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RE: XML Schemas: Best Practices ? Versioning

On 07/09/2001 21:02:00 Michael Brennan wrote:

>I strongly disagree with this. Building a distributed web architecture that
>relies upon explicit specification of system paths to schemas in instance
>documents that may be exchanged among parties over the internet leads to
>more complication than is necessary.

Well, you this is no worse than what we have now, where public identifiers are effectively deprecated in the XML 1.0 spec in favour of system identifiers.  Actually, what I am suggesting as best practice is that schemaLocation URLs should not be treated as literal paths to Schema files which must be dereferenced, but rather as indicative URIs which an entity resolver can than use to grab the local copy of the appropriate Schema.

>Quite apart from that, private conversations you have had with W3C people
>are irrelevant. Implementors of schema processors have to rely upon the
>published specifications. They cannot take into account private
>conversations to which they are not privy. I have serious problems with
>people claiming that private conversations with W3C members are somehow more
>authoritative than the published specifications.

Sadly, and I say this from experience, not all of the collected wisdom and discussions of standards committees makes it into the specs.  Sad but true.  So, while I can understand that literalist interpretation of specs might seem to be the only good and noble thing to do, it can also cause a lot of problems where the context for part of the spec may not be properly represented in the spce itself.  My concern is that this is the case with "schemaLocation".  What is clear, I would argue, is that the usage of "schemaLocation" is not clearly enough defined in the XML Schema spec, because there is too much breadth of interpretation possible, and that breadth risks compromising interoperability.

>The XML Schema specifications are quite explicit on this matter. The
>schemaLocation attribute is optional. Quite apart from that, it is only a
>hint to help a processor to locate the schema. It is not authoritative even
>if it is present.

It is explicit, but does not make it clear
(i) whether the "schemaLocation" is intended for use only where really necessary, or
(ii) whether instead Schema validators are given some latitude in the way they operate, for exceptional cases where the usual "schemaLocation" mechanism cannot be used or is otherwise not appropriate.

Unfortunately, both of the above are consistent with the spec, but not with each other, not from an interoperability and best-practices perspective.

Anthony B. Coates
(1) Content Distribution Architect - Project Gazelle
(2) Leader of XML Architecture & Design - Chief Technology Office
Reuters Plc, London.

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