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

John Aldridge john.aldridge at
Wed Jan 5 18:50:26 GMT 2000

At 09:22 04/01/00 -0500, "Roger L. Costello" <costello at> wrote:

>There has been a considerable amount of discussion (and confusion) on
>how an XML instance document indicates the XML Schema(s) that it
>conforms to.  I am not sure that it is yet clear in people's minds on
>how to do it.  I will take a stab at explaining it, based upon the

(snip very helpful exposition)

I'm still struggling, however to understand how this is all intended to
work in an environment which is not continuously connected to the internet.
 Even on machines which are themseleves well connected, it's surely
unacceptable to have one's data become unusable because the machine in
Outer Mongolia which holds the schema has crashed.

Note that this is not just a matter of validation, because the schema can
supply default attribute values.  The data can become meaningless in the
absence of a schema.

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".

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

(a) Names in the namespace do not have an association to a schema.  No
validation is to be performed (and no attribute defaults are to be supplied).

(b) Unless the processor provides some alternative method of locating the
applicable schema, then the data cannot be interpreted and an error occurs.

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