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In the "OWL and DTDs" thread, some strong claims about schemas being necessary for an XML application to be testable were made. That sounds sensible, and probably is usually very practical, but I'm not at all sure it's logically true. For example, consider a purchase order format definition. It will undoubtably define the element names and attributes and content model(s) that are legal, and at least Schematron is capable of defining some fairly robust models that won't break in the face of change, extra information,etc. But a real test regime will require constraints that aren't expressible in any XML schema language AFAIK: The <part_number> attribute value on each <item> element must match [a product in our catalog]" "The <quantity> element value must be less than or equal to the [number currently in stock according to ERP system]" "The <total_price> element value must [approved by the credit checking service given the credit card number, expiration date, cardholders name]" "The <key> element value must be a prime number represented by at least 20 decimal digits" [OK, that's totally contrived in the purchase order context, sorry] Given that the test suite definition us going to have to specify how to do all this database lookup or web service invocation or calculation or whatever, why is a schema for the syntax really and truly necessary? If one has enough confidence in one's testing regime, it could be the "real" definition of validity, no? --
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