalize it. 
That way you end up with the pretty(?) diagrams, but using guillemots 

>> The problem is that it becomes really difficult to talk about weird or
>> abstract sets. Basing is-a on this would lead to the following
>> counter-intuitive result: every unicorn is a yeti. (There are no
>> documented instances of either, so the set of unicorns and yetis is
>> indistinguishable.)
> True, the empty set is equal to itself, and that leads to some
> counter-intuition, but not to logical inconsistency.  Nor are you
> prevented from saying: "there exists no Unicorn u and Yeti y
> such that y = u."  So empty sets don't force undersired logical
> equivalence, as you imply.

Modeling at this level is actually modeling using axioms; which is what 
I was calling case 3 but should really call axiomatic modeling. The 
unicorn and the yeti is a logical consequence of a strict set element 
notion of is-a.

>> A more serious issue, sticking with birds for the moment, is that it 
>> is
>> similarly hard to talk about properties of birds such as flying: we
>> could not express the fact that all birds except penguins fly.
> Why not?  There are birds that fly and birds that don't.  There
> is the set of individuals that are birds.  There is the set of 
> individuals
> that fly.  There is intersection between the sets, also differences.

Yes, except that when talking about birds, you first of all have to 
find all the birds and then partition them into appropriate sets. That 
begs a pretty important question.

>> An even more serious issue is that we need to capture the following
>> situation:
>> A service has an identifier
>> A Web service is a service
>> A Web service has a URI
>> The Web service's URI counts_as the service identifier
>> It is that counts_as that is beyond the capabilities of UML's
>> generalization. We *could* extend UML's generalization, and that may 
>> be
>> the best overall approach. In fact, we would really need to do that 
>> for
>> all our relationships, use is-a and has-a and *never* rely on 
>> UML's
>> built-in relationships. is-a and has-a could probably be defined 
>> in
>> OCL.
> A service *is identified by* an identifier
> A Web Service *is-a* service
> A URI *is-a(n)* identifier
> A Web Service *is identified by* a URI
> "Counts-as" is bogus, sorry.  It's simply an inherited relation, with
> constraint added.  And "has-a" is wrong in this context, too, because
> identifiers are not part of the composition of the thing identified.

How do you *know* that the Web service's URI is the service's 
identifier. That is what counts-as is intended to capture. It is not at 
all bogus. (A Web service could have a URI that is intended for a 
different `purpose' than the service's identifier: knowing that a URI 
is an identifier is not enough. Also, is-identified-by could be a 
many-many relationship: there is nothing here that is nailing down the 
intuition that a Web service's identifier acts as (a.k.a. counts-as) 
the service identifier. You cannot use higher-order properties amongst 
relationships (such as cardinality) to achieve this.

> If you don't find UML satisfactory for modeling the WSA, perhaps
> I could interest you in Alloy?

Can you provide a pointer to Alloy?

In general, I think that we could use UML. However, the feeling that I 
get is that somehow UML `solves' the modeling question. I wish that 
that were true; but it doesn't. Furthermore, the UML community has 
really resisted becoming formal; I was pretty disappointed in the lack 
of interest personally.

> Walden
Received on Friday, 30 May 2003 16:26:12 UTC

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