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Re: [xml-dev] Not using mixed content? Then don't use XML


On 2013 Mar 25, at 18:03, David Lee <dlee@calldei.com> wrote:

> What miraculous corner of the markup world do you live in?
> Can we expand that corner?
> <<<<<<<<<<<<
> A Corner very near my corner.
> I have worked with XML for 15 years and only *occasionally* have had to touch a schema.
> Even now working deep in the depths of an enterprise XML Database company my use of schema is primarily a QA exercise, [...]


I've been doing SGML and XML things since SGML-on-the-web was the coming thing, and what XML stuff I still do is in the context of an interoperability standards grouping for Astronomy (http://ivoa.net).

The IVOA is rather XSchema-obsessed, most of its Recommendations come with some sort of XSchema document, and many of the data-modelling arguments are articulated (woe is us) in XSchema terms.  Even with all that, however, I feel that the schemas are pretty marginal in practice.

One way they're useful is because you've got to have _some_ way of expressing an agreement about legal XML structure.  They're used to settle arguments ('that's not a valid message because of line xxx of the schema!'; 'oh bugger, you're right, but you know what I mean...').  I think some people do use them for code-generation of one type or another.  But I suspect that most of the systems parsing the schema files are wetware.

In large part, this is because the developers in question are from multiple international institutions collaborating voluntarily, are skilled and independent-minded, and are plugging the XML into existing legacy systems on not enough funding (so, nothing unusual!).  And because everyone knows about Postel's rule, and is generally averse to brittleness.  An XML schema enthusiast would presumably say that strict(er) schema validation would help with each of these things, but... that's not how it's working out.

I suppose that Postel's rule in this context is saying that valid/invalid isn't as binary as one might expect.

All this is separate from the question of what syntax we should be using (is that this thread or the parallel one?).  XML isn't perfect for this (there are several places where (I assert) something like RDF would be a much more natural fit, and some folk push UML-type things), but it's expressive enough, everyone groks it, there's decent tool support, and we use other formats for the real heavy lifting.  This isn't very interesting XML usage -- but XML stopped being Interesting a decade or so ago -- but it works, and schemas are not central to it.

All the best,


Norman Gray  :  http://nxg.me.uk
SUPA School of Physics and Astronomy, University of Glasgow, UK

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