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Re: FW: Transition strategies to xml:id

From: Norman Walsh <Norman.Walsh@Sun.COM>
Date: Mon, 30 Jan 2006 14:18:24 -0500
To: public-xml-core-wg@w3.org
CC: Robin Berjon <robin.berjon@expway.fr>
Message-ID: <87fyn5cynz.fsf@nwalsh.com>
I'll be on vacation this week and next when this is likely to be
discussed. Here are my first thoughts.

/ "Grosso, Paul" <pgrosso@ptc.com> was heard to say:
| However we do run into problems with the following: "An xml:id  
| processor should assure that the following constraint holds: The  
| values of all attributes of type "ID" (which includes all xml:id  
| attributes) within a document are unique." It's only a "should", so  
| that we can specify that we have a good reason to violate this  
| (otherwise there simply is no practical way for us to adopt xml:id).  
| However "An xml:id error occurs for any xml:id attribute that does  
| not satisfy the constraints" and later "A document is conformant to  
| this specification if it generates no xml:id errors." Based on this  
| it is unclear to us that we can specify conformant *content* that has  
| two IDs with the same value on the same element.

I believe that's the case.

| So the options I can see are:
|   a) we drop 'id' and decide that 1.1 content does not work in 1.2  
| processors. This is unacceptable.

Is it? I don't see why 1.1 content would not work in 1.2 processors? A
processor that is aware of 1.2 can surely be aware of the fact that in
1.1 the attribute was named "id" and in 1.2 it's named "xml:id".

I would have thought the problem was in sending 1.2 content to 1.1
processors. Is that expected to work?

|   b) we drop 'xml:id'. That's an option but to be honest it would  
| really, well, be unpleasant. It's a small change but for a simple- 
| minded Perl hacker like me xml:id has been the most useful addition  
| to the XML stack since XSLT 1.0.
|   c) we specify that you can't have both id and xml:id with the same  
| value on the same element. This is impractical in the extreme: it  
| means that you have to do two calls to getElementById to address  
| processors from different versions, and you would need to duplicate  
| every element that references IDs (and we have quite a few of those:  
| all animations, use, all paint servers...). In effect this means that  
| we might support xml:id, but no one will use it and we'll never be  
| able to deprecate 'id' since people will just keep using it. Given  
| this, there's no reason for us to add xml:id support, it just adds to  
| the implementation burden without bringing any benefit.
|   d) we unilaterally decide that we want to violate the xml:id Rec.  
| This isn't optimal (to say the least), and we already have an LC  
| comment to this effect.
|   e) there is an erratum to xml:id that specifies that the uniqueness  
| of the ID is per element and not per attribute. This is logical since  
| it's the element's identity that needs be unique, not the  
| attribute's. (The fact that some schema languages are opposed to  
| elements having multiple identities is an orthogonal issue.)
| As you can see from the above, the only solutions are (b) or (e). I'd  
| really like to avoid (b), so I'd like XML Core to kindly consider (e).

I fear you've overlooked http://www.w3.org/TR/REC-xml/#one-id-per-el
(though perhaps you're not using DTDs so the whole notion of XML "validity"
is irrelevant).

If the rest of the WG is content that (e) is possible and practical, I
won't stand in the way. My concern is that it will make xml:id
somewhat different from XML with respect to the constraint and I think
we wanted to avoid that. OTOH, if you aren't using a DTD, there's
little practical difference between uniqueness per attribute and per

At first glance, however, I think I prefer (d). Maybe you can finesse
this by saying something along the following lines.

  During the transition period, while "id" is being phased out in
  favor of "xml:id", many documents will not be entirely conformant to
  the xml:id Recommendation nor valid with respect to the ID
  uniqueness constraints of XML[1]. In time, when all significant SVG
  software has been updated to support xml:id, the use of attributes
  named "id" can abandonned and conforming documents will be

  [1] http://www.w3.org/TR/REC-xml/#one-id-per-el

| There are several arguments in favour of considering this an error:
|   - the fact that having a "native" id attribute and an xml:id  
| attribute with the same value on the same element is not completely  
| clearly defined as being an error
|   - xml:id intends to replace former native id attributes. Without  
| making it possible to have a transition strategy, it makes it very  
| difficult for languages already largely deployed to adopt it
|   - the problem does not apply only to SVG - I would expect XHTML,  
| VoiceXML, MathML, etc. to suffer from the same problem

Yes. FWIW, DocBook has addressed this problem by saying the attribute
is named "id" in DocBook V4.x and "xml:id" in V5.x and the transition
strategy is to cross that particular chasm in one step :-)

|   - the specification deliberately allows for multiple ID attributes  
| on a given element, allowing them also have the same value seems  
| logical and may actually have been what was intended.

I think we intended to enforce the constraint that elements have one
attribute of type ID.

                                        Be seeing you,

Norman.Walsh@Sun.COM / XML Standards Architect / Sun Microsystems, Inc.
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Received on Monday, 30 January 2006 19:18:32 UTC

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