[sc34wg3] Questions on N0396: (12) [resource] property on occurrence

Graham Moore sc34wg3@isotopicmaps.org
Tue, 22 Apr 2003 19:35:36 +0100

Hi Jan,

We have discussed this very issue. I brought this up with Lars Marius as I
had the same thought.

I think the issue is not necessarily that both things reference a resource,
but that if you model an occurrence as an association then at one end of the
assoc would be a topic that had a SubjectAddress.

The best rationale is that the occurrence is a refinement/ subclass of this
general case. The reason for having an Occurrence structure at all is to
convey explicitly one of the major ideas about topic maps. i.e. the topic
network layer and the information resource layer. One of the main ways of
joining these two layers is through the use of Occurrences. To further make
this an easy to understand concept the SAM presents it as having only one
locator value - i.e. one connection, to one explicit resource.

There are some other reasons perhaps, like the XTM syntax doesnt explain
things like this, so people that have adopted XTM and thus built SAM like
models to support it dont have the concept of occurrence as assocs and thus
dont have occurrences with multiple locators.

The other thing that is interesting from an information science point of
view, is this. My frst thought was that it would make sense to have multiple
locators in occurrences to indicate multiple locations where the resource
can be found. Thus mapping it up to an assoc would imply that 'the same
resource' lived at all of these locations. I think this is a mistake.
Information science makes a difference between the 'concept of the work' and
'instances of the work'. This implies to me that having multiple locators
for the occurrence, and even for the SubjectAddress of a Topic should
resolve to the same stream of bytes from the same server. i.e. they act as
aliases not as alternative locations for a copy of an information resource.
The copy of the SAM stored on my server is a different resource from the one
on Lars Marius's.

Given this, having a occurrence with multiple locators is not the same as
having a SINGLE assoc with one (resource) topic and multiple
SubjectAddresses. It would more sensibly map to multiple occurrences to
multiple associations. This means that having multiple locators is either a)
ambiguous or b) just a model shorthand.

This discussion has got me thinking some more about SubjectAddress now...




Graham Moore, Ontopian               <URL: http://www.ontopia.net >
GSM: +44 (0) 7769658611             moore@ontopia.net

----- Original Message -----
From: "Jan Algermissen" <algermissen@acm.org>
To: <sc34wg3@isotopicmaps.org>
Sent: Tuesday, April 22, 2003 6:48 PM
Subject: [sc34wg3] Questions on N0396: (12) [resource] property on

> Lars, Graham--
> The [resource] property on the occurrence item type is defined
> to have a locator (or null) as its value.
> The [subject addresses] property on the topic item is defined to have
> a set of locators as it's value.
> Since both properties refer to an information resource, I think this
> is a bit inconsistent.
> What is the rationale behind this?
> Jan
> --
> Jan Algermissen                           http://www.topicmapping.com
> Consultant & Programmer                   http://www.gooseworks.org
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