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Why are WebDAV's XML namespace rules different than the W3C's?

From: Yaron Goland (Exchange) <yarong@Exchange.Microsoft.com>
Date: Tue, 28 Dec 1999 16:43:33 -0800
Message-ID: <7DE119D3D0E15543874F7561EECBDBED02619DDE@BEG.platinum.corp.microsoft.com>
To: WebDAV WG <w3c-dist-auth@w3.org>
I was cleaning up my mail box when I found a letter I had written explaining
the differences between the W3C's rules for handling namespaces and
WebDAV's. In the end I believe that everyone adopted the W3C's rules but
this does introduce a very serious interoperability issue that implementers
need to be aware of. Anyone who actually is supporting section 23.4.2
(which, in so far as I know, nobody does) is well advised to re-write their
code to maintain the separation between namespace and element name.

BTW, this is both an old and a very well known issue. I am not posting this
as an alert. Rather this post is yet another entry in the now 50 page or so
WebDAV Book of Why explaining how we got ourselves into this fine mess.

				Yaron

<Introduction>
This letter discusses how WebDAV's rule for combining namespaces came about.
It examines the cultural and technical issues that lead WebDAV to provide
for a namespace unification that is not supported by any other XML community
that we are aware of.
</Introduction>

<Culture Clash>
Having now spent some time reflecting back on our reasoning I suspect the
prime motivator was a culture class. In the world of HTTP in general and
WebDAV in particular things are named with URIs. The idea of a "namespace"
with member "names" is foreign to the general thinking of the culture.
Separation of namespaces and elements is something that is handled by the
person issuing the URI but is not relevant to how one actually handles URIs.
That is, I can create a namespace of the form yaron:namespace/element. When
I handle this URI I do not know, nor do I generally care, that the thing
before the "/" is a namespace name and the thing after the "/" is an element
name. All I see is a single, flat, formless string that I throw to my URI
resolver who then resolves it. Thus the instinct of the group was to reduce
the situation to a known solved problem, in this case, a single URI.
</Culture Clash>

<Technical Reasons>
There were also technical reasons for wanting to just glue the names
together. The primary one was the desire to keep name/value property tables
as simple as possible. Had we not glued together the namespace and name then
property tables would have to have three rows, namespace, element name and
value. This wouldn't work for many existing property systems that only
supported simple name/value pairs. Given that one of the group's major goals
was to be able to layer WebDAV over existing systems it was important that
we be able to use these existing name/value property systems. That is why
WebDAV has such primitive property support. These systems had no hope of
being able to support partial property value replacement and property query.
All they could do was name/value pairs. The value could be a chunk of XML
but that was about it. So we needed a name that was a single string so we
could work with these systems.
</Technical Reasons>

<Some History>
WebDAV had adopted XML before namespaces existed. The assumption of the
WebDAV WG was that elements would be named with URIs but that the SGML
inherited namespace rules didn't allow for element names to include
characters like ":", therefore a hack was needed to work around the
character limitations. When the W3C introduced namespaces we assumed we had
our hack. The only issue was, how to glue the namespace and element name
back together to form the URI?

The WebDAV WG approached the W3C namespace WG with the question of how to do
the gluing but we couldn't get a straight answer. The argument from the W3C
had nothing to do with some theoretical separation between the namespace and
element names. Rather they argued about which magic character one should use
in gluing together the namespace and the name, there were four contenders:
"?", "#", "/" or just simply gluing the elements together. The W3C namespace
WG's interest in this problem was rooted in their desire to be able to take
a namespace and a name and resolve it to some URI they could use to retrieve
the definition of the element, say a DTD.

One faction wanted to use "?" so you would have
http://namespace?elementname. This would be read as a query by the server.
The problem with this solution is that there was no standard for what a
query should look like and one might want to be able to use a more powerful
query format. For example, one might want to ask for a group of element
names instead of just one element name.

Another faction wanted to use "#" so you would have
http://namespace#elementname. This would mean that there was one document
which provided the definition for every element in the namespace and the "#"
would tell you which anchor to navigate to for the element you were
interested in. The problem with this solution was that the definition
document could potentially be huge since it would have to contain every
element in the entire namespace.

Another faction wanted to use "/" so you would have
http://namespace/elementname. This would mean that there was a controlled
namespace that was structured into directories and that one would ask for a
particular file in a particular directory in order to get a name. The
problem with this solution was that it mandated a particular directory
structure that one had to keep to, forever.

As discussed in section 17.7 of RFC 2518, the WebDAV WG didn't believe that
the property names were ever going to be resolved in the real world, can you
imagine everyone, everywhere actually trying to resolve the same single URI
to get the definition of a popular element? This clearly wouldn't work so we
weren't worried about the same problem the W3C guys were worried about. We
couldn't care less about RESOLVING the namespace/element name pair; we just
wanted to somehow get a single string that we could store. As such we were
open to whatever rule the W3C wanted to create. The problem was, the W3C
wasn't willing to pick any of them and WebDAV needed a solution.

At the same time the W3C namespace WG kept missing deadlines for when their
spec was supposed to move forward. They were so far off schedule that it
looked like they would hold up WebDAV's standardization. Having already gone
through so many problems and schedule slips with XML, WebDAV was not going
to wait for the W3C to get namespaces out the door. Therefore the WebDAV WG
decided to copy the namespace spec by value instead of by reference. That
is, the WG copied the entire W3C XML namespace specification into section
23.4 of the WebDAV spec. This freed us up from any W3C dependencies at the
cost of possibly having some differences between the WebDAV namespace
implementation and the officially blessed W3C namespace implementation.
However the W3C swore up and down at the time that there would be no further
changes so our action was fine with them.

Sensing that it wasn't going to get a solution to its property naming
problem from the W3C the WebDAV WG decided to add a new section to 23.4
(23.4.2) which would talk about how to solve the property naming problem.
The simplest solution was "all of the above." That is, just glue the
namespace and the element name together. If you wanted to use "?" then
simply structure your namespace with a "?" in it, the same logic applied to
"#" and "/". By using the glue rule WebDAV was able to support all possible
namespace structure rules. 

Unfortunately the application WG of the IETF got completely slammed and
WebDAV was delayed for six months due to purely administrative reasons.
However the W3C was able to finally get the namespace spec finalized. As
promised, it didn't contain any changes. Unfortunately Tim Berners-Lee
decided he didn't like the syntax and ordered it completely re-written. The
WebDAV WG, thoroughly sick and tired of dealing with the W3C (remind me to
write out the story of XML, from the WebDAV perspective, some time...) was
ready to just say "to hell with it" and ship with our syntax. However cooler
heads prevailed when TBL got his new syntax to recommendation status in
record time (it's good to be the king). So at the very end, WebDAV changed
over to the new syntax and pulled out our copy of namespaces and just
referenced the W3C spec. However this still left section 23.4.2.
Unfortunately the W3C namespace spec still failed to address the issue of
how to combine the names so WebDAV decided to keep the section.
</Some History>

<So, is this an interoperability issue?>
In practice absolutely nobody seems to actually pay attention to 23.4.2.
This means that eventually 23.4.2 will have to be removed when WebDAV moves
on to draft status. However it is important to understand that 23.4.2 does
introduce an interoperability issue when used in conjunction with systems
that don't support 23.4.2.

For example, A client makes a PROPFIND request and gets back the following
property:

<D:prop xmlns:R="z3950:majorschema-originalform-">
	<R:author>
		Alex Hopmann
	</R:author>
</D:prop>
               
The client records the property in the local property table:

Property Name                                       | Value              |
------------------------------------------------------------------------ |
z3950:majorschema-originalform-author               | Alex Hopmann       |

The client then goes off-line and comes back on-line. The client tries to
resynchronize their cache and so makes a PROPFIND request to retrieve the
property value:

<D:prop xmlns:R="z3950:">
	<R:majorschema-originalform-author/>
</D:prop>

RFC 2518 section 23.4.2 specifies that both of these PROPFIND data
structures refer to the same property,
z3950:majorschema-originalform-author. To quote from RFC 2518 section
23.4.2:

	WebDAV compliant XML processors MUST interpret a qualified name as a
	URI constructed by appending the LocalPart to the namespace name
URI.
   
However let's imagine (as is the common case) that the WebDAV server does
not support 23.4.2. In the server's mind it sees the PROPFIND request as
containing a request for Namespace = z3950, Name =
majorschema-originalform-author. Since this obviously doesn't match
Namespace=z3950:majorschema-originalform-, Name=author, the server will not
return the client the property the client was looking for.

The same scenario can be played out in different forms. One client does a
PROPPATCH using the namespace z3950: and the element name
majorschema-originalform-author and another client does a PROPFIND using the
namespace z3950:majorschema- and the element name originalform-author. Using
a WebDAV server that doesn't support 23.4.2 and the client and server will
never be able to talk to each other since the server will treat the requests
as pointing to two unrelated properties.

The problem gets worse and worse as you start to try to push data between
different systems and the namespaces and element names get mangled beyond
all hope as you move from 23.4.2 compliant to 23.4.2 non-compliant clients
and servers. 

Therefore the only solution is for everyone to ignore 23.4.2 and pray really
hard that you never come up against a client or server that actually
supports it.
<So, is this an interoperability issue?>
Received on Tuesday, 28 December 1999 21:06:32 GMT

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