Associating Resources with Namespaces

DRAFT TAG Finding 1 December 2005

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Latest version:
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Norman Walsh, Sun Microsystems, Inc. <Norman.Walsh@Sun.COM>

This document is also available in these non-normative formats: XML.


This Finding addresses the question of how ancillary information (schemas, stylesheets, documentation, etc.) can be associated with a namaespace.

Status of this Document

This document has been produced for review by the W3C Technical Architecture Group (TAG). This finding addresses TAG issue namespaceDocument-8.

This document is an editor's draft without any normative standing.

Additional TAG findings, both accepted and in draft state, may also be available.

The terms MUST, SHOULD, and SHOULD NOT are used in this document in accordance with [RFC 2119].

Please send comments on this finding to the publicly archived TAG mailing list www-tag@w3.org (archive).

Table of Contents

1 Preface
2 The Model
3 Namespace Document Formats
    3.1 RDDL 1.0
    3.2 RDDL 2.0
    3.3 Using GRDDL
4 Identifying Individual Terms
5 Natures
6 Purposes
7 References

1 Preface

The names in a namespace form a collection:

The names in a namespace can, in theory at least, be defined to identify any thing or any number of things.

Given the wide variety of things that can be identified, it follows that an equally wide variety of ancillary resources may be relevant to a namespace. A namespace may have documentation (specifications, reference material, tutorials, etc., perhaps in several formats and several languages), schemas (in any of several forms), stylesheets, software libraries, applications, or any other kind of related resource.

A user, encountering a namespace might want to find any or all of these related resources. In the absence of any other information, a logical place to look for these resources, or information about them, is at the location of the namespace URI itself.

[WebArch Vol 1] says that it is a Good Practice for the owner of a namespace to make available at the namespace URI “material intended for people to read and material optimized for software agents in order to meet the needs of those who will use the namespace”.

The question remains, how can we best provide both human and machine readable information at the namespace URI such that we can achieve the good practice identified by web architecture? One early attempt was [RDDL 1.0]. RDDL 1.0 is an XLink-based vocabulary for identifying the nature and purpose of related resources.

Several attempts were made to simplify RDDL. The TAG's original plan for addressing namespaceDocument-8 was to help define a simpler, standard RDDL format. However, this space has matured somewhat since the TAG's original discussions and RDDL 1.0 is now widely deployed. In addition, some of the proposed alternative formats are also deployed. And it seems likely that over time new variations may arise based on other evolving web standards.

This finding therefore attempts to address the problem by considering it in a more general fashion. We hope to:

  1. Define a conceptual model for identifying related resources that is simple enough to garner community consensus as a reasonable abstraction for the problem.

  2. Show how RDDL 1.0 is one possible concrete syntax for this model.

  3. Show how other concrete syntaxes could be defined and identified in a way that would preserve the model.

2 The Model

For the resource identified by a namespace URI, there may exist other resources related to it. Borrowing on the terminology defined by [RDDL 1.0], we say that each of these other resources has a nature and a purpose. The nature of the resource is a machine-readable label that identifies “what kind of thing” it is. For example, its nature might be “HTML documentation” or “XML Schema” or “CSS Stylesheet”. The purpose of a resource, with respect to the resource identified by the namespace URI, is a machine-readable label that identifies “what use” the thing is. For example, its purpose might be “validation” or “normative reference” or “specification” or “transformation”.

For example, here's a diagram of the model for some DocBook-related resources:

RDDL Model for DocBook

This model indicates that for the purpose of validation there are two schemas, docbook.xsd which has the nature “XML Schema” and docbook.rng which has the nature “RELAX NG”. This model also includes two examples of HTML documentation, defguide.html which has the purpose “reference documentation” and docbook.html which has the purpose “specification”.

If an application can obtain this model from the document that it gets from the namespace URI, then it can find the relevant related resources. For example, a RELAX NG validator could find all the resources that serve the purpose “validation” and identify the one (or one of the ones) with the nature “RELAX NG” and proceed with a validation task. Similarly, a human being could find the resource with the purpose “specification” to locate the specification in a convenient format.

One way to write down the model described above is with RDF. There's nothing about the process of finding related resources that requires the model to be instantiated in RDF or requires any processor to know anything about RDF. But having the model in RDF will allow us to describe how the model can be obtained from specific kinds of documents.

Here's an example of the DocBook model above, expressed in RDF using N3:

# RDDL Model for DocBook

@prefix rddl: <http://www.w3.org/2005/11/rddl#> .
@prefix purpose: <http://www.w3.org/2005/11/rddl/purpose#> .
@prefix nature: <http://www.w3.org/2005/11/rddl/nature#> .
@prefix dc: <http://purl.org/dc/elements/1.1/> .

   purpose:validation <http://docbook.org/xml/5.0b1/rng/docbook.rng> ;
   purpose:validation <http://docbook.org/xml/5.0b1/xsd/docbook.xsd> ;
   purpose:spec <http://docbook.org/specs/wd-docbook-docbook-5.0b1.html> ;
   purpose:reference <http://docbook.org/tdg5/en/html/> .

   rddl:nature nature:RELAXNG .

   rddl:nature nature:XMLSchema .

   rddl:nature nature:HTML .

   rddl:nature nature:HTML .

If we can construct this model from a namespace document, then we know we have all the information we need to locate related resources.

3 Namespace Document Formats

This section describes two formats deployed explicitly to address the question of namespace documents and a third format which can be seen as simultaneously providing a unified view of these two formats and also providing a model to make other new formats available.

3.1 RDDL 1.0

A RDDL 1.0 document encodes the nature and purpose of the related resource in a rddl:resource element. That element uses XLink:

<rddl:resource xlink:title="RELAX NG for validation"
A <a href="http://docbook.org/xml/5.0b1/rng/docbook.rng">schema</a>
for RELAX NG validation.

Extacting the model is a simple matter of reading the xlink:href, xlink:role, and xlink:arcrole attributes of each rddl:resource.

3.2 RDDL 2.0

One of the RDDL 2.0 proposals encodes the nature and purpose of the related resource directly on the HTML a element:

<a rddl:nature="http://www.w3.org/2005/11/rddl/natures/RELAXNG"
for RELAX NG validation.

Extacting the model is a simple matter of reading the rddl:nature, rddl:purpose, and href attributes of each HTML a.

3.3 Using GRDDL

A third approach is to use [GRDDL]. GRDDL provides a mechanism for gleaning resource descriptions from XML. Employing GRDDL allows an author to associate a transformation with a document. The result of applying that transformation is an RDF model (the resource description).

Given that our RDDL model can be expressed in RDF, such a gleaned resource description can be seen as a RDDL model. Consider the following example:

The URI http://www.w3.org/2000/10/swap/pim/usps is the namespace name for an RDF vocabulary that describes United States postal addressing terms. It's namespace document is an XHTML document that uses GRDDL to identify the transformation http://www.w3.org/2000/07/hs78/html2rdfs.

If this transformation is applied, the resulting resource description includes the following model fragment:

 purpose:normative-reference <http://pe.usps.gov/cpim/ftp/pubs/Pub28/pub28.pdf>,
                             <http://pe.usps.gov/cpim/ftp/pubs/Pub63/Pub63.pdf> .

In other words, this RDDL model:

RDDL Model for USPS

It's important to note that processing a GRDDL document only requires XSLT processing in the general case. For well-known transformation URIs, an application can simply understand the markup.

In fact, a pair of well-known GRDDL transformation URIs for RDDL 1.0 and RDDL 2.0 will allow us to unify both RDDL variants and the GRDDL case.

The use of GRDDL would allow arbitrary, perhaps non-human readable, documents to function as namespace documents. However, that's counter to the [WebArch Vol 1] good practice. Hopefully that will be sufficient discouragement.

4 Identifying Individual Terms

For many applications of namespaces, it's valuable not only to be able to point to the namespace as a whole, but also to be able to point to terms within that namespace. Fragment identifiers are the obvious mechanism for achieving this. For example, http://www.w3.org/2005/xpath-functions/#matches is the URI for the “matches” function, a term in the XQuery 1.0, XPath 2.0, and XSLT 2.0 Functions and Operators Namespace, http://www.w3.org/2005/xpath-functions/.

If it would be valuable to directly address specific terms in a namespace, namespace owners SHOULD provide identifiers for them. It follows that if the namespace document is available in multiple forms (RDDL 1.0, RDF through GRDDL, etc.) that consistent fragment identifiers MUST be made available.

5 Natures

@@ list of natures, borrow from RDDL.

6 Purposes

@@ list of purposes, borrow from RDDL.

7 References

RFC 2119
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. IETF. March, 1997. (See http://www.ietf.org/rfc/rfc2119.txt.)
RDDL 1.0
Jonathan Borden and Tim Bray. Resource Directory Description Language (RDDL). February, 2002. (See http://www.rddl.org/.)
WebArch Vol 1
Ian Jacobs and Norman Walsh, editors. Architecture of the World Wide Web, Volume 1. World Wide Web Consortium, 2004. (See http://www.w3.org/TR/webarch/.)
Dominique Hazaël-Massieux and Dan Connolly, editors. Gleaning Resource Descriptions from Dialects of Languages (GRDDL). World Wide Web Consortium, 2004. (See http://www.w3.org/TR/grddl/.)