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[XUS] An XML URN Scheme

 
 
Philippe Poulard
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      02-25-2005
I don't know if anybody already designed an URN scheme for XML; here is
a proposal: I call it the "XML URN Scheme", or XUS.

=========

URNs are logical names used to identify resources. XUS defines an URN
Scheme for XML documents that works on a delegation model.
As XUS (XML URN Scheme) is used exclusively for XML documents, it uses
namespace URIs to identify this scheme and the delegate scheme.

1 Using XUS
An URI that intends to follow the XML URN Scheme is called a XUS URN or
a XUS URI. XUS URNs are working on a delegation model : when an URN is
recognized to belong to the XML URN Scheme defined here, the last part
of the URN is interpreted with a delegate scheme.
The XUS scheme and the delegate scheme are both identified with a
namespace URI, and expressed in a XUS URN with a related prefix, as
explained below.

* XUS syntax:
The syntax of an URI is the following :
scheme:scheme-specific-part#fragment
where the characters : and # stands for themselves.
The syntax of a XUS URI is the following :
urnus-prefix:scheme-prefix:scheme-part#fragment
where the characters : and # stands for themselves and:
* urn is a string that stands for itslef and denotes that the URI
is an URN
* xus-prefix is a prefix bound to the XUS namespace URI :
http://www.inria.fr/xml/xus/xml-urn-scheme. The usual prefix is xus, but
any other prefix bound to the XUS namespace URI suits.
* scheme-prefix is a prefix bound to the namespace URI of the
delegate scheme.
* scheme-part is the part scheme that depends on the delegate
scheme identified by its namespace URI.

2 Namespace binding
XML URN Schemes are found generally within XML documents inside which
the appropriate namespace declarations can be found. In this case,
theses declarations must be used if the XUS URN is in the scope of the
declarations, as explained in the relevant specification.
However, when used outside the scope of an XML document, a XUS
interpreter may use a given map of prefixes linked to namespace URIs, or
with a component that resolves prefixes, usually known as "namespace
resolver".

* Precautions
A XUS URI may be used anywhere within an XML document with no particular
precaution, except when used in the DOCTYPE declaration, in an external
entity, or in the DTD internal subset. In fact, each time a reference is
made with a XUS URI before reading the XML content may cause troubles.
In this cases, the namespace binding must be provided in addition to the
document.
For this reasons, it is recommended to use XUS URIs within the XML content.

* Namespace expansion
A XUS URI may be resolved to a raw expanded URI for comparison purpose.
The expansion consist on replacing the prefixes by their corresponding
namespaces ; the expansion is completed by escaping the characters of
the namespaces as specified in RFC 2396.

3 XUS interpretation
A XUS interpreter is a program that decodes XUS URNs.
A XUS interpreter used to decode a XUS URN fails to resolve such an URN
when :
* The xus-prefix prefix is not bound to the XUS namespace URI ; the
XUS interpreter must report that the URN to resolve is not a XUS URN.
* The scheme-prefix prefix is not bound to a namespace URI ; the
XUS interpreter must report that the delegate scheme is missing.
* The scheme-prefix prefix is bound to the XUS namespace URI ; the
XUS interpreter must report that the delegate scheme is invalid.
* The delegate scheme is not known by the interpreter ; the XUS
interpreter must report that the delegate scheme is unknown.
* The delegate scheme fails ; the XUS interpreter must report that
the delegate scheme fails.
A delegate scheme is known by the XUS interpreter if this scheme has
itself an interpreter that can be invoked by the XUS interpreter.
If the XUS interpreter succeed in decoding the URI, it returns the
following components :
scheme the string urn
scheme-specific-part the string that follows urn:
xus-prefix the prefix used for the XML URN Scheme
xus-uri the XUS namespace URI http://www.inria.fr/xml/xus/xml-urn-scheme
scheme-prefix the prefix used for the delegate scheme
scheme-uri the namespace URI of the delegate scheme
scheme-part the part scheme that follows scheme-prefix:
scheme-components the components returned by the delegate scheme

4 Example
<?xml version="1.0" encoding="iso-8859-1"?>
<document
xmlnsus="http://www.inria.fr/xml/xus/xml-urn-scheme">
xmlns:msps="http://www.acme.org/my-special-purpose-scheme">
<content>
<p><a href='urnus:msps:acme-fields-here'>My document</a></p>
</content>
</document>
--
Cordialement,

///
(. .)
-----ooO--(_)--Ooo-----
| Philippe Poulard |
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