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Web arch: proposed text for 4.3.2

From: Norman Walsh <Norman.Walsh@Sun.COM>
Date: Sun, 01 Jun 2003 16:38:03 -0400
To: www-tag@w3.org
Message-ID: <87isrph6tw.fsf@nwalsh.com>

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To backup the proposed principle that designers SHOULD use namespaces,
I suggest:

The Web is significantly a <emph>networked</emph> information system.
Authors and applications can use URIs uniformly to identify different
resources on the Web. After representations of these resources have
been retrieved, they may be processed in a variety of ways. Some
applications (and some users) will undoubtably build new resources by
combining several representations together. This is particularly easy,
and potentially useful, when XML representations are available for all
the resources.

However, combining representations in this way moves them out of their
original context and places them in a new context. This change of
context introduces the possibility of information loss. Any
information that depended on the local context will no longer be
available.

What is needed is a mechanism for establishing a global context for
the elements and attributes in the XML resources. This problem bears a
strong resemblance to the distinction between relative and absolute
URI. While the many hundreds of relative URI references to
"index.html" on a typical web server may be entirely unambiguous in
their respective contexts, they have no unambiguous global meaning.
But each such relative URI has an unambiguous absolute URI that can be
established in its local context and used when a document is moved. This
solves the problem for URI references.

For elements and attributes, their names can be seen as analogous to
relative URI. Within their original context, they have meanings that
are clear and entirely unambiguous. Namespaces in XML provides a
mechanism for establishing a globally unique name that can be
understood in any context.

The "absolute" form of an XML element or attribute name is the
combination of its namespace URI and its local name. This is
represented lexically in documents by associating namespace names with
(optional) prefixes and combining prefixes and local names with a colon
as described in [Namespaces in XML].

Designers that use namespaces are thus providing a global context for
documents authored with their schema. Establishing this global context
allows their documents (and portions of their documents) to be reused
and combined in novel ways not yet imagined. Failure to provide a
namespace makes such reuse more difficult, perhaps impractical in some
cases.

The most significant technical drawback to using namespaces is that
they do not play well with DTDs. DTDs perform validation based on the
lexical form of the name, making prefixes semantically significant in
ways that are not desirable. As other schema language technologies
become widely deployed, this drawback will diminish in significance.

                                        Be seeing you,
                                          norm

- -- 
Norman.Walsh@Sun.COM    | "Bother", said Pooh, as he deleted his root
XML Standards Architect | directory.
Web Tech. and Standards |
Sun Microsystems, Inc.  | 
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Received on Sunday, 1 June 2003 16:38:11 GMT

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