RE: [xsl] Request help in understanding: node instance of schema-element(node)

Subject: RE: [xsl] Request help in understanding: node instance of schema-element(node)
From: Wendell Piez <wapiez@xxxxxxxxxxxxxxxx>
Date: Mon, 28 Jan 2008 11:59:43 -0500
At 05:28 AM 1/27/2008, Mike wrote:
To be honest, even though Saxon does use xsi:schemaLocation for the limited
purpose of locating a schema once validation has been requested, I don't
think it's a good idea to use it. If you are validating a document then
that's probably because you don't trust it, and if you don't trust it, then
why should you trust its schemaLocation attribute?

I agree with the conclusion here. The reasoning, however, does open a deeper set of issues.

In my experience, schemas (of whatever variety) have been used for two purposes, which have historically been joined at the hip, but which are actually quite distinct.

One is "validation", which is to say, determine whether a document or parts of a document conform to an expected or required type. ("Schema as gauge.")

The other is for type annotation or even for binding of XML data (which natively takes the form of sequences of characters) to data types. ("Schema as jig.")

Sadly, even the latter process is still commonly referred to as "validation", even when the first process is redundant and nugatory, since the file is already "valid" in the important sense of "known to be valid" (i.e. trusted).

If these two functions of a schema were better distinguished, many common design problems in systems and workflow might be remediable.

I like the idea of allowing either process to be abstracted away from xsi:schemaLocation precisely for this reason (which is not to say I'm not also in favor of supporting that attribute as specified, in tools conforming to the relevant standards).


(Back in 2001 I delivered a paper that touched on these issues in a theoretical way; I'm now surprised by how dated it isn't.

Wendell Piez                            mailto:wapiez@xxxxxxxxxxxxxxxx
Mulberry Technologies, Inc.      
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