[sc34wg3] The interpretation of facets

Martin Bryan sc34wg3@isotopicmaps.org
Sat, 26 Apr 2003 08:44:04 +0100

Steve Pepper wrote

> The draft HyTM specification in N391 contains the first complete
> attempt to explain the interpretation of facets with respect to
> the model implicit in XTM. This was way past due and is a sign
> that the restatement of 13250 really is leading to positive
> results. However, I don't believe that the interpretation given
> in N391 is correct. This posting outlines what I believe is a
> more correct interpretation and a couple of problems I see.

and later he wrote

> I believe that each (locator,facet,fvalue) triple should, in
> principle, give rise to an association whose type reflects the
> facet and where the role playing topics reflect the locator and
> the facet value.

The naming of facets is much more complicated than the simplistic model
proposed by Steve implies. The facet name may be defined in another topic,
which has multiple names. If it is not specified using a reference to a
topic the name can be defined either as the value of the linktype attribute
or as the name of the generic identifier of the element. For these to be
used as part of an association they have to be turned into addressable
topics first.  (I'm presuming that when Steve refers to the type of an
association he is refering to the topic identified by the type attribute of
the association, which becomes the AssociationType property of the
association in SAM, rather than suggesting that the linktype or GI of the
facet become the linktype of the association.)

It is the fvalue name, not the facet name, that needs to be linked to the
location(s) identified by the contents of the fvalue element. Once again the
fvalue name can be identified in three ways: by pointing to a topic with
multiple names, by entering a specific value for the facetval attribute or
by using the name of the generic element of the conforming element as the
default value name. Again the last two need to be defined as topics before
they can form part of an association.

As for associating a SAM locator with the facet value (which is what Steve
seems to be proposing in his triple), this is possible only if the thing
being located is part of a topic map. There is no requirement that the
location defined in a facet value be within the topic map. A topic map can
assign properties to a resource without that resource being an element in a
topic map. Like RDF, it is a simple mechanism for association property/value
pairs to resources. A topic map can validly contain just facet definitions.
In other words it can simply be used, like any RDF resource, to assign
metadata to properties. So, for example, I could use facets to distinguish
the set of documents at a particular website that were in English from those
that were in French. (Remember that topic maps preceded RDF: at the time we
did this topic maps were the only way of doing this.)

So the question comes: should facet-property-name/facet-property-value pairs
associated with locations within topic maps be treated differently from
those associated with locations that do not refer to topic maps? At the
present moment N391 treats the location as a string, and then makes this the
value of a SAM Occurrence item. The reason for doing this is simply to allow
for matching of locations identified by facets with those used by topics,
which have the same structure. Basically what facets do is identify
occurrences of resources to which a specific property value should be
applied, in the same way that occurrence elements identify resources that
cover a particular topic within a specific role.

I do not see any other way in which we can create associations with items
that are not otherwise part of the topic map using the existing set of
components in the SAM. I feel it should not be necessary to abuse the SAM
Occurrence item in this way, but given the SAM team's refusal to add a
proper Facet item to the model the only choice I had was to force the
creation of topics for facet-property-name/facet-property-value pairs and to
associate these with SAM Occurrence items. I have always objected against
needing to create topics specifically to record the names of facets and the
values assigned to them. The proposals in N391 are anethema to me, but they
are the only way I can see of applying the current set of SAM information
items to the recording of facets that allows all possible uses of facets to
be covered.

Martin Bryan