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- From: Curt Arnold <CurtA@techie.com>
- To: firstname.lastname@example.org
- Date: Fri, 22 Sep 2000 07:22:38 -0500
This is in response to
html on XML Schema last call issue LC-117
Noah Mendelsohn wrote:
> >> there is not a mechanism to identify a schema
> >> resource to be used to validate an XML 1.0
> >> (pre-namespace) compatible document.
> In fact, as described in section 6.3.2 of the
> structures specification, the noNamespaceSchemaLocation
> attribute is provided for exactly this purpose.
Using xsi:noNamespaceSchemaLocation to specify a schema location hint
cause the document to be invalid according to a pre-existing DTD. The
way that I could see that a document could declare that it adheres to a
specific schema while retaining validity to an existing DTD would be
use of a processing instruction.
> There is no need to explicitly import (since you are
> talking about multiple namespaces, I presume you meant
> import rather than include) namespace B into the schema
> for namespace A, except in the case where constructions
> from B are explicitly and directly used in creating
> declarations for A.
I did mean import (sorry) and I was precisely talking about instances
constructions from multiple namespaces are used in creating a schema
resource that combines elements from multiple namespaces.
For example, if I have an "http://www.example.com/namespace/automobile"
namespace that imports distinct namespaces for major subsystems
("..drivetrain", "..engine", "..interior", "..frame", "..suspension",
"..tires", "..transport", "..finance", etc) plus generic namespaces like
XHTML and uses elements from these distinct namespaces to describe the
overall picture of a car. Each individual namespace may be owned by a
different organization (either an internal division or WG or an external
organization) that may independently revised.
When I assume version "1.0" of the schema for "../automobile", I specify
through my <xsd:import>'s specific hints that indicate that version
this namespace uses elements from version "1.15" of "drivetrain", "1.7"
"engine", "1.3" of interior, etc.
For me to use schemaLocation to specify that this schema resource should
used to validate its 15 or so imported namespaces, I have to do
In general such a schemaLocation would be prone to error (omitting or
mistyping one or more namespaces), that it would typically be omitted.
However, at that point, you effectively lost the ability of the document
assert (though it could always be lying) that it was valid against a
specific schema resource.
>The specification makes clear that schemaLocations are
>hints, on imports and in instance documents. Processors
>are therefore free to use either of the hints, or to ignore
The tension comes from a dual nature of schemaLocation. It tries to act
both as a retrieval location for a resource (a file:// URL that an XML
editor validates during development) and an assertion of conformance to
known universally identified schema resource (a URN or http:// URL that
would rarely be dereferenced) that a server would typically ignore
validating with a schema resource of its choosing.
Definitely, the "assertion of arbitrary conformance but verification
expectations" scenario is an extremely important scenario that you had
dirty parser tricks to accomplish with DTD's. However, most authors
expectations when they hear "schemaLocation" would be to expect DTD-like
behavior. Renaming "schemaLocation" to "schemaIdentifier" might clarify
expectations. That might minimize the need to say, "it is only a
is an identifier whose optional, last-ditch resolution mechanism is
I had in my original message has suggested use of system identifiers,
however I understand now that use of URI's is preferred for
with RDF among other things.
Based on your message, I going state that I'm not satisfied with the
proposed resolution (basically to leave schema resource location
substantially unchanged with the exception of LC-116) in that there are
substantial use cases (XML 1.0 compatible documents and documents where
large number of namespaces are governed by a small set of schema
where the suggested mechanism of asserting schema conformance is
unsatisfactory to the point that alternative solutions will be used.
I would recommend (in order of significance):
1. Renaming xsi:schemaLocation to xsi:schemaIdentifier (or maybe
xsi:assert). Tools that want to check the assertion may (but are not
required) to dereference the URI if they cannot resolve the identifier
another mechanism. Primer and other text should suggest that
xsi:schemaIdentifier should be a URN or at least an absolute http: URL.
2. Recasting "locating schema resources" sections to "asserting schema
conformance" or similar wording.
3. (Optional) Add back xsi:schemaLocation (or call it something
say xsi:map) but as pairs of schema identifiers (not namespaces) and
retrieval URL's that can be used in resolving schema identifiers. For
example (ignoring my issues with using NS-Schema pairs for the moment)
"xsi:schemaLocation" attributes would be expected to be vanishingly rare
production systems and would be ignored by default, but could be used
4. (Optional) Define a processing instruction that XML 1.0 compatible
documents can use to assert conformance to a schema without compromising
their validity with an existing DTD. Something like ( indicate
Which would be interpreted as asserting the document conforms the schema
identified as http://.../schema1.xsd that has some probability of being
found a c:/myschemas/schema1.xsd when http://www.example.com/ns1 is used
unqualified element names. (ns="" might be a little over the top). An
transform could use the info in the processing instruction to convert
document to a namespaced XML document with good, old
5. Provide an mechanism to declare that a schema identifier covers
namespaces. This could be as simple as making the odd position elements
xsi:schemaIdentifier either a namespace URI or "##any", "##other" or
"##none" (none would eliminate the need to
Our automotive example could then be: