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Re: Anomaly in the DAV:prop element usage

From: Geoffrey M. Clemm <geoffrey.clemm@rational.com>
Date: Thu, 6 Jan 2000 22:42:08 -0500
Message-Id: <10001070342.AA19317@tantalum>
To: esedlar@us.oracle.com
Cc: geoffrey.clemm@rational.com, w3c-dist-auth@w3.org, anovosel@us.oracle.com, dbeech@us.oracle.com, vkrishna@us.oracle.com, jjc@jclark.com

   From: "Eric Sedlar" <esedlar@us.oracle.com>

   This is an interesting issue--how do specify some set of XML elements that
   you want out of a document not available to you.  This is basically the XML
   equivalent of a SELECT list in SQL.

Yes!  (It is comforting to know that at least one other person out
there worries about this kind of thing :-).  In the versioning
context, it comes up in the REPORT method.  An important report is a
DAV:available-reports report (analagous to an OPTIONS call, but
returning the reports available, rather than the methods applicable).

   I raised this issue in the database context with the W3C people a few months
   ago, to raise the issue of a simplified form of XPath for this purpose.  I
   proposed basically using a list of XPath identifiers, omitting elements in
   the predicate that reference the value of any node.

   I'm copying this to some relevant XML & XPath people to highlight WebDAV
   PROPFIND requests as another application for this usage in addition to
   database data retrieval.  It's kind of disturbing to me to see invalid XML
   used for this--I'd prefer a comma separated list of XPath specifications,
   omitting predicates, like:

   <D:prop xmlns:R="http://www.foo.bar/boxschema/">
    R:bigbox, R:author, D:supportedlock/lockentry/lockscope
   </D:prop>

   I think this will avoid confusion, since a list of XML element names
   shouldn't appear as empty elements.

My concern with this approach is that the parser won't know that this
identifier is an element name, so the application would have to do the
processing itself (e.g. keeping track of the namespaces defined in that
context, and replacing the namespace prefix with the appropriate URL).
This is feasible, but does require the application to internally contain
parsing knowledge.

The alternative, using empty elements that violate the element definition
avoids this problem, at the cost of being invalid XML.

For the DAV:available-report REPORT, I have avoided the problem by
defining DAV:xxx-report to be the element returned in the
DAV:available-reports response, DAV:xxx-report-request be the element
in the request body for that kind of report, and
DAV:xxx-report-response be the element in the response body for that
kind of report.

But for the (recently added) DAV:property-report REPORT (which does
a PROPFIND on the resources referenced by properties that are a
list of href's), I can't avoid the "XML-select" problem.

Cheers,
Geoff
Received on Thursday, 6 January 2000 22:42:12 GMT

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