[JDEV] namespaces, cont.

[C. Scott Ananian]

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On Tue, 21 Dec 1999, C. Scott Ananian wrote:

> However, *anything* can be included inside the <x> tag, and by convention
> that anything is a valid XML document in the namespace given by the xmlns
> attribute.

Thinking about this, I should have said:
  "by convention that 'anything' is a valid XML document *conforming to
   the DTD* specified in the xmlns attribute."

The fact that all interior tags are in a different namespace is important
when evaluating validity and such, but the key to interpreting the
contents is to go off and fetch the DTD specified.  The *namespace*
specification doesn't require that the contents of the xmlns attribute
point to a DTD; xmlns="foo" and xmlns="http://slashdot.org" are quite
acceptable from the *namespace validity* standpoint, even though they
don't give you any information at all about how to interpret the contents
of the <x> tag.

Also, given a close reading of the name-space spec, it should be understood
that the namespaces of http://protocol.jabber.org/ns.html are ill-defined.
The real 'namespace's defined should be canonical URL pointing to DTDs for,
say, the client authorization protocol.  The namespace you want to use,
according to W3C recommentations, is
    http://protocol.jabber.org/jabber:iq:auth/auth.dtd
(i.e., a URI), *not*
    jabber:iq:auth

If I want to refer to an element as
  <jabber:iq:auth:query>
I just set the following attribute (in the query tag or any enclosing)
  xmlns:jabber:iq:auth="http://protocol.jabber.org/jabber:iq:auth/auth.dtd"
The actual "universal name" of the tag (using James Clark's syntax) is
then
  {http://protocol.jabber.org/jabber:iq:auth/auth.dtd}query

With all this in mind, we can see how the examples at
http://protocol.jabber.org/jabber:iq:auth/examples.txt are
incorrect.  I'll repeat the first one here:

<iq type="set">
  <query xmlns="jabber:iq:auth">
    <username>foo</username>
    <password>bar</password>
    <resource>FooBarGuy</resource>
  </query>
</iq>

Now, the <iq> tag can only have <query> as a member.  But as written above,
the <iq> tag has a member called <{jabber:iq:auth}query>, which is *not*
in the DTD --- only the default-namespace <query> tag is in the DTD.  The
example is thus not valid XML.  We could rewrite it as:

<iq type="set">
  <query xmlns:auth="http://protocol.jabber.org/jabber:iq:auth/auth.dtd">
    <auth:username>foo</auth:username>
    <auth:password>bar</auth:password>
    <auth:resource>FooBarGuy</auth:resource>
  </query>
</iq>

where the desired interpretation of the <query> tag is that the
contents form a valid XML document using the DTD specified by the xmlns
attribute.

An alternative is to change the DTD for the server protocol such that
<iq> just encapsulates #PCDATA (no interior <query> tags) -- but then it
becomes very difficult to specify precisely how the contents of the <iq>
tag should be interpreted (the contents don't stand-alone as a valid XML
document any more, because there's no DTD specified).

It might be mentioned that Bert Bos has made an effort at a formal
definition for nested XML documents that uses <!doctype> instead of
relying on namespace hacks.  What detail there is on this is found at
    http://www.w3.org/XML/simple-XML.html
Work on XML-Schema (the eventual replacement for DTDs) also tries to
handle extensibility in a cleaner manner.  For example, the XML-Data
proposal submitted to the XML-Schema working group allows 'open content
models' (http://www.w3.org/TR/1998/NOTE-XML-data-0105/#OpenClosed).
But for now standard XML and namespace hacks are the way to go.
  --s
                                                         @ @
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