Object-oriented serialization (Was Re: Some questions)

Matthew Gertner matthew at praxis.cz
Sun Dec 5 16:28:02 GMT 1999

Dan Brickley wrote:
> I believe it will be possible to annotate XML schemas with information
> for mapping into (generic or domain specific) application datamodels
> such as RDF. I don't think it is right to expect the hard-pressed XML
> Schema group to define all these mappings within that working group.
> But that doesn't matter; all we need is a placeholder for such
> information.

I totally agree. As long as these considerations are being taken into
account, I'm sure there will be plenty of people experimenting with
various approaches. This will certainly lead to a better understanding
of how to address these issues than simply mandating something that was
worked out by a committee.

> My understanding of the Cambridge Communique meeting was that we reached
> agreement on just this. See points 1-6 under '3. Observations and
> Recommendations' in http://www.w3.org/TR/1999/NOTE-schema-arch-19991007

The need to develop an abstract schema for representing objects and
properties is very clear; one of the problems people have with
understanding RDF is that this need seems so obvious that they assume
XML already fills it. The real question is whether a separate RDF syntax
is the appropriate way to do this. I see a lot of value in seeing this
information as an extension of the information currently provided in an
XML schema (i.e. basically a serialization of the XML infoset). The
overlap is great because both RDF schemas and XML schemas are working
with the same basic informational units (assuming you accept the mapping
of class -> element type).

The RDF model also seems awfully complex for normal mortals. If the
stated target were knowledge management specialists, then there is
clearly an important niche market for a very complete mechanism for
specifying semantic relationships between resources. If we are talking
about the standard mechanism for object interchange on the Web, a
simpler mechanism (adding, for example, only the notion of properties
and strong datatyping) implemented inside an XML schema has a much
greater chance of being widely accepted. Of course, I'm only guessing
that XML schemas will be very widespread anyway, so there's plenty of
room for disagreement.

> Sure, you could do this. My hunch is that the urge to do this won't be
> as strong when we have more abstract (objects and properties) interfaces
> to XML content, rather than our current APIs that obsess on detail of
> particular serialisations rather than on what those serialisations have
> told us about the objects. If we could get to a world where generic
> rather than domain interfaces being useful to even 10% instead of 5% of
> applications (to borrow your figure), that'd be a huge win.

Interesting insight. I see your point, but I also see this also
supporting the argument for making any XML instance a potential object
by using the associated schema for conveying information about object
properties. This would mean that there would be a "new DOM" only works
on valid instances. If you do have a schema, it should be possible to
exploit this directly by having better generic interfaces, rather than
trying to treat well-formed and valid instances in the same way.

> There is also a need to know the objects'n'properties view of the data
> without going to fetch (or having advance knowledge of) the
> syntactic schema or serialisation policy. RDF's
> initial syntax was one approach; there have been and will be
> others. The Microsoft folks were for a while throwing around some
> interesting ideas on mapping more 'colloquial' XML syntax into directed
> labelled graphs. There's a version at http://www.biztalk.org/Resources/canonical.asp
> for example.

Cool, I will have to take a much closer look at that. It seems to be
very close to what I am talking about. Thanks for the tip.

> I've also heard that some folks want to use it for structured hypertext
> documents...
> (One consequence of XML's document heritage is that document order is
> generally treated as meaningful and in need of preservation. This can be
> a pain in the butt for data-centric apps.)

Fair enough, but the potential for object interchange is what is getting
people really excited. Nothing about having object facilities in the XML
schema language precludes the use of XML for structured documents. But
if we are talking about a web "vision", the potential for easy
interchange of data between applications is more likely to have a
revolutionary impact than the use of structured documents and
stylesheets. I also think that many of these object facilities will
actually turn out to be very useful for what are normally considered to
be documents.

> I don't see a conflict here. RDF is happy with multiple ways of shipping
> I'd love to see examples of an annotated XML Schema that shows how to
> derrive an objects'n'properties view of instance data.

I am going to have to write up something about this. Stay tuned...


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