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Subject: XPath expressions in UBL / Signatures
It would be premature to make a comment on this to the ubl-comment list so I thought I'd bounce it off ubl-dev and see if anyone can give it a sanity check: We just had some debate on xml-dev about the goods and the bads about the growing use of XPath expressions. http://lists.xml.org/archives/xml-dev/201105/threads.html#00007 One of the upshots of this was that I put forward a view and supported it and it didn't get completely shot down that XPath expressioons aren't yet fully portable across different applications/implementations. The problem is mainly with XPath expressions used without any formally defined binding of the namespaces, i.e. if standalone and with an underspecified binding of a) default namespaces and b) any prefixes/namespaces. The XPath spec does not define how applications must cater for this as it expects this to be specified in other standards which make use of XPath (such as XSLT). Does this have ramifications for UBL and its use of XPath, particularly in the XML signatures / signature extensions to UBL? Is it clear enough what an application has to do about any default namespace in such an expression and about prefix and namespace bindings? If not, I wonder if a comment is in order. Without anything specific enough about how to execute/evaluate an XPath expression, different applications may (validly) return different results for the same expression and the same target/context, it seems. (I note too that UBL uses XPath expressions in elements besides signatures but these have been around since before 2.1.) At the same time, it seems another upshot of the xml-dev discussion was the news that XPath 3.0 may go some way to solving portability issues by allowing fully qualified element names http://lists.xml.org/archives/xml-dev/201105/msg00065.html so maybe future UBL specs could recommend these once they become standard and adequately supported; but that day might be some way off. ---- Stephen D Green
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