W3C home > Mailing lists > Public > www-dom@w3.org > January to March 2000

Re: Namespace treatment

From: <keshlam@us.ibm.com>
Date: Fri, 3 Mar 2000 14:20:19 -0500
To: "www-dom@w3.org" <www-dom@w3.org>
Message-ID: <85256897.006A388A.00@D51MTA03.pok.ibm.com>
>There is a peculiar asymmetry in the way the DOM models namespaces. It
>is said, for example, that a given element node (as any node) is bound
>to a namespace. This although it is not the node which stands in a
>relation to a namespace, but the node's name. Why was it deemed
>necessary to close this transitive relation in the model?

It's really an early-binding/late-binding issue. We could have bound the
URI to the name, and said that every time you try to get the namespace URI,
we would resolve it in the context of the namespace declaration attributes
at and above that node. This might have been more elegant, but would
definitely have involved more computation.

Also, it's unclear that's the right behavior. What if you move a node from
one point to another, and thereby into a context where the same prefix is
bound to a different URI? It's "the same node object", so one can argue
that its behavior shouldn't change -- and that therefore it should stay in
the original namespace. (This is more important if you have a DOM which
subclasses nodes based on which namespace they belong to -- we aren't sure
whether anyone will do that, but we didn't want to rule it out.)

We really did consider trying to define things such that the behavior would
always be consistent with writing out to XML text, making a change at that
level, and reading it back in. But our conclusion was that the DOM
represents the infoset -- the data model of the XML document's contents,
rather than its syntax -- and that the early-binding approach was most
practical for most DOM applications. Complications only arise if you really
want to change a node's namespace URI (in which case we feel you should
create a new node, since the URI is part of the node's identity in the same
sense that localname is), or at serialization time (see below).

>Any "namespace" attributes
>are superfluous. They could well be trashed upon element modification
>and generated (with caching if so desired) as requested.

True. Note that the Infoset says that the namespace declarations are not
really attributes, and some folks did argue that they shouldn't appear in
the DOM at all. But we wanted to allow Level 1 "pseudo-namespace" code to
continue working with Level 2 DOMs, and that meant retaining the
declarations.

>Why normalize? ever?

Valid question. But there were requests -- e.g. from those who (like
Deiter) don't like the look of attributes with qualified-name collisions --
for such a mechanism. Since the Load/Save chapter of Level 3 is going to
have to deal with that anyway, our current guess is that exposing it as a
separate method ought to be easy. If it isn't, we'll have to do a more
serious cost/benefit analysis.

>>     In Level 2, that normalization task is left as an exercise for
>> the reader,
> poor soul...

In fact, I expect that most folks will use a serializer that was packaged
with their parser... so this is mostly an exercise for the experts, who
probably have a pretty good handle on how to deal with it.

Shouldn't be bad. During the tree-walk, if you see a prefix-to-URI binding
that isn't currently declared, issue a declaration at that node (or, in the
case of an Attr, at the owningNode -- yes, this requires a tiny bit of
lookahead). While you're at it, you can reconcile any Attr qualified-name
conflicts. There's room to get more sophisticated about it but that
just-do-it-locally approach will work, and the fancier you get the more
likely someone will quibble about how your style doesn't match theirs.
<grin/> At the other extreme, you can discard _all_ the existing
declarations and create new ones at the top of the document, with entirely
synthetic prefixes... but the prefixes might be meaningful to a human
reading the document (xsl:, for example), and DTD validation breaks down if
you don't use the prefixes the DTD expects (comment withheld).

______________________________________
Joe Kesselman  / IBM Research
Received on Friday, 3 March 2000 14:20:50 GMT

This archive was generated by hypermail 2.2.0+W3C-0.50 : Friday, 22 June 2012 06:13:47 GMT