XSchema Spec, Section 3, Draft 1 (Namespaces)

[Simon St.Laurent]

Section 3 of the XSchema Specification follows.  Note that there are a 
considerable number of questions raised.  One guy with a whiteboard can only 
make so many connections.

As always, a prettier HTML version of this will be posted shortly at 
http://purl.oclc.org/NET/xschema.

Simon St.Laurent
Dynamic HTML: A Primer / XML: A Primer / Cookies

3.0 XSchema and Namespaces

XSchema uses namespaces for its own operations and also supports schemas that 
take advantage of namespace facilities.  XSchema processors are responsible 
only for elements that use the XSchema namespace appropriate to the version of 
XSchema they are processing.  (This namespace is identified with the prefix 
XSC: throughout this document; see 3.3 below for alternatives.)  Information 
in other namespaces may be passed to other applications as the processor deems 
appropriate.

Please note: the information in this section is based on the 18 May 1998 
"Namespaces in XML" Working Draft and is subject to change if needed to 
maintain conformance with that specification.

3.1 The XSchema Namespace

XSchemas created using XSchema 1.0 should include the following namespace 
declaration after the XML prolog but before the appearance of DTD information 
and the XSC:XSchema element.  This namespace declaration identifies elements 
prefixed with XSC: as elements that should be processed by an XSchema 
processor and also identifies the version number of the XSchema specification 
used.

<?xml:namespace ns="http://www.purl.org/NET/XSchema/v1" prefix="XSC"?>

This declaration is required. XSchema processors must always check for this 
declaration to properly recognize the prefix that will identify XSchema 
elements.

3.2 Interaction with Other Namespaces

XSchemas will often support document sets that have their own namespaces.  
Because prefixes may change from document to document, and to provide the 
simplest possible solution, XSchema provides a mechanism for declaring 
namespaces within XSchemas.  This allows documents and the XSchemas used to 
verify them to track the same namespace using different prefixes if necessary.

<!ELEMENT XSC:Namespace (Doc?)>
<!ATTLIST XSC:Namespace
	prefix CDATA #REQUIRED
	ns CDATA #REQUIRED
	src CDATA #IMPLIED>

The XSC:Namespace element and its contents very similar to the 
<?xml:namespace?> processing instruction.  The attributes of the XSC:Namespace 
element correspond to the prefix, ns, and src productions in the Namespaces 
Working Draft.  The ns attribute for the namespace element is the most 
important part, containing a unique identifier that will be used to link the 
elements declared in an XSchema to their counterparts in the document.  The ns 
attribute will contain a URI, typically a URL. (Note that #-separated 
identifiers are prohibited in the namespaces draft.) The src attribute can 
supplement the ns attribute if the unique identifier provided is not a URL but 
needs (perhaps) to be retrieved.  The prefix attribute provides an identifier 
prefix that will be used throughout the XSchema to identify elements belonging 
to this particular namespace.

The ns attribute contains the key value that will be used to map XSchema 
element declarations to elements instance appearing in documents.  So long as 
the ns attributes underlying the (possibly different) prefixes used in the 
XSchema and the document are the same, the mapping should work correctly.  If 
namespaces are used which have different ns names, the mapping will be broken 
and the processor should report errors.

*** - Should namespaces be forced to appear at the beginning of the XSchema 
document, as they are required to appear before DTDs?

*** - Should namespace declarations in a parent XSC:XSchema element hold in 
XSC:XSchema subelements, or should they start over?

3.3 Note Regarding Namespace Usage

Namespaces are still in development at this time, and as such are subject to 
dramatic change.

This specification was written making several assumptions, which may or may 
not prove to be true as the Namespaces draft evolves:

1. The only part of a namespace declaration that genuinely identifies the 
namespace is the ns production.  The src is for additional information, and 
the prefix only serves as a proxy for the full ns production.  This has 
several implications, the simplest of which allows developers to create 
XSchemas using prefixes other than XSC by specifying a different prefix in the 
namespace declaration described in Section 3.1.

2. The URLs in a namespace declaration will not need to be retrieved.  The 
XSchema namespace declaration uses a PURL (permanent URL) provided by the 
OCLC. (For details on PURLs, visit http://purl.org.) PURLs use redirection to 
maintain a permanent address for sites that may change address. While XSchema 
specification information may be stored at the location to which the PURL 
server redirects visitors, XSchema applications should not rely on any of that 
information being there.


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