RE: [xml-dev] My report on experiments with unused namespaces

[Amelia A Lewis <amyzing@talsever.com>]

On Tue, 21 Sep 2010 16:25:46 -0400, Costello, Roger L. wrote:
> I wish to verify that I correctly understand Amelia's post.

Errrrrrr.

> ---------------------------------------------------------
> <Root xmlns:soap="http://www.w3.org/2001/09/soap-envelope";> 
> 
>     <faultcode>soap:Client</faultcode>
> 
> </Root>
> ---------------------------------------------------------
> 
> Notice the value of <faultcode> is a QName.
> 
> If the application that receives the XML document is _not_ "SOAP 
> aware" then the application _may_ treat soap:Client as simply data, 
> not as a QName. That application _may_ consider this to be an unused 
> namespace:
> 
>     http://www.w3.org/2001/09/soap-envelope 
> 
> If the application that receives the XML document _is_ "SOAP aware" 
> then the application _must_ process soap:Client as a QName. Thus, 

If that's an RFC 2119 "MUST", then a random "soap-aware" application 
(whatever that may be) MUST follow the specification for SOAP-aware 
applications.

Since this isn't a SOAP message, I have no idea where your "MUST" comes 
from.

> that application _must_ consider this to be a used namespace:
> 
>     http://www.w3.org/2001/09/soap-envelope
> 
> Is this correct?

To the best of my knowledge and belief, there is no specification or 
schema governing the message supplied above, so processors are free to 
do whatever they would like to do with it.

> <soap:Envelope xmlns:soap="http://www.w3.org/2001/09/soap-envelope";> 
> 
>     ...
> 
> <soap:Envelope> 

Ah, now, we have a specification that discusses the contents of SOAP 
messages.

> Notice that the element name is a QName.
> 
> According to the XML REC, element names may contain colons. Thus, 
> soap:Envelope could simply be the name of the element. However, the 
> REC also says this:
> 
>     Note:
> 
>     The Namespaces in XML Recommendation [XML Names] 
>     assigns a meaning to names containing colon characters. 
>     Therefore, authors should not use the colon in XML names 
>     except for namespace purposes, but XML processors must 
>     accept the colon as a name character.
> 
> I conclude from that note that all XML-aware applications which 
> receive the above XML document _must_ treat soap:Envelope as a QName. 

Good heavens.  How silly.  All of those pre-namespace XML 
implementations are thus condemned as not-XML, just because they 
predate the concept of QName?

> Therefore all applications (SOAP-aware applications and SOAP-unaware 
> applications) _must_ consider this namespace as being used: 
> 
>     http://www.w3.org/2001/09/soap-envelope 

What does:

"this namespace is in use"

and

"this namespace is not in use"

mean?

There are several different ways to answer each of those questions 
(although answering one "true" should *generally* mean that the other 
is answered "false").  Is "in use" defined by the Namespaces in XML 
specification?

Before the concept can be useful, it must be meaningful.  On the other 
hand, because there are dragons here, it behooves one to have a use in 
mind before taking the time to map this territory.  Why does it 
*matter* whether you label a namespace "in use" or "not in use"?  What 
are you trying to achieve?

The Namespaces in XML specification *does* define what it means to 
declare a namespace to prefix mapping.  It is easy to say "this 
namespace is mapped to this prefix in this element scope."  It is at 
least possible to say "if this namespace declaration were removed (and 
no other change were made), this document would no longer be 
well-formed."  That seems to correspond to your "in use as part of 
element name" scenario (and also corresponds to the similar, though not 
identical, "in use as part of attribute name" scenario--there's an 
interesting distinction there, since it's guaranteed that xmlns="" can 
only affect element well-formedness, whereas xmlns:[x]="" can affect 
attributes as well).

Now, for the case where a QName is used in element or attribute 
content, but not as part of element or attribute names, suppression of 
the controlling namespace declaration can produce invalid XML, but not 
ill-formed XML.  That's an important distinction, because validation 
errors are not required to be fatal.  Make a document ill-formed 
(including the special case "namespace-ill-formed"), and the parser is 
*required* to produce a fatal error and *stop parsing*.  Now, there's 
an interesting parallel to the above distinction between namespace 
declarations that can only affect elements, and never attributes: those 
declarations (declarations of the mapping to the default/nil prefix) 
can never be used in element and attribute content, either (well ... in 
fact, they could, potentially, but no specification that uses QNames in 
content permits that, so far as I am aware).

Given these potentially significant characterizations, what's the goal 
of categorizing namespace declarations as "in use" or "not in use"?

Amy!
-- 
Amelia A. Lewis                    amyzing {at} talsever.com
Being your slave, what should I do but tend
upon the hours and times of your desire?
I have no precious time at all to spend,
nor services to do, till you require.