CR-44 response: should xsi:type be dropped?

Dear Alexander,

The W3C XML Schema Working Group has spent the last several weeks working
through the comments received from the public on the Candidate
Recommendation (CR) of the XML Schema specification. We thank you for the
comments you made on our specification during our CR comment period, and
want to make sure you know that all comments received during the CR comment
period have been recorded in our CR issues list (

You raised the point registered as issue CR-44:

Issue Title: Drop xsi:type?

You specifically said in your note [1]:

"5) the CR also asks for explicit feedbck on xsi:type -- while we do
already support substitutionGroups in XML Spy, we have not yet implemented
xsi:type, because in contrast to a substitution group (which simply allows
a global element to be substituted for another global element) xsi:type
requires the validation engine to keep information about complexTypes in
memory at all times, which is both a performance and memory size issue. Our
current implementation of the validation engine discards all complexType
and simpleType information, once the element content model has been built
in memory, which allows for much more efficient processing. Furthermore,
the ability for the XML instance document author to directly access/change
the underlying type of an element introduces an entirely new level of
complexity or risk that is - in our opinion - unwarranted."

The workgroup has considered your concerns, but has decided that on balance
the utility of xsi:type outweighs the associated implementation complexity.
The group has therefore decided to retain the xsi:type feature.  As you
know, xsi:type is useful for a variety of purposes, including serialization
of typed object graphs.

It would be helpful to us to know whether you are satisfied with the
decision taken by the WG on this issue, or wish your dissent from the WG's
decision to be recorded for consideration by the Director of the W3C.


Noah Mendelsohn
XML Schema Working Group


Noah Mendelsohn                                    Voice: 1-617-693-4036
Lotus Development Corp.                            Fax: 1-617-693-8676
One Rogers Street
Cambridge, MA 02142

Received on Monday, 29 January 2001 19:00:05 UTC