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Re: [xml-dev] Caution using XML Schema backward- or forward-compatibility as a versioning strategy for data exchange

Roger's point about the distinction  between validation and processing is entirely on-point.  If the term "compatible" only means "causes limited pain to whoever maintains XML validation code" then its not a very useful word. 

I think that both "semantic" and "compatible" are problematic terms.  What is "compatibility"?  Is it breakage to a system or pain experienced by a user?  Is it something else?   Defining "compatible" too narrowly and too technically makes the word useless because it causes too many changes to be defined as non-compatible, and presumably we need the identification of "non-compatible" changes to drive some action.  The usual use of the term compatible suggests that validation is more important than processing.

There is a change case that has not been talked about, and that is where a schema is used as the basis for another schema or is incorporated into another schema.  This has a different set of sources of pain than changes to XML documents because the impact of a schema change may not alter the documents that are produced but is likely to impact the schemas that incorporate it.  It seems that there is a different (and more useful) set of meanings of the term "compatible" in that use-case. 


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