[Tdwg-tag] A very simple question stated again.

Roger Hyam roger at tdwg.org
Sun Mar 26 15:30:11 CEST 2006

Hi All,

Gregor posted my rather long winded description of confusion about 
semantics in XML Schema to the list and it may have confused you. It can 
be summed up with a simple question to which a simple answer is all that 

*"Are the semantics encoded in the XML Schema or in the structure of the 
XML instance documents that validate against that schema? Is it possible 
to 'understand' an instance document without reference to the schema?"*

Possible answers are:

   1. *Yes:* you can understand an XML instance document in the absence
      of a schema it validates against i.e. just from the structure of
      the elements and the namespaces used.
   2. *No*: you require the XML Schema to understand the document.

This is not a trivial question. The answers may require different 
approaches to an overall architecture.

Versioning of schemas, for example, becomes irrelevant if the answer is 
Yes - as the meaning is implicit in the structure you can throw the 
schema away and not loose anything. XML is 'self describing' so you 
would think this must be true. The schema is just a useful device to 
help you construct XML in the correct format.

If the answer in No then we need clear statements about how all 
instances must always bear links to a permanently retrievable schema - 
or they become meaningless. We need very tight version control of 
schemas and a method of linking between the versions so we can track how 
the meaning has changed. We also need clear statements on what happens 
when you can validate a document with multiple schemas? Does this imply 
multiple meanings? Schemas must be archived with any data etc.

If you respond to this message please state a preference for either 1 or 
2. There is no middle road on this one!

All the best,



 Roger Hyam
 Technical Architect
 Taxonomic Databases Working Group
 roger at tdwg.org
 +44 1578 722782

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