Associating Resources with Namespaces

1 Preface

The names in a namespace form a collection. Sometimes it is a collection of element names (DocBook and XHTML, for example), sometimes it is a collection of attribute names (XLink, for example), sometimes it is a collection of functions (XQuery 1.0 and XPath 2.0 Data Model), sometimes it is a collection of properties (FOAF), sometimes it is a collection of concepts (WordNet). 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 “in the wild” 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 [rddl10]. 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, time has passed, 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 taking a step back. 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 [rddl10], 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:

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 ash 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/> .

<http://docbook.org/ns/docbook>
   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/> .

<http://docbook.org/xml/5.0b1/rng/docbook.rng>
   rddl:nature nature:RELAXNG .

<http://docbook.org/xml/5.0b1/xsd/docbook.xsd>
   rddl:nature nature:XMLSchema .

<http://docbook.org/specs/wd-docbook-docbook-5.0b1.html>
   rddl:nature nature:HTML .

<http://docbook.org/tdg5/en/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 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"
	       xlink:arcrole="http://www.w3.org/2005/11/rddl/purposes#validation"
	       xlink:role="http://www.w3.org/2005/11/rddl/natures/RELAXNG"
	       xlink:href="http://docbook.org/xml/5.0b1/rng/docbook.rng">
A <a href="http://docbook.org/xml/5.0b1/rng/docbook.rng">schema</a>
for RELAX NG validation.
</rddl:resource>

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

4 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
<a rddl:nature="http://www.w3.org/2005/11/rddl/natures/RELAXNG"
   rddl:purpose="http://www.w3.org/2005/11/rddl/purposes#validation"
   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 rddl:nature, rddl:purpose, and href attributes of each HTML a.

5 Using GRDDL

A third approach is to use [GRDDL].

@@describe grddl@@

@@incorporate Dan's USPS example: http://www.w3.org/2000/10/swap/pim/usps

@@note that grddl doesn't actually require xslt except in the general case@@

@@note that the preceding examples use grddl too.@@

@@note that this technique would allow for non-human readable namespace documents but that's counter to the spirit of the webarch good practice.@@

6 Natures

@@ list of natures, borrow from RDDL.

7 Purposes

@@ list of purposes, borrow from RDDL.

8 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.)

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/.)

GRDDL

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/.)