[Standards-JIG] sequence vs. choice in Jabber/XMPP schemas

[Peter Saint-Andre]

Traditionally, old-style Jabber implementations did not care about the 
order of elements. When we began to define XML schemas rather than DTDs 
for JEPs and (later) the XMPP RFCs, we used xs:sequence (prioritizing 
more careful specification of the number of allowable child elements) 
rather than xs:choice (which would have prioritized allowing any order 
of child elements). Even though the existing schemas are described as 
informative rather than normative, people have used them for 
compliance-testing purposes. The predictable result is that existing 
implementations have failed the compliance tests. I see several possible 
approaches to remediation:

1. Keep using xs:sequence and require existing implementations to pay 
attention to the order of child elements.

2. Switch to xs:choice and care less about the number of allowable child 
elements.

3. Define two different sets of schemas: a strict one following (1) and 
a loose one following (2).

4. Switch from W3C XML Schema to a different schema language, such as 
RELAX NG, Schematron, or Examplotron.

Thoughts?

</psa>

-- 
Peter Saint-Andre
Jabber Software Foundation
http://www.jabber.org/people/stpeter.shtml