W3C home > Mailing lists > Public > xmlschema-dev@w3.org > July 2001

Validation/element reference/default elements questions

From: <Holger.Joukl@LBBW.de>
Date: Tue, 3 Jul 2001 14:28:22 +0200
Message-Id: <200107031215.OAA23104@bank.de>
To: xmlschema-dev@w3.org
Hi there,
I am really new to XML Schema (and XML, anyway) and I hope this is the
right place
for my questions... and forgive me for the beginner´s stuff - I guess I
the Primer quite well, but Part 1 makes my head explode sometimes.

Q1: Suppose a schema declares 3 top level elements A, B, C. Can an instance
that only contains
        A be valid against that schema? If yes, are empty document
instances always valid?
        Is it even correct to say a document instance is valid, or is the
only thing you can say that
        an instance consists of elements which are all valid?

Q2: Do you need to declare top-level elements in the schema for validation,
or is it sufficient to
        define types?

Q4: Can you reference (ref=...) elements that are not top-level?

Q5: XML Schema recommendation does not allow creation of missing elements
in instances if they
       are declared with the default property.
       Is there a special reason for not allowing such behaviour?

Q6: And is a document instance without such a schema-defaulted element
valid or invalid against the schema?
       (Would be a yes if Q1 part 1 is a yes)

Here is some background for my curiosity:
What we want to do here is using XML Schemas to validate
Rendezvous-middleware messages we
are sending between various applications in a trading environment.
These are self-describing data offering name, type and value for the
message fields.
A message would look like:

msg = {  f1 = 'hello'
                 f2 = 4.7
                 f3 = { ...}
(carrying additional type information; actually, on the application side we
are using
class-like python RvMsg types and a python wrapper around the Rv-C/C++ API)

A straightforward translation to an XML representation would be:

<msg type="RvMsg">
  <f1 type="string">hello</f1>
  <f2 type="float">4.7</f2>
  <f3 type="RvMsg>

Regarding the interfaces we are building, there might well be incoming
messages lacking one field or the other in the message. We´d like
to either reject such messages or default the missing (or empty) fields.

Of course, we could just put a field with default value into the message
if the corresponding element in the schema has the default=... property
(which feels
like cheating, as the schema does not allow element creation for document

Furthermore, translating the message to a XML doc instance, it could never
have the missing field
represented by empty tags; thus, we could not validate this
with the same schema (provided the answer to Q2 is "invalid").

Though, I do not see another possibility than the "cheating" way for us
right now.

Q7: Am I on the right track at all with what I´m thinking?

Your help is greatly appreciated.
Best regards
  Holger Joukl


Holger Joukl
2342 Z - IT/Org II Handel
Tel. +49 (711) 124 - 7078


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Received on Tuesday, 3 July 2001 08:34:07 UTC

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