xml-dev - URNs as SYSTEM IDs

["Simon St.Laurent" <simonstl@simonstl.com>]

While writing an xmlhack story [1] on an IETF Internet-Draft [2], I
encountered a 'feature' of the XML 1.0 Recommendation [3] that may cause
more difficulties in practice.

SYSTEM identifiers, or more properly, the SystemLiteral which contains the
content of the SYSTEM identifier, are defined as URIs, conforming to RFC
2396.  These URIs are "meant to be dereferenced to obtain input for the XML
processor to construct the entity's replacement text."

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.

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.

I'm worried about this providing yet another interoperability issue inside
of XML 1.0, but I'm not sure there's a hell of a lot we can do about it,
except perhaps enjoy a 'lucky dip' (as Rick Jelliffe called it) every time
we encounter a SystemLiteral.

[1] - http://www.xmlhack.com/read.php?item=913
[2] -
http://www.ietf.org/internet-drafts/draft-mealling-iana-xmlns-registry-00.txt
[3] - http://www.w3.org/TR/REC-xml#NT-ExternalID

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