Round 2: How an XML instance document references an XML Schema

John Aldridge john.aldridge at
Thu Jan 6 12:20:19 GMT 2000

At 23:18 05/01/00 +0000, ht at (Henry S. Thompson) wrote:
>John Aldridge <john.aldridge at> writes:

>> I'd hoped to find a statement such as "a general-purpose schema-aware
>> processor must provide some catalogue facility which allows the
>> specification of a location from which to fetch the schema corresponding to
>> an NS URI.  Only in the absence of such a catalogue entry may the processor
>> attempt to dereference the URI given by the schemaLocation attribute".
>As I've tried to convey in other messages in this and related threads, 
>the XML Schema design is VERY concerned with precisely the issue you
>raise above, namely, schema validation should not be a hostage to
>connectivity and/or URL stability.  Our approach was, however, NOT to
>design YACM (Yet Another Catalog Mechanism), but allow for ANY
>alternative schema location mechanism which people come up with.  I
>hope a careful reading of chapter 4 of the PWD [1] will clarify this
>for you.

I did carefully read Chapter 4, honest, but still struggled to understand
the way the flexibility it includes should be used.  Note that I did not
suggest above that the document should include a specific catalogue design;
just that I'd hoped it would mandate the existence of _some_ catalogue.

>For myself, I envisage schema validators working the in a similar way
>to XT, James Clark's XSLT implementation: you will be able to invoke a
>schema validator with explicit specification of the schema(s) you wish

By which you mean (I think) "explicit specification of _how to locate_ the
schema(s) you wish applied,".  Presumably you are not intended to be able
to request that elements be validated against a schema with a
targetNamespace which does not match the namespace from which the elements
to be validated are drawn?

>         or you can leave it to the validator (Not an option XT
>provides).  The XML Schema PWD allows for one, the other, or both, but
>observes that only the schemaLocation approach gives interoperability
>(at the price of fragility).

OK, that's very helpful.  So, when writing an XML file, I should start it:

<?xml version="1.0">

And then say to the customers for this data: 

   You must process this data either

   (a) in an environment with reliable access to (in which case you
       may use any "general-purpose schema-aware" XML processor), or,

   (b) you are constrained to use only those XML processors which
       allow you to specify that the schema for the namespace is to be found in some other
       location accessible to you.

In the context of the obligation "...unless directed otherwise
general-purpose schema-aware processors must attempt to dereference each
schema URI...", the existance of a catalogue or other mechanism for
locating a schema counts as "directed otherwise".

I guess I'm just suspicious that, in the absence of specific requirements,
processors will not bother to implement an such alternative mechanism.
After all, the language quoted in the previous paragraph is very similar to
that describing DTD links:  "An XML processor ... may use the public
identifier to try to generate an alternative URI.  If the processor is
unable to do so, it must use the URI specified in the system literal".

How many DTD-aware processors actually do provide a public identifier based
lookup mechanism?

I also wrote, on what I now believe to be a different topic:

>> I'm also puzzled about the semantics of a namespace declaration without a
>> corresponding schemaLocation attribute.  Does it mean:


>> (b) Unless the processor provides some alternative method of locating the
>> applicable schema, then the data cannot be interpreted and an error occurs.
>That will always be true, regardless of how things are specified:  a
>schema validator confronted with a document with elements in a
>namespace for which it neither is given nor can discover a schema will 
>necessarily declare defeat.

I guess I was really confused about the relation between schemas and

I understand your answer to mean that by using a name from a namespace, and
then using a schema-aware processor, you are automatically claiming that
the element conforms to the schema for that namespace.

There is no such thing, to a schema-aware processor, as a namespace without
an associated schema.

Thanks for your help, both here and on other topics to which I've not
contributed but have followed with interest.

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