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Re: [xml-dev] Need a language whiz: An XML Schema "specifies" howdata is to be structured? "describes"? "constrains"?


On 4 Jan 2018, at 14:46, John Cowan wrote:

But it
seems to me that this is only half of assessment: if a document is valid,
you may still decide to reject it for any of a variety of reasons:
This, many times!

I think people often become obsessed by XML validation, apparently forgetting that in any non-trivial document/workflow there will be multiple layers of validation of the type which John illustrates, which cannot reasonably be articulated with any schema, and in comparison to which schema validation is merely an initial sanity check, or possibly contract check.

If, for example, a postal code has a mistaken format, and this is caught by a schema check, then the resulting error ('computer says no') is likely to be much less intelligible to the 'sender' than if the error is caught at a higher level, where there is more semantic context. This is a type of 'brittleness', which in my experience is often associated with complicated schemas.

It may therefore be a wise general position to avoid designing intricate schemas as much as possible, and to perform accept/reject decisions relatively late (I can easily think of cases where this would not be appropriate, of course).

Best wishes,


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

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