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   Re: URNs as SYSTEM IDs

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  • From: "Simon St.Laurent" <simonstl@simonstl.com>
  • To: xml-dev@lists.xml.org
  • Date: Wed, 22 Nov 2000 15:22:12 -0500

At 02:57 PM 11/22/00 -0500, Norman Walsh wrote:
>/ "Simon St.Laurent" <simonstl@simonstl.com> was heard to say:
>| In common practice, that's meant using URLs, typically HTTP-based URLs.
>| Validating (and some non-validating) XML parsers tend to report errors when
>| they can't retrieve the content referenced by a SystemLiteral, since
>| effectively it means that they can't validate the document.
>They can retrieve the content if they have some practical entity
>management system in place, such as OASIS TR9401 Catalogs (through,
>for example, a SAX entityResolver).

The problem is that this kind of infrastructure is barely deployed, and
many developers don't even know of its existence.  Like many validating
parsers, they're just going to punt when they hit a URN SystemLiteral.

>| I can't find a validity constraint which mandates this behavior, however.
>| It seems like dereferencing is a fundamental quality of SystemLiterals, but
>| that dereferencing is somewhat, well, variable.  Using URNs - when there
>| isn't a whole lot of infrastructure for processing them - strikes me as
>| foolhardy, but I can see where they might be attractive from an abstract
>| perspective, at least.
>At the very least, you should use a PUBLIC identifier as well since
>that allows an entity manager to do the right thing even in the
>presence of varying system identifiers.

This draft seems to be an attempt to create a parallel to PUBLIC without
using PUBLIC:

>It is standard practice within W3C
>standards to forego the use of the PUBLIC identifier in favor of
>'well known' SYSTEM identifiers. There have been several IETF
>standards that have simply created non-existent URIs in order to
>simply identify but not resolve the SYSTEM identifier for some given
>XML document. 

I'm not sure where this leaves us - in the land of rarely-deployed
architecture, I suspect.

Simon St.Laurent
XML Elements of Style / XML: A Primer, 2nd Ed.
XHTML: Migrating Toward XML
http://www.simonstl.com - XML essays and books


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