[topicmapmail] binary associatinons: Two roleSpecs or many more?

[Steve Pepper]

Hi Johannes,

At 09:14 20.01.2003 +0100, Johannes Busse wrote:
>in defining an ontology I got the following
>question: I am wondering, how many roleSpecs can (or
>should be) "attached" to an association with *two*
>members: Two roleSpecs or many more?

If I have understood you correctly, you are asking
whether a given association type implies one (and only
one) corresponding set of role types (expressed using
roleSpec elements in XTM).

I have wondered about this myself and I did actually
experiment with it in the Italian Opera topic map.
There you will find an association type "written-by"
that is used for *both* associations between operas
and librettists *and* associations between literary
works in general and writers. (Search for the string
"AT: written-by" in either opera.hytm or opera.xtm.)

To be honest, I'm not too happy with this solution. My
feeling is that if the set of role types are different
then we are really talking about a different association
type as well (even though its semantics may be very
similar).

Is it a question of taste or of good modelling practice?
What do others think?

(One argument for sticking with a single set of role
types is that it makes the definition of contraints on
the association type much simpler, of course.)

>Once we know the type of an association (here:
>authorship) we also know the roleSpecs of the
>"first" and "second" assoc-member. The whole
>semantic information is included in the
>assoc-name.

Yes. Except that "first" and "second" is misleading
since there is no notion of direction in an association
(which is perhaps why you used "inverted commas").

>It seems to me that this often is a reminiscence to the
>database-notion of an relation, where the
>table headings are semantically not very interesting.

Yes - except that without the notion of direction you
*must* have the role types in order to make sense of
the association. (Was Tosca composed by Puccini, or was
Puccini composed by Tosca?)

>Iff this is right, there are several consequences:
>
>(1) The topic-view of an XTM-Browser should show
>not only associated topics, but also the attached
>roles *for each topic*. (To my knowledge eg. the
>omnigator does this not. Is this a feature or a
>lack of function?) - Do you know, where this
>slightly complex problem of visualisation is
>solved?

The Omnigator does do this actually, but not too
obtrusively. What we discovered was that in binary
associations, specifying the role types was almost
always unnecessary.

We encourage people to assign multiple base names to
the association type, using scopes that are defined
by the role types, thus:

<topic id="born-in">
   <baseName>
     <baseNameString>born in</baseNameString>
   </baseName>
   <baseName>
     <scope><topicRef xlink:href="#place"/></scope>
     <baseNameString>birthplace of</baseNameString>
   </baseName>
</topic>

This will display as "Puccini (born in) Lucca" and
"Lucca (birthplace of) Puccini". Displaying the role
types (person and place) is therefore not strictly
necessary (although to aid debugging the Omnigator
does show them using mouseover events). In fact, showing
the role types in parentheses (as we did in earlier
versions of the Omigator) significantly increased the
clutter.

For associations with more than three role types, the
roles *are* always displayed in the Omnigator.

>(2) the notion of transitivity of associations
>must keep in mind also the roleSpecs.

Yes - because they tell you how to "chain" the
associations in the absence of directionality.

>(3) an association can be seen as a "sparse"
>relation:
>- each row has indefinitely many columns, which
>- most of them are empty or "undefined", whereby
>- only very few of them are  used
>- these which are used have to be made explicit by
>   means of the roleSpec

I'll leave that to someone with more database expertise
to answer!

Steve
--
Steve Pepper, Chief Executive Officer <pepper@ontopia.net>
Convenor, ISO/IEC JTC1/SC34/WG3  Editor, XTM (XML Topic Maps)
Ontopia AS, Waldemar Thranes gt. 98, N-0175 Oslo, Norway.
http://www.ontopia.net/ phone: +47-23233080 GSM: +47-90827246