Protocol for Web Description Resources (POWDER): Description Resources

W3C Recommendation 1 September 2009

This version

http://www.w3.org/TR/2009/REC-powder-dr-20090901/

Latest version

http://www.w3.org/TR/powder-dr/

Previous version

http://www.w3.org/TR/2009/PR-powder-dr-20090604/

Editors:

Phil Archer, Institute of Informatics & Telecommunications (IIT), NCSR "Demokritos" (formerly with FOSI)

Kevin Smith, Vodafone Group R & D

Andrea Perego, Università degli Studi dell'Insubria

Please refer to the errata for this document, which may include some normative corrections.

See also translations.

Copyright © 2009 W3C^® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use rules apply.

---

Abstract

The purpose of the Protocol for Web Description Resources (POWDER) is to provide a means for individuals or organizations to describe a group of resources through the publication of machine-readable metadata, as motivated by the POWDER Use Cases [USECASES]. This document details the creation and lifecycle of Description Resources (DRs), which encapsulate such metadata. These are typically represented in a highly constrained XML dialect that is relatively human-readable. The meaning of such DRs are underpinned by formal semantics, accessible by performing a GRDDL Transform.

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This document is a W3C Recommendation that was developed by the POWDER Working Group.

Please see the Working Group's implementation report and Disposition of Last Call Comments. The disposition of comments received during previous calls are also available. Changes since the previous version of this document are minor in nature and are fully documented in the Change log.

Publication of this Recommendation is synchronized with several other documents:

• POWDER: Grouping of Resources (Recommendation)
• POWDER: Formal Semantics (Recommendation)
• POWDER: Primer (Working Group Note)
• POWDER: Test Suite (Working Group Note)

The W3C Membership and other interested parties are invited to review the document and send comments to public-powderwg@w3.org (with public archive).

This document has been reviewed by W3C Members, by software developers, and by other W3C groups and interested parties, and is endorsed by the Director as a W3C Recommendation. It is a stable document and may be used as reference material or cited from another document. W3C's role in making the Recommendation is to draw attention to the specification and to promote its widespread deployment. This enhances the functionality and interoperability of the Web.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

Table of Contents

1 Introduction

The Protocol for Web Description Resources (POWDER) facilitates the publication of descriptions of multiple resources such as all those available from a Web site. These descriptions are always attributed to a named individual, organization or entity that may or may not be the creator of the described resources. This contrasts with more usual metadata that typically applies to a single resource, such as a specific document's title, which is usually provided by its author.

This document sets out how Description Resources (DRs) can be created and published, how to link to DRs from other online resources, and, crucially, how DRs may be authenticated and trusted. The aim is to provide a platform through which opinions, claims and assertions about online resources can be expressed by people and exchanged by machines. POWDER has evolved from the data model developed for the final report [XGR] of the Web Content Label Incubator Group [WCL-XG], from which we define a Description Resource as: "a resource that contains a description, a definition of the scope of the description and assertions about both the circumstances of its own creation and the entity that created it."

The next section introduces the division between the operational semantics of POWDER (designed to be easy and practical to use in everyday situations) and the more formal semantics (designed to make POWDER available on the broader Semantic Web). This division means that there are several components to the Protocol for Web Description Resources which in turn leads to there being several documents in the set.

The method of defining the scope of a DR, that is, defining what is being described, is provided in Grouping of Resources [GROUP]. The layered semantics of POWDER is defined in a Formal Semantics document [FORMAL]. It is hoped that the provision of those documents allows the present one to give a clearer, more concise definition of the structure and use of Description Resources. The full set of POWDER documents also includes its Use Cases, Primer and Test Suite, together with the namespace documents [WDR, WDRS and WDRD].

POWDER takes a very broad approach so that it is possible for both the resource creator and third parties to make assertions about all kinds of things, with no architectural limits on what they are making claims about. For example, medically proficient organizations might be concerned with properties of the agencies and processes that produce Web content (e.g. companies, people, and their credentials). Equally, a 'Mobile Web' application might need to determine the properties of various devices, such as their screen dimensions, and those device types might be described with such properties by their manufacturer or by others. Although the broad approach is supported, we have focused on Web resources rather than trying to define a universal labelling system for objects. In practice, POWDER associates a description with one or more IRIs [IRI] that must then be interpreted as being descriptions of the resources dereferenced from those IRIs.

Trust is a central theme of POWDER; however, we do not prescribe a single method through which trust must be conferred on Description Resources. By its very nature, trust is a human judgment that can only be made by weighing the likelihood that the data is true against the consequences of it being false. This judgment is highly dependant on the circumstances under which the need to extend trust arises. POWDER does, however, provide support for, and is amenable to, a variety of methods through which users and user agents can establish trust.

Reference is made throughout this document to POWDER documents. Unless otherwise stated, these are XML documents that have their root element in the POWDER namespace. A POWDER processor is software that can process POWDER documents, the minimum specification for which is set out in Section 3.

1.1 Operational and Formal Semantics

The Protocol for Web Description Resources has been designed with a variety of content production workflows in mind. It offers a practical contribution to such areas as content discovery, personalization and access control; one that can be implemented readily by non-specialists. There is, however, a tension between this operational approach and the broader goal of making the data available in a format that can be processed on the Semantic Web. By making data from potentially disparate sources interoperable, and by enabling machines to process the meaning of that data, the Semantic Web offers exciting possibilities in precisely the areas of interest to POWDER.

The tension between what can be processed by humans and what can be processed by machines is resolved by defining both operational and formal semantics of Description Resources, and by bridging much of the gap between the two by way of a GRDDL transform that is associated with the root POWDER namespace. POWDER documents are written in a highly constrained dialect of XML that may also include RDF/XML constructs within some elements. Such documents may be processed directly in specialized systems; however, for more general environments, it is appropriate to perform the GRDDL transform to render the data as syntactically valid RDF/OWL (see Section 2.2 below). The semantics of such documents, known as POWDER-S documents, are defined in a companion Formal Semantics document [FORMAL]. This defines the GRDDL transform from POWDER to POWDER-S and includes a Semantic Extension to RDF that must be understood if the OWL data is to be processed effectively.

1.2 Namespaces and Terminology.

The POWDER vocabulary namespace is http://www.w3.org/2007/05/powder# for which we use the prefix wdr. The POWDER-S namespace is http://www.w3.org/2007/05/powder-s# for which we use the prefix wdrs. All prefixes used in this document, together with their associated namespaces, are shown in the table below.

In this document, the words MUST, MUST NOT, SHOULD, SHOULD NOT, RECOMMENDED and MAY are to be interpreted as described in RFC2119.

POWDER makes substantial use of XML, the processing rules for which MUST be followed faithfully. The processing rules for attribute-value normalization are particularly relevant when considering the white space separated lists of values that occur in POWDER. A space-separated list is a string of which the items are separated by one or more space characters (in any order). The string may also be prefixed or suffixed with zero or more of those characters. To obtain the values from a space-separated list user agents MUST replace any sequence of space characters with a single #x20 character, dropping any leading or trailing #x20 character, and then chopping the resulting string at each occurrence of a #x20 character, dropping that character in the process.

The (unqualified) terms POWDER, POWDER Document and Description Resource (DR) refer to operational representations that are encoded largely in XML. The term POWDER-S refers to RDF/OWL representations, the full semantics of which are expressed when account is taken of the extension defined in the Formal Semantics document [FORMAL].

2 POWDER Structure and Semantics

2.1 Operational Semantics

The following natural language statement:

On 14th December 2007, the entity described at http://authority.example.org/company.rdf#me said that everything on example.com is red and square.

is encoded in POWDER as shown in the example below, which in turn is followed by explanatory notes.

   <?xml version="1.0"?>
1  <powder xmlns="http://www.w3.org/2007/05/powder#" 
           xmlns:ex="http://example.org/vocab#">

2    <attribution>
3      <issuedby src="http://authority.example.org/company.rdf#me" />
4      <issued>2007-12-14T00:00:00</issued>
5    </attribution>

6    <dr>
7      <iriset>
8        <includehosts>example.com</includehosts>
9      </iriset>

10     <descriptorset>
11       <ex:color>red</ex:color>
12       <ex:shape>square</ex:shape>
13       <displaytext>Everything on example.com is red and square</displaytext>
14       <displayicon src="http://authority.example.org/icon.png" />
15     </descriptorset>
16   </dr>

17  </powder>

Explanation:

Line 1

All POWDER documents have the root element of powder. The ex namespace is used to exemplify a descriptive vocabulary and is fictitious.

Lines 2 - 5

All POWDER documents MUST have exactly one attribution element. This contains, or points to, information about who has provided the description and typically will also include information about when it was created and any validity period. The GRDDL transform renders the attribution element as an OWL ontology header [OWL] and each child element of attribution is therefore transformed into an owl:AnnotationProperty as discussed below. The exception to this is the abouthosts element which is introduced in Section 2.3.

Line 3

Exactly one issuedby element MUST be included and it MUST use the src attribute to point to an RDF resource that describes the entity that created the POWDER document. It is RECOMMENDED that this be done using an instance of the Agent class from either the FOAF or Dublin Core vocabularies.

Line 4

The issued element within the attribution element is optional and is simply mapped to wdrs:issued by the GRDDL transform. Likewise the validfrom and validuntil elements discussed in Section 5.2 may be included in the attribution element and the dates and times given SHOULD be conformant with the XML dateTime datatype. Arbitrary OWL annotation properties that describe the POWDER document may also be included.

Lines 6 - 16

This particular POWDER document contains a single Description Resource (dr).

Lines 7 - 9

The scope of the DR — i.e. what is described — is defined as set out in the Grouping of Resources document [GROUP]. In this case, the scope is 'everything on example.com'. All Description Resources MUST contain at least one iriset element and, as described in the Grouping document, this MUST NOT be empty and MUST NOT contain any elements from any other namespace. If more than one iriset element is included then the scope of the DR is the union of the resources identified by the IRIs in all sets.

Lines 10 - 15

The description itself. A DR MUST contain at least one descriptorset element (and/or tagset see Section 2.7) that MUST NOT be empty and MAY contain RDF/XML that describes the IRIs in the IRI set. Since the subject of the triples is not explicit and is only accessed through processing, there are limitations on the RDF that can safely be included, specifically, only RDF properties that take literals (including XML Literals) or values for the rdf:resource attribute so long as this does not point to a blank node. Other RDF/XML constructs, particularly blank nodes, MUST NOT be included. Section 2.8 shows how to assert the rdf:type relationship. The Formal Semantics document [FORMAL] provides full details (and warnings) concerning the semantics of the descriptor set.

As shown in lines 13 and 14, a textual and/or graphic summary that can be displayed to end users may be included using displaytext and displayicon respectively. The GRDDL transform maps these to wdrs:text and wdrs:logo respectively. Further optional elements are introduced throughout this document and summarized in the Appendix.

Operationally, a user (or user agent) should begin the examination of a POWDER document by considering the attribution element which, if the document is valid in the XML-sense of the term, will be present and contain an issuedby element. This element is mandatory as, for most people, deciding whether to trust a given description is typically most dependent on answering the question "who says?" If the IRI that is the value of the src attribute of the issuedby element points to a description of a trusted individual or entity the data provided in the remainder of the document will usually be trusted, subject to integrity-checking mechanisms such as those discussed in Section 5.

If the user (or user agent) confers his/her trust in the document, then the remainder of the data can be processed, either in an entirely operational manner or by performing the GRDDL transform associated with the POWDER namespace and merging the resulting RDF graph (see Section 2.2 below).

Either way, example 2-1 means that the processor can be confident that any resource identified by an IRI with a host having 'example.com' as its last two components will be red and square. Conversely, an agent seeking resources that are red and square will trust example.com to provide them.

User agents MAY display the text and/or icon to end users in any way deemed appropriate. Since a DR can contain any number of descriptorset elements and therefore any number of displaytext and displayicon elements, exactly how, or whether, these are displayed is up to the application. As a result, DR publishers are strongly advised to include no more than one displaytext and displayicon per DR except where differentiated using xml:lang attributes.

A POWDER document may contain any number of DRs, each describing its own IRI set(s), each of which will be attributed to the same person or entity. Where those IRI sets overlap, the descriptions are additive. In Example 2-2 below, the opinion of the entity described at http://authority.example.org/company.rdf#me is that everything on example.com is red; IRIs on example.com where the path starts with /foo can be dereferenced to resources that are BOTH red AND shiny.

   <?xml version="1.0"?>
1  <powder xmlns="http://www.w3.org/2007/05/powder#" 
2          xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
3          xmlns:ex="http://example.org/vocab#">

4    <attribution>
5      <issuedby src="http://authority.example.org/company.rdf#me" />
6    </attribution>

7    <dr>
8      <iriset>
9        <includehosts>example.com</includehosts>
10     </iriset>
 
11     <descriptorset>
12       <ex:color>red</ex:color>
13     </descriptorset>
14   </dr>

15   <dr>
16    <iriset>
17       <includehosts>example.com</includehosts>
18       <includepathstartswith>/foo</includepathstartswith>
19     </iriset>

20     <descriptorset>
21       <ex:finish rdf:resource="http://example.org/vocab#shiny" />
22     </descriptorset>
23   </dr>

24 </powder>

It is possible to create sets of Description Resources that are exclusive, i.e. where only one of a given list applies, as detailed in Section 2.3. Other variations on the basic POWDER model set out in the preceding examples are discussed in later sections of this document.

2.2 Formal Semantics: POWDER-S

The operational semantics of POWDER, as defined above, are underpinned by more formal semantics. A GRDDL Transformation is associated with the POWDER namespace. When the transformation is performed on a POWDER document, a Semantic POWDER document (POWDER-S) is its output. A POWDER-S document is an OWL ontology and the attribution information in the source document is encoded as an ontology header. Both IRI sets and descriptor sets are represented as OWL classes. To complete the encoding of a DR, a sub-class relationship is then asserted between the IRI set and the descriptor set from which it can be deduced that all instances of the IRI set are also instances of the descriptor set.

The aim of POWDER-S is to make POWDER data available to more general Semantic Web tools, not to create an alternative encoding. All POWDER-S documents are syntactically valid RDF/OWL documents; however, logical inferences may only be drawn by applications that implement the semantic extension defined in the Formal Semantics document [FORMAL]. That document provides full details of the transformation, which is split into two steps. In brief: the first step reformulates the IRI set definition purely in terms of regular expressions that are matched against a candidate IRI. The regular expression syntax used is defined by XML schema as modified by XQuery 1.0 and XPath 2.0 Functions and Operators [XQXP]. Other elements of the document are unaffected in this intermediate stage, which is known as POWDER-BASE. The second step transforms POWDER-BASE into POWDER-S. This document is not concerned with the intermediate step and presents only POWDER and POWDER-S examples.

1  <?xml version="1.0"?>
2  <rdf:RDF
3     xmlns:wdrs="http://www.w3.org/2007/05/powder-s#"
4     xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
5     xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
6     xmlns:owl="http://www.w3.org/2002/07/owl#"
7     xmlns:ex="http://example.org/vocab#">
8   
9    <owl:Ontology rdf:about="">
10     <wdrs:issuedby rdf:resource="http://authority.example.org/company.rdf#me" />
11     <wdrs:issued>2007-12-14T00:00:00</wdrs:issued>
12   </owl:Ontology>
13  
14   <owl:Class rdf:nodeID="iriset_1">
15     <owl:equivalentClass>
16       <owl:Class>
17         <owl:intersectionOf rdf:parseType="Collection">
18           <owl:Restriction>
19             <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
20             <owl:hasValue rdf:datatype="http://www.w3.org/2001/XMLSchema#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(example\.com)(:([0-9]+))?\/</owl:hasValue>
21           </owl:Restriction>
22         </owl:intersectionOf>
23       </owl:Class>
24     </owl:equivalentClass>
25   </owl:Class>
26
27   <owl:Class rdf:nodeID="descriptorset_1">
28     <rdfs:subClassOf rdf:resource="http://www.w3.org/2002/07/owl#Thing"/>
29     <rdfs:subClassOf>
30       <owl:Class>
31         <owl:intersectionOf rdf:parseType="Collection">
32           <owl:Restriction>
33             <owl:onProperty rdf:resource="http://example.org/vocab#color" />
34             <owl:hasValue>red</owl:hasValue>
35           </owl:Restriction>
36           <owl:Restriction>
37             <owl:onProperty rdf:resource="http://example.org/vocab#shape" />
38             <owl:hasValue>square</owl:hasValue>
39           </owl:Restriction>
40         </owl:intersectionOf>
41       </owl:Class>
42     </rdfs:subClassOf>
43     <wdrs:text>Everything on example.com is red and square</wdrs:text>
44     <wdrs:logo rdf:resource="http://example.org/icon.png" />
45   </owl:Class>
46
47   <owl:Class rdf:nodeID="iriset_1">
48     <rdfs:subClassOf rdf:nodeID="descriptorset_1"/>
49   </owl:Class>
50
51 </rdf:RDF>

It is anticipated that POWDER-S documents will typically be generated by performing the GRDDL transform associated with POWDER after validity and trust has been conferred on the input document. However, a POWDER-S document may be created independently, in which case the question of trust remains open.

If trust is conferred on a POWDER-S document directly, or is inherited from the POWDER document that was transformed to generate it, then the graph can be merged with other graphs available to the processor.

The wdrs:issuedby property (line 10) is an OWL annotation property with a domain of wdrs:Document but no defined range. As noted above, however, it is RECOMMENDED that the filler be an instance of the Agent class from either the dcterms or foaf namespace. For the avoidance of doubt: it is the POWDER document that is issuedby the Agent not the resources that the POWDER document describes.

The matchesregex property restriction in the IRI set class (lines 14-25) draws on a semantic extension defined in the Formal Semantics document [FORMAL] (the notmatchesregex property is also defined). This supports the critical matching of an IRI against a regular expression to confer class membership on the resource denoted by that IRI. For emphasis we repeat that if the POWDER semantic extension is not understood, a generic RDF/OWL tool will not be able to ascribe any meaning to the properties of the IRI set class.

Notice that an rdf:nodeID is used to identify the IRI set and descriptor set classes, meaning that they cannot be referenced by external documents. This is deliberate and ensures that further properties cannot be added that might render the assertions made the POWDER document author incorrect.

The descriptor set class is pure RDF/OWL and is effectively a re-expression of the descriptorset element in a POWDER document with full semantics. Lines 27 - 45 of example 2-3 are derived directly from example 2-1 lines 10 - 15.

2.3 Exclusive Description Resources

As noted in Section 2.1, a POWDER document may contain any number of DRs. These may offer overlapping descriptions of the same resources. It is often the case though that a description of one set of resources must not be applied to other resources on the same Web site.

A modified version of example 2-2 is shown below as example 2-4. Note that the property in line 19, part of the second DR, is now the same as that in line 10 which is part of the first DR — but with a different value. Taken together, the two DRs state that in the opinion of the person or entity described at http://authority.example.org/company.rdf#me, all resources on example.com are red, but that those on example.com where the path starts with /foo are BOTH red AND blue.

   <?xml version="1.0"?>
1  <powder xmlns="http://www.w3.org/2007/05/powder#" 
           xmlns:ex="http://example.org/vocab#">

2    <attribution>
3      <issuedby src="http://authority.example.org/company.rdf#me" />
4    </attribution>

5   <dr>
6     <iriset>
7        <includehosts>example.com</includehosts>
8      </iriset>
 
9      <descriptorset>
10       <ex:color>red</ex:color>
11     </descriptorset>
12   </dr>

13   <dr>
14     <iriset>
15       <includehosts>example.com</includehosts>
16       <includepathstartswith>/foo</includepathstartswith>
17     </iriset>

18     <descriptorset>
19       <ex:color>blue</ex:color>
20    </descriptorset>
21   </dr>

21  </powder>

One way to avoid this paradox would be to add an extra definition to the first IRI set that excluded IRIs beginning with /foo as shown in example 2-5, thus creating two DRs with scopes that are disjoint.

   <?xml version="1.0"?>
1  <powder xmlns="http://www.w3.org/2007/05/powder#" 
           xmlns:ex="http://example.org/vocab#">

2    <attribution>
3      <issuedby src="http://authority.example.org/company.rdf#me" />
4    </attribution>

5   <dr>
6     <iriset>
7        <includehosts>example.com</includehosts>
8        <excludepathstartswith>/foo</excludepathstartswith>
9      </iriset>
 
10     <descriptorset>
11       <ex:color>red</ex:color>
12     </descriptorset>
13   </dr>

14   <dr>
15     <iriset>
16       <includehosts>example.com</includehosts>
17       <includepathstartswith>/foo</includepathstartswith>
18     </iriset>

19     <descriptorset>
20       <ex:color>blue</ex:color>
21    </descriptorset>
22   </dr>

23  </powder>

POWDER supports this structure; however, in a commercial content production environment it can be impractical to keep track of which exclusions should be written into which IRI sets, even with the aid of tools. This is particularly so where content is being produced in multiple departments or bought in from multiple suppliers. In such situations, the more typical need, and the more natural way of working, is to create a description that applies to all content on a given Web site except where indicated.

For this reason, POWDER uses the concept of an ordered list to create a set of DRs, only one of which applies to a given IRI.

   <?xml version="1.0"?>
1  <powder xmlns="http://www.w3.org/2007/05/powder#" 
           xmlns:ex="http://example.org/vocab#">

2    <attribution>
3      <issuedby src="http://authority.example.org/company.rdf#me" />
4      <issued>2007-12-14T00:00:00</issued>
5      <abouthosts>example.com</abouthosts>
6    </attribution>

7    <ol>
8      <dr>
9        <iriset>
10          <includehosts>example.com</includehosts>
11         <includepathstartswith>/foo</includepathstartswith>
12       </iriset>

13       <descriptorset>
14         <ex:color>blue</ex:color>
15       </descriptorset>
16     </dr>

17     <dr>
18       <iriset>
19         <includehosts>example.com</includehosts>
20       </iriset>
 
21       <descriptorset>
22         <ex:color>red</ex:color>
23       </descriptorset>
24     </dr>
25   </ol>

26 </powder>

In the POWDER namespace, the ol tag indicates an ordered list of Description Resources. If a given IRI is within the scope of the first DR in the list, then that DR applies; if not, move on to the second DR and see if the given IRI is within its scope, and so on, until either a match is made or the end of the list is reached. As such, it is important to place any exclusions before other DRs that include that resource in their scope, or else the exclusion will never be matched. An IRI can only be described by 0 or 1 DRs from an ordered list.

The first DR in the list in example 2-6 covers resources on example.com with a path starting with /foo and describes them as being blue. Any resource available from example.com that does not have a path starting with /foo is described as being red. Further DRs may be added to the list without the need to edit either of the existing DRs, although order may be important. This would be the case if, for instance, resources available from example.com with a path starting with /foo and a query string containing material=natural were green. Such a DR would need to go above the first one in the example.

It is noteworthy that if the content provider at example.com wishes to link to this document, he/she can arrange for all resources to include exactly the same pointer — the POWDER document contains all the necessary information to discover how the red and blue resources are arranged. Linkage is discussed in more detail in Section 4. Moreover, the disposition of content with different characteristics across a given Web site or set of Web sites can be discovered by processing the one document. If processed alongside a site map, for example, this becomes a powerful content discovery mechanism.

In order to minimize the processing of a sequence of DRs like this, we introduce the abouthosts element in line 5. This takes a white space separated list of hosts and can be included in the attribution element of any POWDER document, whether it contains an ordered list of DRs or not. In this context, abouthosts provides a processing hint, quickly identifying whether a candidate resource might be described by the DRs in the document. The element also has a bigger role to play, which is discussed in Section 2.5; for now we note that if the host component of a given IRI is included in the list given as the value for the abouthosts element, then the POWDER document may, but is not guaranteed to, contain a description of that IRI. If the host component of a given IRI is not included in the list given as the value for the abouthosts element, then the processor MUST assume that the POWDER document does not contain a description of that IRI. From a discovery point of view, it provides an efficient method of hinting at where to find the resources that match the given description(s).

N.B. IRI sets within a POWDER document do not inherit the value of abouthosts as part of their definition and SHOULD include elements such as includehosts to ensure that they are self-contained. Semantically, it is an error if an IRI set defines a set of resources that is inconsistent with an abouthosts element. In POWDER-S, this is captured by making descriptorset-derived classes of resources a subset of the abouthosts-derived class of resources. A logical inconsistency arises if a descriptor is applied to a resource that is not within the scope specified by abouthosts.

POWDER-S does not directly support ordered lists, but their semantics are captured by creating classes that exclude defined resources in such a way as to create the necessary semantics as shown in example 2-7 below.

1   <?xml version="1.0"?>
2   <rdf:RDF
3      xmlns:wdrs="http://www.w3.org/2007/05/powder-s#"
4      xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
5      xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
6      xmlns:owl="http://www.w3.org/2002/07/owl#"
7      xmlns:ex="http://example.org/vocab#">
8  
9      <owl:Ontology rdf:about="">
10       <wdrs:issuedby rdf:resource="http://authority.example.org/company.rdf#me" />
11       <wdrs:issued>2007-12-14T00:00:00</wdrs:issued>
12     </owl:Ontology>
13 
14     <owl:Class rdf:nodeID="aboutset">  <!-- from the abouthosts element -->
15       <owl:equivalentClass>
16         <owl:Class>
17           <owl:intersectionOf rdf:parseType="Collection">
18             <owl:Restriction>
19               <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
20               <owl:hasValue rdf:datatype="http://www.w3.org/2001/XMLSchema#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(example\.com)(\:([0-9]+))?\/</owl:hasValue>
21             </owl:Restriction>
22           </owl:intersectionOf>
23         </owl:Class>
24       </owl:equivalentClass>
25     </owl:Class>
26  
27     <owl:Class rdf:nodeID="iriset_1">
28       <owl:equivalentClass>
29         <owl:Class>
30           <owl:intersectionOf rdf:parseType="Collection">
31             <owl:Restriction>
32               <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
33               <owl:hasValue rdf:datatype="http://www.w3.org/2001/XMLSchema#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(example\.com)(\:([0-9]+))?\/</owl:hasValue>
34             </owl:Restriction>
35             <owl:Restriction>
36               <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
37               <owl:hasValue rdf:datatype="http://www.w3.org/2001/XMLSchema#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]*)(\:([0-9]+))?(\/foo)</owl:hasValue>
38             </owl:Restriction>
39           </owl:intersectionOf>
40         </owl:Class>
41       </owl:equivalentClass>
42     </owl:Class>
43 
44     <owl:Class rdf:nodeID="iriset_1_not">
45       <owl:equivalentClass>
46         <owl:Class>
47           <owl:unionOf rdf:parseType="Collection">
48             <owl:Restriction>
49               <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#notmatchesregex" />
50               <owl:hasValue rdf:datatype="http://www.w3.org/2001/XMLSchema#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(example\.com)(\:([0-9]+))?\/</owl:hasValue>
51             </owl:Restriction>
52             <owl:Restriction>
53               <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#notmatchesregex" />
54               <owl:hasValue rdf:datatype="http://www.w3.org/2001/XMLSchema#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]*)(\:([0-9]+))?(\/foo)</owl:hasValue>
55             </owl:Restriction>
56           </owl:unionOf>
57         </owl:Class>
58       </owl:equivalentClass>
59     </owl:Class>
60 
61     <owl:Class rdf:nodeID="descriptorset_1">
62       <rdfs:subClassOf rdf:nodeID="aboutset"/>
63       <rdfs:subClassOf>
64         <owl:Class>
65           <owl:intersectionOf rdf:parseType="Collection">
66             <owl:Restriction>
67               <owl:onProperty rdf:resource="http://example.org/vocab#color" />
68               <owl:hasValue>blue</owl:hasValue>
69             </owl:Restriction>
70           </owl:intersectionOf>
71         </owl:Class>
72       </rdfs:subClassOf>
73     </owl:Class>
74    
75     <rdf:Description rdf:nodeID="iriset_1">
76       <rdfs:subClassOf rdf:nodeID="descriptorset_1"/>
77     </rdf:Description>
78 
79     <owl:Class rdf:nodeID="iriset_2">
80       <owl:equivalentClass>
81         <owl:Class>
82           <owl:intersectionOf rdf:parseType="Collection">
83             <owl:Restriction>
84               <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
85               <owl:hasValue rdf:datatype="http://www.w3.org/2001/XMLSchema#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(example\.com)(\:([0-9]+))?\/</owl:hasValue>
86             </owl:Restriction>
87             <owl:Class rdf:nodeID="iriset_1_not" />
88           </owl:intersectionOf>
89         </owl:Class>
90       </owl:equivalentClass>
91     </owl:Class>
92  
93     <owl:Class rdf:nodeID="descriptorset_2">
94       <rdfs:subClassOf rdf:nodeID="aboutset"/>
95       <rdfs:subClassOf>
96         <owl:Class>
97           <owl:intersectionOf rdf:parseType="Collection">
98             <owl:Restriction>
99               <owl:onProperty rdf:resource="http://example.org/vocab#color" />
100              <owl:hasValue>red</owl:hasValue>
101            </owl:Restriction>
102          </owl:intersectionOf>
103        </owl:Class>
104      </rdfs:subClassOf>
105    </owl:Class>
106 
107    <rdf:Description rdf:nodeID="iriset_2">
108      <rdfs:subClassOf rdf:nodeID="descriptorset_2"/>
109    </rdf:Description>
110 
111 </rdf:RDF>

The key feature of example 2-7 is the creation of the 'iriset_1_not' (lines 44-59). This differs from the IRI set in lines 27-42 in that it is defined using the notmatchesregex property, and a union of property restrictions, cf. their intersection. This allows the second IRI set to create a logical closed world for IRI set 2 (line 87). A full example with a third DR in the list and generalized rules is provided in the Formal Semantics document [FORMAL].

Example 2-7 also shows that the abouthosts element from the original POWDER document (line 5 in Example 2-6) is transformed into a class very similar to those for the IRI sets, this time with a node ID of 'aboutset' (line 14 - 25). Both descriptorset_1 and descriptorset_2 are defined as the intersection of this set, and the various descriptive properties which ensures that the intended operational semantics of abouthosts are preserved. A logical inconsistency is detected if an IRI is asserted to be a sub class of a descriptor set that intersects with an about set of which it is not a member. The Formal Semantics document [FORMAL] has further detail on the processing of abouthosts.

2.4 DRs with Multiple IRI Sets

As noted in Section 2.1, the scope of a DR may be the union of multiple IRI sets. Example 2-8 below shows a DR for which the scope is all resources on example.com that have a path starting with /foo AND those on example.org that have a path starting with /bar. In POWDER-S, the sub-class relationship between each of the IRI sets and the descriptor set is asserted independently.

1  <?xml version="1.0"?>
2  <powder xmlns="http://www.w3.org/2007/05/powder#" 
3          xmlns:ex="http://example.org/vocab#">

4    <attribution>
5      <issuedby src="http://authority.example.org/company.rdf#me" />
6      <issued>2007-12-14T00:00:00</issued>
7    </attribution>

8    <dr>
9      <iriset>
10       <includehosts>example.com</includehosts>
11       <includepathstartswith>/foo</includepathstartswith>
12     </iriset>

13     <iriset>
14       <includehosts>example.org</includehosts>
15       <includepathstartswith>/bar</includepathstartswith>
16     </iriset>

17     <descriptorset>
18       <ex:color>red</ex:color>
19       <ex:shape>square</ex:shape>
20       <displaytext>Everything on example.com/foo, and everything on example.org/bar, is red and square</displaytext>
21       <displayicon src="http://example.org/icon.png" />
22     </descriptorset>
23   </dr>

24 </powder>

1  <?xml version="1.0"?>
2  <rdf:RDF
3    xmlns:wdrs="http://www.w3.org/2007/05/powder-s#"
4    xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
5    xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
6    xmlns:owl="http://www.w3.org/2002/07/owl#"
7    xmlns:dcterms="http://purl.org/dc/terms/"
8    xmlns:ex="http://example.org/vocab#">
9  
10   <owl:Ontology rdf:about="">
11     <wdrs:issuedby rdf:resource="http://authority.example.org/company.rdf#me" />
12     <wdrs:issued>2007-12-14T00:00:00</wdrs:issued>
13   </owl:Ontology>
14  
15   <owl:Class rdf:nodeID="iriset_1">
16     <owl:equivalentClass>
17       <owl:Class>
18         <owl:intersectionOf rdf:parseType="Collection">
19           <owl:Restriction>
20             <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
21             <owl:hasValue rdf:datatype="http://www.w3.org/2001/XMLSchema#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(example\.com)(\:([0-9]+))?\/</owl:hasValue>
22           </owl:Restriction>
23           <owl:Restriction>
24             <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
25             <owl:hasValue rdf:datatype="http://www.w3.org/2001/XMLSchema#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]*)(\:([0-9]+))?(\/foo)</owl:hasValue>
26           </owl:Restriction>
27         </owl:intersectionOf>
28       </owl:Class>
29     </owl:equivalentClass>
30   </owl:Class>
31 
32   <owl:Class rdf:nodeID="iriset_2">
33     <owl:equivalentClass>
34       <owl:Class>
35         <owl:intersectionOf rdf:parseType="Collection">
36           <owl:Restriction>
37             <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
38             <owl:hasValue rdf:datatype="http://www.w3.org/2001/XMLSchema#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(example\.org)(\:([0-9]+))?\/</owl:hasValue>
39           </owl:Restriction>
40           <owl:Restriction>
41             <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
42             <owl:hasValue rdf:datatype="http://www.w3.org/2001/XMLSchema#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]*)(\:([0-9]+))?(\/bar)</owl:hasValue>
43           </owl:Restriction>
44         </owl:intersectionOf>
45       </owl:Class>
46     </owl:equivalentClass>
47   </owl:Class>
48  
49   <owl:Class rdf:nodeID="descriptorset_1">
50     <wdrs:text>Everything on example.com/foo, and everything on example.org/bar, is red and square</wdrs:text>
51     <wdrs:logo rdf:resource="http://example.org/icon.png" />
52     <rdfs:subClassOf>
53       <owl:Class>
54         <owl:intersectionOf rdf:parseType="Collection">
55           <owl:Restriction>
56             <owl:onProperty rdf:resource="http://example.org/vocab#color" />
57             <owl:hasValue>red</owl:hasValue>
58       </owl:Restriction>
59           <owl:Restriction>
60             <owl:onProperty rdf:resource="http://example.org/vocab#shape" />
61             <owl:hasValue>square</owl:hasValue>
62           </owl:Restriction>
63         </owl:intersectionOf>
64       </owl:Class>
65     </rdfs:subClassOf>
66   </owl:Class>
67 
68   <owl:Class rdf:nodeID="iriset_1">
69     <rdfs:subClassOf rdf:nodeID="descriptorset_1"/>
70   </owl:Class>
71 
72   <owl:Class rdf:nodeID="iriset_2">
73     <rdfs:subClassOf rdf:nodeID="descriptorset_1"/>
74   </owl:Class>
75 
76 </rdf:RDF>

2.5 Direct Resource Description

POWDER is predicated on the concept of IRI sets — applying descriptions to more than one IRI at a time. This relies on there being some structure to the IRIs used. To a human reader, it would be fairly obvious that all resources with an IRI on composers.example.org that have a path starting with /Mahler would be about a different composer to those with a path starting with /Beethoven. IRI sets allow these relationships to become machine processable.

However, not all Web content is arranged in such clearly delineated fashion, and as a result it may be impossible to define a suitable IRI set. For example, a content management system may assign a simple numerical IRI to newly added content irrespective of the type or the subject matter of content available from that IRI. In such a situation, the usefulness of POWDER as a content discovery mechanism is reduced, but not eliminated.

The POWDER document in example 2-9 below contains two descriptions that can be applied to example.org and example.com (defined in the abouthosts element), but those descriptions are not tied to particular IRI sets within a Description Resource.

1  <?xml version="1.0"?>
2  <powder xmlns="http://www.w3.org/2007/05/powder#" 
3          xmlns:ex="http://example.org/vocab#">

4    <attribution>
5      <issuedby src="http://authority.example.org/company.rdf#me" />
6      <abouthosts>example.org example.com</abouthosts>
7    </attribution>

8    <descriptorset xml:id="red">
9      <ex:color>red</ex:color>
10   </descriptorset>

11   <descriptorset xml:id="blue">
12     <ex:color>blue</ex:color>
13   </descriptorset>

14 </powder>

From a discovery point of view, this tells us that there are red and blue resources available on example.org and example.com but can go no further. Associating a description set with a specific IRI can only be done by linking from a resource to a description, hence the inclusion of the identifiers on the descriptor set elements (which are preserved in POWDER-S). An HTML document at http://www.example.org/page.html might include a link element thus:

<link rel="describedby" href="/powder.xml#red" type="application/powder+xml" />

Although example 2-9 does not contain any Description Resources, it does preserve two key elements of POWDER:

Firstly, the description is attributed and therefore an assessment of trust can be made in the same way as any other POWDER document.

Secondly, the attribution element includes an abouthosts element. This effectively limits the scope of the descriptions to the declared hosts, since if a resource is not available from a listed host, then, as noted in the discussion of Example 2.6 above, the processor MUST NOT apply the data in the document to that resource.

This is an important point, since it would be possible to claim that any resource on the Web is 'red' by including the link element shown above. This may be what is required (see following section); however, the abouthosts element allows POWDER document authors to set an outer limit on where the descriptions can be applied. It is for this reason that we specify that if the abouthosts element is included in a POWDER document, then processors MUST NOT, rather than SHOULD NOT or MAY NOT, apply descriptions within the document to any host other than those listed. This has implications for the specification of a POWDER Processor. See Section 3 below.

It is emphasized that providing descriptions in this way is inferior to the preferred method of explicitly associating them with IRI sets within a DR. If a POWDER document is to describe all the resources on a given Web site in the same way, then authors should include a Description Resource with an IRI set defined using the appropriate value of the includehosts element. This is because the semantics are different in a subtle but important way:

The first of these is clearly a better aid to content discovery.

2.6 Pre-Defined Descriptors

As shown in the previous section, descriptorset elements can exist outside a DR. There are situations where defining descriptor sets independently of any DR but within the same POWDER document can be very convenient, and this is discussed in Section 2.8. This section is concerned with the definition of descriptor sets that can be referred to by DRs in other POWDER documents.

If a POWDER document does not include an abouthosts element in its attribution, then a descriptor set within it that has an identifier of its own may be used to describe any resource. However, this is the job that RDF vocabularies and OWL ontologies are designed for. Therefore, authors are strongly advised to create or use existing examples of either of those in preference to a POWDER document without any restriction on where the descriptions can be applied.

Bearing in mind that warning, some content providers and their suppliers may find it convenient to split Description Resources across several POWDER documents. In example 2-10 below, powder1.xml defines two types of book published by the organization described at http://education.example.org/company.rdf#me related to the UK National Curriculum. The publisher sells their books through two online outlets: books.example.com and archive.example.net and therefore restricts its descriptions to those hosts. powder2.xml describes one of those outlets (books.example.com), which brands its Key Stage 1 products as 'robin' and its Key Stage 2 products as 'starling.' By referring to powder1.xml, books.example.com is able to associate the publisher's relevant icon with the correct pages of its own e-commerce portal.

A POWDER processor MUST take full account of the abouthosts element in powder1.xml when processing the DRs in powder2.xml.

1  <?xml version="1.0"?>
2  <powder xmlns="http://www.w3.org/2007/05/powder#" 
3          xmlns:ex="http://education.example.org/vocab#">

4    <attribution>
5      <issuedby src="http://education.example.org/company.rdf#me" />
6      <abouthosts>books.example.com archive.example.net</abouthosts>
7    </attribution>

8    <descriptorset xml:id="ks1">
9      <displaytext>This material is suitable for UK National Curriculum Key Stage 1</displaytext>
10     <displayicon src="http://education.example.org/ks1.png" />
11   </descriptorset>

12   <descriptorset xml:id="ks2">
13     <displaytext>This material is suitable for UK National Curriculum Key Stage 2</displaytext>
14     <displayicon src="http://education.example.org/ks2.png" />
15   </descriptorset>

16 </powder>

1  <?xml version="1.0"?>
2  <powder xmlns="http://www.w3.org/2007/05/powder#">

3    <attribution>
4      <issuedby src="http://books.example.com/company.rdf#me" />
5      <issued>2008-03-03T00:00:00</issued>
6    </attribution>

7    <dr>
8      <iriset>
9        <includehosts>books.example.com</includehosts>
10       <includepathstartswith>/robin/</includepathstartswith>
11     </iriset>
12     <descriptorset src="http://education.example.org/powder1.xml#ks1" />
13   </dr>

14   <dr>
15     <iriset>
16       <includehosts>books.example.com</includehosts>
17       <includepathstartswith>/starling/</includepathstartswith>
18     </iriset>
19     <descriptorset src="http://education.example.org/powder1.xml#ks2" />
20   </dr>

21 </powder>

Given powder2.xml, a POWDER Processor SHOULD use the descriptor sets defined in powder1.xml (or report an error if it is not available, not valid etc.). The GRDDL transform uses the value of the src attribute in the sub class assertion such that the POWDER-S encoding of powder2.xml includes:

<owl:Class rdf:nodeID="iriset_1">
  <rdfs:subClassOf rdf:resource="http://education.example.org/powder1.xml#ks1"/>
</owl:Class>

1  <?xml version="1.0"?>
2  <rdf:RDF
3    xmlns:wdrs="http://www.w3.org/2007/05/powder-s#"
4    xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
5    xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
6    xmlns:dcterms="http://purl.org/dc/terms/"
7    xmlns:owl="http://www.w3.org/2002/07/owl#">
8  
9    <owl:Ontology rdf:about="">
10     <wdrs:issuedby rdf:resource="http://books.example.com/company.rdf#me" />
11     <wdrs:issued>2008-03-03T00:00:00</wdrs:issued>
12   </owl:Ontology>
13 
14   <owl:Class rdf:nodeID="aboutset">
15     <owl:equivalentClass>
16       <owl:Class>
17         <owl:intersectionOf rdf:parseType="Collection">
18           <owl:Restriction>
19             <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
20             <owl:hasValue>\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(books\.example\.com|archive\.example\.net)(\:([0-9]+))?\/</owl:hasValue>
21           </owl:Restriction>
22         </owl:intersectionOf>
23       </owl:Class>
24     </owl:equivalentClass>
25   </owl:Class>
26 
27   <owl:Class rdf:ID="ks_1">
28     <wdrs:text>This material is suitable for UK National Curriculum Key Stage 1</wdrs:text>
29     <wdrs:logo rdf:resource="http://education.example.org/ks1.png" />
30     <owl:equivalentClass>
31       <owl:Class>
32  <owl:intersectionOf rdf:parseType="Collection">
33    <owl:Class rdf:nodeID="aboutset"/>
34  </owl:intersectionOf>
35       </owl:Class>
36     </owl:equivalentClass>
37   </owl:Class>
38 
39   <owl:Class rdf:ID="ks_2">
40     <wdrs:text>This material is suitable for UK National Curriculum Key Stage 2</wdrs:text>
41     <wdrs:logo rdf:resource="http://education.example.org/ks2.png" />
42     <owl:equivalentClass>
43       <owl:Class>
44  <owl:intersectionOf rdf:parseType="Collection">
45    <owl:Class rdf:nodeID="aboutset"/>
46  </owl:intersectionOf>
47       </owl:Class>
48     </owl:equivalentClass>
49   </owl:Class>
50   
51 </rdf:RDF>

1  <?xml version="1.0"?>
2  <rdf:RDF
3     xmlns:wdrs="http://www.w3.org/2007/05/powder-s#"
4     xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
5     xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
6     xmlns:owl="http://www.w3.org/2002/07/owl#">
7 
8    <owl:Ontology rdf:about="">
9      <wdrs:issuedby rdf:resource="http://education.example.org/company.rdf#me" />
10     <wdrs:issued>2008-03-03T00:00:00</wdrs:issued>
11   </owl:Ontology>
12 
13   <owl:Class rdf:nodeID="iriset_1">
14     <owl:equivalentClass>
15       <owl:Class>
16     <owl:intersectionOf rdf:parseType="Collection">
17           <owl:Restriction>
18             <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
19             <owl:hasValue rdf:datatype="http://www.w3.org/2001/XMLSchema#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(books\.example\.com)(\:([0-9]+))?\/</owl:hasValue>
20           </owl:Restriction>
21           <owl:Restriction>
22             <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
23             <owl:hasValue  rdf:datatype="http://www.w3.org/2001/XMLSchema#string">\:\/\/([^\:\/\?\#\@]+\.)*[^\:\/\?\#\@]+(\:([0-9]+))?(\/robin\/)</owl:hasValue>
24           </owl:Restriction>
25         </owl:intersectionOf>
26       </owl:Class>
27     </owl:equivalentClass>
28   </owl:Class>
29   
30   <owl:Class rdf:nodeID="iriset_1">
31     <rdfs:subClassOf rdf:resource="http://education.example.org/powder1.rdf#ks_1"/>
32   </owl:Class>
33 
34   <owl:Class rdf:nodeID="iriset_2">
35     <owl:equivalentClass>
36       <owl:Class>
37         <owl:intersectionOf rdf:parseType="Collection">
38           <owl:Restriction>
39             <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
40             <owl:hasValue rdf:datatype="http://www.w3.org/2001/XMLSchema#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(books\.example\.com)(\:([0-9]+))?\/</owl:hasValue>
41           </owl:Restriction>
42           <owl:Restriction>
43             <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
44             <owl:hasValue rdf:datatype="http://www.w3.org/2001/XMLSchema#string">\:\/\/([^\:\/\?\#\@]+\.)*[^\:\/\?\#\@]+(\:([0-9]+))?(\/starling\/)</owl:hasValue>
45           </owl:Restriction>
46         </owl:intersectionOf>
47       </owl:Class>
48     </owl:equivalentClass>
49   </owl:Class>
50     
51   <owl:Class rdf:nodeID="iriset_2">
52     <rdfs:subClassOf rdf:resource="http://education.example.org/powder1.rdf#ks_2"/>
53   </owl:Class>
54 
55 </rdf:RDF>

Although abouthosts can provide a useful processing hint in a purely operational POWDER environment as shown in Example 2-6, taking account of it when checking that a particular descriptor set can be applied to a given resource can require significant processing, especially in POWDER-S. Furthermore, it can readily lead to mistakes. Use only when necessary.

2.7 Free Text Tags, Comments, Labels and "See Also"

In the examples given so far, controlled vocabularies have been used to describe the resources. POWDER also supports free text 'tags' too — that is, key words or short phrases that describe the content — as shown in example 2-12 below.

1  <?xml version="1.0"?>
2  <powder xmlns="http://www.w3.org/2007/05/powder#" 
3          xmlns:ex="http://example.org/vocab#">

4    <attribution>
5      <issuedby src="http://authority.example.org/company.rdf#me" />
6      <issued>2007-12-14T00:00:00</issued>
7    </attribution>
  
8    <dr>
9      <iriset>
10       <includehosts>example.com</includehosts>
11     </iriset>

12     <tagset>
13       <tag>London</tag>
14       <tag>Swiss Re</tag>
15       <tag>gherkin</tag>
16     </tagset>
17   </dr>
18 </powder>

The DR in this example does not have a descriptor set; rather, it has a tag set (it could have both). Any number of tags may be included as elements within the tag set and these may contain any text (subject to usual XML data encoding rules).

A DR MUST contain at least one tagset or descriptorset element, or both.

The tag element can only be used within a tagset and specifically MUST NOT be used within a descriptorset. This is because the descriptor set element is used to contain values for properties taken from controlled vocabularies expressed in RDF.

Tag sets and descriptor sets can also be associated with other descriptive or informative resources (not necessarily POWDER documents) through the use of the seealso element. This has a src attribute that takes an IRI as shown in the example below. Associating tags with other resources in this way can help to disambiguate them. Each seealso in a descriptorset or tagset is transformed into an rdfs:seeAlso annotation on the relevant OWL class.

Two further rdfs annotation properties are accessible directly in POWDER: label and comment. The usage and transformation from POWDER to POWDER-S of all these properties are shown in the following example.

1  <?xml version="1.0"?>
2  <powder xmlns="http://www.w3.org/2007/05/powder#" 
3          xmlns:ex="http://example.org/vocab#">

4    <attribution>
5      <issuedby src="http://authority.example.org/company.rdf#me" />
6      <issued>2007-12-14T00:00:00</issued>
7    </attribution>

8    <dr>
9      <iriset>
10       <includehosts>example.com</includehosts>
11     </iriset>

12     <tagset>
13       <label>Tags for the London landmark</label>
14       <tag>London</tag>
15       <tag>Swiss Re</tag>
16       <tag>gherkin</tag>
17       <seealso src="http://encyclopaedia.example.com/gherkin.html" />
18       <seealso src="http://photo.example.com/gherkin.jpg" />
19       <comment>Tags are linked to specific resources that contextualize them</comment>
20     </tagset>
21   </dr>
22 </powder>

1  <?xml version="1.0"?>
2  <rdf:RDF
3    xmlns:wdrs="http://www.w3.org/2007/05/powder-s#"
4    xmlns:foaf="http://xmlns.com/foaf/0.1/"
5    xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
6    xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
7    xmlns:owl="http://www.w3.org/2002/07/owl#"
8    xmlns:dc="http://purl.org/dc/elements/1.1/"
9    xmlns:dcterms="http://purl.org/dc/terms/"
10   xmlns:ex="http://example.org/vocab#">
11  
12   <owl:Ontology rdf:about="">
13     <wdrs:issuedby rdf:resource="http://authority.example.org/company.rdf#me" />
14     <wdrs:issued>2007-12-14T00:00:00</wdrs:issued>
15   </owl:Ontology>
16  
17   <owl:Class rdf:nodeID="iriset_1">
18     <owl:equivalentClass>
19       <owl:Class>
20         <owl:intersectionOf rdf:parseType="Collection">
21           <owl:Restriction>
22             <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
23             <owl:hasValue rdf:datatype="http://www.w3.org/2001/XMLSchema#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(example\.com)(:([0-9]+))?\/</owl:hasValue>
24           </owl:Restriction>
25         </owl:intersectionOf>
26       </owl:Class>
27     </owl:equivalentClass>
28   </owl:Class>
29  
30   <owl:Class rdf:nodeID="tagset_1">
31     <rdfs:label>Tags for the London landmark</rdfs:label>
32     <rdfs:comment>Tags are linked to specific resources that contextualize them</rdfs:comment>
33     <rdfs:seeAlso rdf:resource="http://encyclopaedia.example.com/gherkin.html" />
34     <rdfs:seeAlso rdf:resource="http://photo.example.com/gherkin.jpg" />
35     <rdfs:subClassOf>
36       <owl:Class>
37         <owl:intersectionOf rdf:parseType="Collection">
38           <owl:Restriction>
39             <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#tag" />
40             <owl:hasValue>London</owl:hasValue>
41           </owl:Restriction>
42           <owl:Restriction>
43             <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#tag" />
44             <owl:hasValue>Swiss Re</owl:hasValue>
45           </owl:Restriction>
46           <owl:Restriction>
47             <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#tag" />
48             <owl:hasValue>gherkin</owl:hasValue>
49           </owl:Restriction>
50         </owl:intersectionOf>
51       </owl:Class>
52     </rdfs:subClassOf>
53   </owl:Class>
54 
55   <owl:Class rdf:nodeID="iriset_1">
56     <rdfs:subClassOf rdf:nodeID="tagset_1"/>
57   </owl:Class>
58 
59 </rdf:RDF>

The seealso, label and comment elements are provided simply as shortcuts to avoid having to declare the rdfs namespace in a POWDER document. The relevant rdfs properties MAY be used directly within a tag set or descriptor set and these receive exactly the same semantics in the GRDDL transformation. For clarity, the following two statements are semantically equivalent in a POWDER document:

<tagset>
  <seealso src="http://encyclopaedia.example.com/gherkin.html" />
<tagset>

<tagset>
  <rdfs:seeAlso rdf:resource="http://encyclopaedia.example.com/gherkin.html" />
<tagset>

xml:lang attributes are retained by the GRDDL transformation for the label and comment elements.

2.8 POWDER Flexibility

2.8.1 Multiple Descriptor Sets

In much the same way that a Description Resource may contain multiple IRI sets, it may also contain multiple descriptor sets. This is particularly useful where two distinct and independent types of description are applied to different sections of a single Web site. For instance, an editorial policy or trustmark might apply to a whole Web site, whereas a description of the content's suitability for children might vary in different sections, as the following example shows. Here the whole of movie.example.com is covered by the same trustmark with resources that have IRIs with a path beginning with /after9pm identified as only being suitable for adults. By defining descriptor sets separately within the POWDER document, these can be referenced as needed within the DRs without duplicating data. Separately from the ordered list, the entity described at http://authority.example.org/company.rdf#me has tagged the /latest section with the phrases New Releases and What's Hot.

This example introduces some new elements and attributes, which are detailed below.

1  <?xml version="1.0"?>
2  <powder xmlns="http://www.w3.org/2007/05/powder#" 
3          xmlns:ex="http://example.org/vocab#">

4    <attribution>
5      <issuedby src="http://authority.example.org/company.rdf#me" />
6      <issued>2007-12-14T00:00:00</issued>
7    </attribution>
  
8    <ol>
9      <dr>
10       <iriset>
11         <includehosts>movie.example.com</includehosts>
12         <includepathstartswith>/after9pm</includepathstartswith>
13       </iriset>

14       <descriptorset include="adult" />
15       <descriptorset include="trustmark" />
16     </dr>

17     <dr>
18       <iriset>
19         <includehosts>movie.example.com</includehosts>
20       </iriset>

21       <descriptorset include="allages" />
22       <descriptorset include="trustmark" />
23     </dr>
24   </ol>

25   <descriptorset node="adult">
26     <ex:agemin>18</ex:agemin>
27     <displaytext>Adults Only</displaytext>
28     <displayicon src="http://authority.example.org/adult.png" />
29   </descriptorset>

30   <descriptorset node="allages">
31     <ex:agemin>0</ex:agemin>
32     <displaytext>All Ages</displaytext>
33     <displayicon src="http://authority.example.org/all.png" />
34   </descriptorset>

35   <descriptorset node="trustmark">
36     <typeof src="http://trust.example.org/vocab#trustedsite" />
37   </descriptorset>

38   <dr>
39     <iriset>
40       <includehosts>movie.example.com</includehosts>
41       <includepathstartswith>/latest</includepathstartswith>
42     </iriset>

43     <tagset>
44       <tag>New Releases</tag>
45       <tag>What's Hot</tag>
46       <seealso src="http://cinema.example.com/nowplaying/" />
47     </tagset>
48   </dr>
49  </powder>

1   <?xml version="1.0"?>
2   <rdf:RDF
3      xmlns:wdrs="http://www.w3.org/2007/05/powder-s#"
4      xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
5      xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
6      xmlns:owl="http://www.w3.org/2002/07/owl#"
7      xmlns:ex="http://example.org/vocab#">
8  
9      <owl:Ontology rdf:about="">
10       <wdrs:issuedby rdf:resource="http://authority.example.org/company.rdf#me" />
11       <wdrs:issued>2007-12-14T00:00:00</wdrs:issued>
12     </owl:Ontology>
13 
14    <owl:Class rdf:nodeID="adult">
15      <wdrs:text>Adults Only</wdrs:text>
16      <wdrs:logo rdf:resource="http://authority.example.org/adult.png" />
17      <rdfs:subClassOf>
18        <owl:Class>
19          <owl:intersectionOf rdf:parseType="Collection">
20            <owl:Restriction>
21              <owl:onProperty rdf:resource="http://example.org/vocab#agemin" />
22              <owl:hasValue>18</owl:hasValue>
23            </owl:Restriction>
24          </owl:intersectionOf>
25        </owl:Class>
26      </rdfs:subClassOf>
27    </owl:Class>
28 
29    <owl:Class rdf:nodeID="allages">
30      <wdrs:text>All Ages</wdrs:text>
31      <wdrs:logo rdf:resource="http://authority.example.org/all.png" />
32      <rdfs:subClassOf>
33        <owl:Class>
34          <owl:intersectionOf rdf:parseType="Collection">
35            <owl:Restriction>
36              <owl:onProperty rdf:resource="http://example.org/vocab#agemin" />
37              <owl:hasValue>0</owl:hasValue>
38            </owl:Restriction>
39          </owl:intersectionOf>
40        </owl:Class>
41      </rdfs:subClassOf>
42    </owl:Class>
43 
44    <owl:Class rdf:nodeID="trustmark">
45      <rdfs:subClassOf>
46        <owl:Class>
47          <owl:intersectionOf rdf:parseType="Collection">
48            <owl:Class rdf:about="http://trust.example.org/vocab#trustedsite" />
49          </owl:intersectionOf>
50        </owl:Class>
51      </rdfs:subClassOf>
52    </owl:Class>
53 
54    <owl:Class rdf:nodeID="iriset_1">
55      <owl:equivalentClass>
56        <owl:Class>
57          <owl:intersectionOf rdf:parseType="Collection">
58            <owl:Restriction>
59              <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
60              <owl:hasValue rdfdatatype="http://www.w3.org/2001/XMLSchema#string">\\/\/(([^\/\?\#]*)\@)?([^\\/\?\#\@]+\.)?(movie\.example\.com)(([0-9]+))?\/</owl:hasValue>
61            </owl:Restriction>
62            <owl:Restriction>
63              <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
64              <owl:hasValue rdfdatatype="http://www.w3.org/2001/XMLSchema#string">\\/\/(([^\/\?\#]*)\@)?([^\\/\?\#\@]*)(\([0-9]+))?(\/after9pm)</owl:hasValue>
65            </owl:Restriction>
66          </owl:intersectionOf>
67        </owl:Class>
68      </owl:equivalentClass>
69    </owl:Class>
70 
71    <owl:Class rdf:nodeID="iriset_1_not">
72      <owl:equivalentClass>
73        <owl:Class>
74          <owl:unionOf rdf:parseType="Collection">
75            <owl:Restriction>
76              <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#notmatchesregex" />
77              <owl:hasValue rdfdatatype="http://www.w3.org/2001/XMLSchema#string">\\/\/(([^\/\?\#]*)\@)?([^\\/\?\#\@]+\.)?(movie\.example\.com)(([0-9]+))?\/</owl:hasValue>
78            </owl:Restriction>
79            <owl:Restriction>
80              <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#notmatchesregex" />
81              <owl:hasValue rdfdatatype="http://www.w3.org/2001/XMLSchema#string">\\/\/(([^\/\?\#]*)\@)?([^\\/\?\#\@]*)(\([0-9]+))?(\/after9pm)</owl:hasValue>
82            </owl:Restriction>
83          </owl:unionOf>
84        </owl:Class>
85      </owl:equivalentClass>
86     </owl:Class>
87 
88    <rdf:Description rdf:nodeID="iriset_1">
89      <rdfs:subClassOf rdf:nodeID="adult"/>
90      <rdfs:subClassOf rdf:nodeID"trustmark"/>
91    </rdf:Description>
92 
93    <owl:Class rdf:nodeID="iriset_2">
94      <owl:equivalentClass>
95        <owl:Class>
96          <owl:intersectionOf rdf:parseType="Collection">
97            <owl:Restriction>
98              <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
99              <owl:hasValue rdfdatatype="http://www.w3.org/2001/XMLSchema#string">\\/\/(([^\/\?\#]*)\@)?([^\\/\?\#\@]+\.)?(movie\.example\.com)(([0-9]+))?\/</owl:hasValue>
100           </owl:Restriction>
101           <owl:Class rdf:nodeID="iriset_1_not" />
102         </owl:intersectionOf>
103       </owl:Class>
104     </owl:equivalentClass>
105    </owl:Class>
106 
107   <rdf:Description rdf:nodeID="iriset_2">
108     <rdfs:subClassOf rdf:nodeID="allages"/>
109     <rdfs:subClassOf rdf:nodeID="trustmark"/>
110   </rdf:Description>
111 
112   <owl:Class rdf:nodeID="iriset_3">
113     <owl:equivalentClass>
114       <owl:Class>
115         <owl:intersectionOf rdf:parseType="Collection">
116           <owl:Restriction>
117             <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
118             <owl:hasValue rdfdatatype="http://www.w3.org/2001/XMLSchema#string">\\/\/(([^\/\?\#]*)\@)?([^\\/\?\#\@]+\.)?(movie\.example\.com)(([0-9]+))?\/</owl:hasValue>
119           </owl:Restriction>
120           <owl:Restriction>
121             <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
122             <owl:hasValue rdfdatatype="http://www.w3.org/2001/XMLSchema#string">\\/\/(([^\/\?\#]*)\@)?([^\\/\?\#\@]*)(\([0-9]+))?(\/latest)</owl:hasValue>
123           </owl:Restriction>
124         </owl:intersectionOf>
125       </owl:Class>
126     </owl:equivalentClass>
127   </owl:Class>
128 
129   <owl:Class rdf:nodeID="tagset_1">
130     <rdf:seeAlso rdf:resource="http://cinema.example.com/nowplaying/" />
131     <rdfs:subClassOf>
132       <owl:Class>
133         <owl:intersectionOf rdf:parseType="Collection">
134           <owl:Restriction>
135             <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#tag" />
136             <owl:hasValue>New Releases</owl:hasValue>
137           </owl:Restriction>
138           <owl:Restriction>
139 	        <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#tag" />
140 	        <owl:hasValue>What's Hot</owl:hasValue>
141 	      </owl:Restriction>
142         </owl:intersectionOf>
143       </owl:Class>
144     </rdfs:subClassOf>
145   </owl:Class>
146  
147   <rdf:Description rdf:nodeID="iriset_3">
148     <rdfs:subClassOf rdf:nodeID="tagset_1"/>
149   </rdf:Description>
150 
151 </rdf:RDF>

As was shown in Example 2-10 above, if the src attribute is set on a descriptorset element in a POWDER document, then the IRI given becomes the subject of the sub-class assertion. Example 2-14 presents a different situation where the POWDER author has created some descriptor sets and then included them in the DRs. Hence the include attribute is set in lines 14, 15, 21 and 22 of the POWDER document.

Notice also that, rather than using xml:id to identify the descriptor sets, the node attribute is used (lines 25, 28 and 31). The transformation uses these attributes to create rdf:nodeID identifiers for the respective descriptor classes, as is the usual case with POWDER-S documents, so that they can only be referenced (and influenced) within the POWDER document and not from outside.

Line 32 of the POWDER document introduces the typeof element. This is POWDER's method for asserting that all the resources identified in the IRI set are instances of a particular class - i.e. asserting the rdf:type property. In POWDER-S this is effected by asserting a sub-class relationship between the descriptor class and the referred-to class (line 45-51). In common with the seealso, label and comment elements, typeof is provided as a shortcut. Writing rdf:type directly in a descriptor set has exactly the same semantics.

2.8.2 XML Data in POWDER

We noted in Section 2.1 that the value of a descriptor may be an XML Literal. By embedding XML data directly in a DR, it is associated with all the resources in the IRI set as shown in the following example. Note the use of the rdf:parseType="Literal" attribute in line 15 of the POWDER instance and line 36 of the POWDER-S instance.

1  <?xml version="1.0"?>

2  <powder xmlns:ex="http://example.org/vocab#"
3          xmlns:ex2="http://example.org/palette#"
4          xmlns="http://www.w3.org/2007/05/powder#"
5          xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">

6    <attribution>
7      <issuedby src="http://authority.example.org/company.rdf#me" />
8      <issued>2007-12-14T00:00:00</issued>
9    </attribution>

10   <dr>
11     <iriset>
12       <includehosts>example.com</includehosts>
13     </iriset>

14     <descriptorset>
15       <ex:color rdf:parseType="Literal">
16         <ex2:hex>ff0000</ex2:hex>
17         <ex2:red>255</ex2:red>
18         <ex2:green>0</ex2:green>
19         <ex2:blue>0</ex2:blue>
20       </ex:color>
21       <displaytext>Everything on example.org is red</displaytext>
22       <displayicon src="http://example.org/icon.png" />
23     </descriptorset>
24   </dr>

25 </powder>

1  <?xml version="1.0"?>
2  <rdf:RDF
3     xmlns:wdrs="http://www.w3.org/2007/05/powder-s#"
4     xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
5     xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
6     xmlns:owl="http://www.w3.org/2002/07/owl#"
7     xmlns:ex="http://example.org/vocab#"
8     xmlns:ex2="http://example.org/palette#">
9
10    <owl:Ontology rdf:about="">
11      <wdrs:issuedby rdf:resource="http://authority.example.org/company.rdf#me" />
12      <wdrs:issued>2007-12-14T00:00:00</wdrs:issued>
13    </owl:Ontology>
14  
15    <owl:Class rdf:nodeID="iriset_1">
16     <owl:equivalentClass>
17       <owl:Class>
18         <owl:intersectionOf rdf:parseType="Collection">
19           <owl:Restriction>
20             <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
21             <owl:hasValue rdf:datatype="http://www.w3.org/2001/XMLSchema#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(example\.com)(:([0-9]+))?\/</owl:hasValue>
22           </owl:Restriction>
23         </owl:intersectionOf>
24       </owl:Class>
25     </owl:equivalentClass>
26    </owl:Class>
27 
28    <owl:Class rdf:nodeID="descriptorset_1">
29      <wdrs:text>Everything on example.com is red</wdrs:text>
30      <wdrs:logo rdf:resource="http://example.org/icon.png" />
31      <owl:equivalentClass>
32        <owl:Class>
33          <owl:intersectionOf rdf:parseType="Collection">
34            <owl:Restriction>
35              <owl:onProperty rdf:resource="http://example.org/vocab#color" />
36              <owl:hasValue rdf:parseType="Literal">
37                <ex2:hex>ff0000</ex2:hex>
38                <ex2:red>255</ex2:red>
39                <ex2:green>0</ex2:green>
40                <ex2:blue>0</ex2:blue>
41              </owl:hasValue>
42            </owl:Restriction>
43          </owl:intersectionOf>
44        </owl:Class>
45      </owl:equivalentClass>
46    </owl:Class>
47  
48    <owl:Class rdf:nodeID="iriset_1">
49      <rdfs:subClassOf rdf:nodeID="descriptorset_1"/>
50    </owl:Class>
51  
52 </rdf:RDF>

2.8.3 Extensibility

As the previous two sub sections aim to demonstrate, POWDER is designed to be flexible, imposing as few constraints as possible on its structure to allow easy integration into a variety of workflows. However, where constraints do exist, for example in only allowing literal values for RDF properties in the descriptor set, they do so to allow at least the functional processing of POWDER documents without an RDF toolkit. Hence, although DRs are highly flexible, they are not extensible. The Grouping document [GROUP] does define a mechanism for extending that aspect of DRs however.

2.8.4 Localization

Textual content of POWDER documents may be subject to linguistic-related processes, such as translation, spell-checking, etc. The xml:lang attribute can be added to any textual POWDER element within a valid document. However; its use is only recommended for the displaytext, comment, and label elements. The use of xml:lang on tag elements is not recommended since there is no way to identify different tags as being linked. For example:

  <tag xml:lang="fr">bleu</tag>
  <tag xml:lang="en">square</tag>

are both valid tags for something that is square and blue but the two are not alternative linguistic realizations of the same concept. This becomes even more apparent when POWDER is transformed into POWDER-S.

The W3C Internationalization Tag Set [ITS] provides a convenient method to define which parts of a document have textual content, using a rules document. POWDER's ITS Rules Document [ITS-RULES] declares that the displaytext, tag, comment, and label elements contain text that may be subject to linguistic-related processes. However, such processing is not part of POWDER and the ITS attributes are not expressed within the POWDER data model/semantics. The ITS information is necessarily lost when performing the GRDDL transformation to POWDER-S.

2.9 Conformance Statement: POWDER and POWDER-S Documents

A conformant POWDER document complies with the requirements set out in sections 2.1 - 2.8 plus sections 5.2 and 5.3 of this document.

This document does not set any conformance criteria for POWDER-S which is encoded using OWL plus the semantic extension that is defined in the Formal Semantics document [FORMAL].

3 The POWDER Processor

The Protocol for Web Description Resources is designed first and foremost as a simple and efficient method of publishing metadata. A POWDER document contains sufficient information to yield a description of any resource that is within the scope of the DR(s) to which it has access. A key function of a POWDER Processor is therefore to return RDF triples that describe candidate resources.

If u is the URI or IRI of a resource, then as a minimum, a POWDER Processor MUST support the function describe(u) by returning RDF triples that describe u.

A minimal set of formal constraints are placed on how a POWDER Processor must be realized. It may be a component in a user agent or some other application that is able to process the descriptions it returns or a standalone online service. It may use HTTP to collect POWDER documents from the Web or it may act as a gateway to a repository of Description Resources. It will be able to process XML and probably RDF, and it is highly likely that a real application will support a variety of different functions and options, in particular, one or more of those related to trust as discussed in Section 5; however, a conformant POWDER Processor will support the key describe(u) function.

As an example, imagine a POWDER Processor that has access to the document shown in Example 2-1. The function call describe('http://www.example.com/') would return:

<rdf:Description rdf:about="http://www.example.com/">
  <ex:color>red</ex:color>
  <ex:shape>square</ex:shape>
  <wdrs:describedby rdf:resource="http://...example2-1.xml" />
</rdf:Description>

Notice that the description includes a link to the POWDER document from which the data was derived (this is optional). The wdrs:describedby property is discussed further in Section 4.1.3 below.

Where a POWDER Processor has a Web interface, describe(u) SHOULD be the default function available by sending an HTTP GET request to its IRI and including the candidate resource as the value for the parameter u in the query string. The query string should be URL-encoded as defined by XForms 1.0 [XF] such that, if ipp is the IRI of the POWDER Processor, then the above request for a description of http://www.example.com/ would be made by dereferencing the following IRI:

ipp?u=http%3A%2F%2Fwww.example.com%2F

which would return the RDF/XML document shown below

<?xml version="1.0"?>
<rdf:RDF
  xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
  xmlns:ex="http://example.org/vocab#"
  xmlns:wdrs="http://www.w3.org/2007/05/powder-s#">
 
  <rdf:Description rdf:about="http://www.example.com/">
    <ex:color>red</ex:color>
    <ex:shape>square</ex:shape>
    <wdrs:describedby rdf:resource="http://...example2-1.xml" />
  </rdf:Description>
 
</rdf:RDF>

If the processor can find no information about the candidate resource within the data available at the time of the request, then it uses the wdrs:notknownto property to return the 'not known' message shown below.

<?xml version="1.0"?>
<rdf:RDF
  xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
  xmlns:wdrs="http://www.w3.org/2007/05/powder-s#">
 
  <rdf:Description rdf:about="http://www.example.com/">
    <wdrs:notknownto rdf:resource="ipp" />
  </rdf:Description>
 
</rdf:RDF>

The range of wdrs:notknownto is the class wdrd:Processor, defined as the class of POWDER Processors.

The value for u may be replaced by the word 'referer', in which case the POWDER Processor SHOULD return a description of the IRI given in the HTTP REFERER field (or the not known response). This allows a POWDER Processor to return descriptions of resources that point to it. For example, an XHTML document published at http://www.example.com/ might include the following link element:

<link rel="meta" href="ipp?u=referer" type="application/rdf+xml" />

User agents following that link would receive the response shown in Example 3-1. (N.B. Only the behavior of the POWDER processor is specified here; the relationship type of meta is non-normative in this context).

The describe(u) function SHOULD also support an additional optional parameter, d. This is the IRI of a POWDER document and may include a fragment identifier as discussed in Section 2.5. Referring to Example 2-9 we can envisage a document at http://www.example.com/page.html that includes a link element pointing to /powder.xml#red and construct the function call:

describe(http://www.example.com/page.html, http://www.example.com/powder.xml#red)

or over HTTP as

ipp?u=http%3A%2F%2Fwww.example.com%2F&d=http%3A%2F%2Fwww.example.com%2Fpowder.xml%23red

Alternatively, the document at http://www.example.com/page.html might simply point directly to the POWDER processor with the following link:

<link rel="meta" href="ipp?u=referer&d=http%3A%2F%2Fwww.example.com%2Fpowder.xml%23red" type="application/rdf+xml" />

Dereferencing that IRI returns RDF triples about that specific document.

3.1 Error Handling

In addition to the descriptive or 'not known' responses, a POWDER Processor SHOULD also report errors of which there are two distinct types: data errors and processing errors. We reserve the generic error codes 100 and 200 respectively for these two types of error, with implementers free to define appropriate codes in the range 1xx and 2xx. A client will therefore always be able to determine which type of error has been reported even if it cannot understand the precise nature of that error.

Only one specific error is defined for all conformant POWDER processors: error 101 is raised when a candidate resource is within the scope of a DR that refers to a descriptor set that, through an abouthosts restriction, cannot apply. This can happen, for example, where the descriptor set and DR are defined separately as shown in Section 2.6.

Errors are reported using the wdrs:data_error, wdrs:proc_error and wdrs:err_code properties. If data is the IRI of the POWDER document containing the erroneous data, then the processor should return:

<rdf:Description rdf:about="data">
  <wdrs:err_code>101</wdrs:err_code>
  <wdrs:data_error>Description Resource refers to a descriptor set that is out of scope</wdrs:data_error>
</rdf:Description>

In similar fashion, if ipp is the identifier for the processor itself, then an internal error might be reported thus:

<rdf:Description rdf:about="ipp">
  <wdrs:err_code>214</wdrs:err_code>
  <wdrs:proc_error>Error in subroutine foo at line x</wdrs:proc_error>
<rdf:Description>

3.2 POWDER Processor Conformance Statement

A conformant POWDER Processor will support the describe(u) function in accordance with the specifications set out in this document and those to which it refers as follows. The POWDER processor:

(see the Formal Semantics document for details of each POWDER format).

The distinctions between how the different encodings of Description Resources arise because all POWDER documents can be transformed into POWDER-BASE. As the name suggests, this is the fundamental format. However, IRI sets are much more easily defined by humans using POWDER, and it is anticipated that it will be the predominant format. Where the processor is associated with a data source that does not make use of POWDER, perhaps using a different transform to create POWDER-BASE from a proprietary format and, due to its application environment, it is known with a high degree of certainty that it will never have to process POWDER documents from other sources, then it clearly will not have to support it (or POWDER-S).

POWDER-S is defined to allow Description Resources to be processed as part of the more general Semantic Web. The application context must therefore determine whether a POWDER processor should support it.

There is no requirement for a processor to use the XSLTs provided by the Working Group to effect the transforms associated with the POWDER namespaces. The Formal Semantics document [FORMAL] is normative, not the XSLTs.

As noted, there are no other formal conformance criteria for a POWDER Processor; however, good implementations will go significantly further. For example, they may:

• return results as RDF in alternative serializations or as HTML;
• support lists of POWDER documents as a data source;
• implement a variety of policies to take account of the attribution data when responding to requests;
• return richer results detailing the processing that has been carried out;
• provide an option to include IRIs discovered when one that is in the scope of a DR redirects to one that is not.

The inverse of the describe(u) function — 'find resources with these properties' — would be very useful in several of the scenarios set out in the POWDER use cases. Where the processor is acting as a gateway to a specific repository it is likely to be possible to specify a simple syntax for queries that would return DRs or IRI sets that include descriptor sets matching given properties. Since a descriptor set may contain RDF/XML with few constraints, a fully flexible function of this type could only be provided by supporting SPARQL queries against POWDER-S documents, noting that this requires an implementation of the POWDER semantic extension by the requesting agent.

4 Associating Resources and DRs

4.1 Linking a Resource to a POWDER Document: Relationship and Media types

A given resource (hereafter known as the described resource) may relate itself to a POWDER document. There are several methods to define such a relationship, detailed in the following sub-sections. To facilitate linking between a described resource and a POWDER document we define a relationship type of describedby for use in (X)HTML link elements, HTTP Link Headers and ATOM feeds; and a textually identical term as part of the POWDER-S vocabulary. This is a generic relationship type that does not of itself imply that the link points to a POWDER document — that is done by the specific Media type. The formal definition of describedby is given in Appendix D.

We define the following Media types for POWDER and POWDER-BASE documents:

application/powder+xml [Appendix B]

and for POWDER-S documents:

application/powder-s+xml [Appendix C]

4.1.1 (X)HTML link Elements

In HTML and XHTML documents, the link element can be used to point to a POWDER document with the relationship type of 'describedby' as shown in Example.4-1 below. The relationship type is defined in the POWDER profile document, which should therefore be referenced in the head tag when using a version of HTML that supports it.

Documents MAY also include any of the attribution data from the POWDER document in meta tags. In particular, the issuedby field is likely to be useful to user agents deciding whether or not to fetch the full POWDER document. Any attribution data encoded in meta tags within an HTML document should be the same as that in the POWDER document. In case of discrepancy, the POWDER document should be taken as more authoritative.

N.B. the value of the rel attribute is a white space separated list so other relationship types may be included too. For example, it may be desirable to give processors a clue as to which descriptive vocabulary or vocabularies is/are used in the DR(s) in the POWDER document. Again, such relationship types may need to be defined in a profile document.

<html xmlns="http://www.w3.org/1999/xhtml">
   <head profile="http://www.w3.org/2007/11/powder-profile">
      <meta name="wdr.issuedby" content="http://authority.example.org/company.rdf#me"/>
      <link rel="describedby" href="powder.xml" type="application/powder+xml"/>
      <title>Welcome to example.com </title>
   </head>
   <body>
      <p>Today's content is …</p>
   </body>
</html>

This approach requires the described resource to be requested and parsed in order to find the relevant POWDER document's location. This may be acceptable for data warehousing, for example retrieving DRs asynchronously to a user request, so that the metadata about the described resource can be inspected and usage trends compiled. However a more efficient method of associating resources with a POWDER document is to use the HTTP Link Header.

4.1.2 ATOM link Elements

POWDER may provide useful annotations in ATOM syndication feeds, again with the describedby property providing the link's relationship type. Example 4-2 shows such an annotated ATOM feed.

In accordance with the ATOM specification [RFC4287], the describedby relationship is a relative URI, the base of which is http://www.iana.org/assignments/relation/ - i.e. the full URI of describedby is http://www.iana.org/assignments/relation/describedby - and this is included in the ATOM registry [AREG].

<?xml version="1.0" encoding="utf-8"?>
  <feed
    xmlns="http://www.w3.org/2005/Atom">
    <link rel="describedby" href="http://ecw.example.org/powder1.xml" />
    <title>The English Civil War</title>
    <link href="http://ecw.example.org/"/>
    <updated>2007-10-30T15:00:00Z</updated>
    <author>
      <name>John Doe</name>
    </author>
    <id>urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6</id>

    <entry>
      <title>Charles I</title>
      <link href="http://ecw.example.org/charles1.html"/>
      <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
      <updated>2007-06-04T09:39:21Z</updated>
      <summary>Charles I came to the throne believing in his Divine Right to rule...</summary>
    </entry>

    <entry>
      <link rel="describedby" href="http://monarchy.example.org/powder2.xml" />
      <title>Divine Right</title>
      <link href="http://monarchy.example.org/divine_right.html"/>
      <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6b</id>
      <updated>2007-06-06T13:43:54Z</updated>
      <summary>Divine Right was claimed by several English monarchs, notably Charles I...</summary>
    </entry>

  </feed>

The feed element is linked to powder1.xml which is a hint, not a guarantee, that it describes many or all of the items in the feed. The second item in the feed is described by a different POWDER document.

It is worth noting that the structure of the ATOM feed plays no part in determining what is described by the POWDER documents — this is only defined in the IRI sets within the DRs they contain. For the avoidance of doubt, powder1.xml does not fulfil the role of 'default' description that is then overridden by powder2.xml.

4.1.3 HTTP Link Headers (Informative)

This sub-section is marked informative (cf. normative) in recognition that, at the time of writing, Link Relations and HTTP Header Linking [HLINK] is an Internet Draft. It is anticipated that, following further review, it will become an RFC.

The HTTP Link Header [HLINK] is an alternative to the HTML link element and has very similar semantics such that the HTML link shown in Example 4-1 above can be expressed as follows:

Link: <powder.xml>; rel="describedby" type="application/powder+xml";

User agents may discover the location of the POWDER document by inspecting the HTTP Response headers and passing it to a POWDER processor without having to parse the resource itself. This has several distinct advantages:

• A HEAD request would reveal the location of the POWDER document, which could then be processed before deciding whether to fetch the described resource itself. This has particular applicability to access control and content personalization.
• The POWDER document may be fetched asynchronously by the user agent before the described resource has been processed, maximizing processing efficiency where this is important.
• The HTTP Link header can be applied to any type of resource, not just formats that support a link element of their own such as (X)HTML.
• Server configuration is often a very efficient way to link all resources on a Web site to a central resource (including stylesheets and script libraries as well as POWDER documents).

4.1.4 Semantic Linkage Using the describedby Property

We define the RDF property wdrs:describedby with a domain of rdf:Resource and a range of wdrs:Document. This is the class of POWDER documents and is a sub class of owl:Ontology. The meaning of wdrs:describedby is identical to the describedby relationship type defined above so that:

1  <html
2    xmlns="http://www.w3.org/1999/xhtml" version="XHTML+RDFa 1.0"
3    xmlns:wdrs="http://www.w3.org/2007/05/powder-s#">
4    <head>
5      <title>The English Civil War</title>
6      <link rel="wdrs:describedby" href="http://ecw.example.org/powder1.xml" type="application/powder+xml" />
7    </head>
8    <body>
9      …
10     <p>Charles I came to the throne believing in his 
11     <a href="http://monarchy.example.org/divine_right.html">Divine Right</a> to rule…
12     …
13   </body>
14 </html>

Here the active document is linked to a description through the link element in the head with the relationship type defined using the wdrs:describedby property (line 6). The POWDER document (powder1.xml) SHOULD describe the active document and may or may not provide a description of other resources linked from it. For the purposes of this example we'll assume that powder1.xml describes the whole of the Web site that includes this document as being a source of information that is recommended for school study of the English Civil War.

Notice that the text in example 4-3 includes a link to another document on another host in line 11 (divine_right.html). Suppose that document, and others on the monarchy.example.org Web site, are described by a different POWDER document. It is possible to use RDFa to link to a POWDER document describing divine_right.html as shown in lines 11-13 in example 4-4 below.

1  <html
2    xmlns="http://www.w3.org/1999/xhtml" version="XHTML+RDFa 1.0"
3    xmlns:wdrs="http://www.w3.org/2007/05/powder-s#">
4    <head>
5      <title>The English Civil War</title>
6      <link rel="wdrs:describedby" href="http://ecw.example.org/powder1.xml" />
7    </head>
8    <body>
9      …
10     <p>Charles I came to the throne believing in his 
11        <a about="http://monarchy.example.org/powder2.xml"
12        rev="wdrs:describedby"
13        href="http://monarchy.example.org/divine_right.html">Divine Right</a> to rule...
14     …
15   </body>
16 </html>

Note the use of the rev relationship type in line 12 which effectively switches round the subject and object of the triple. An RDFa processor will extract the following two triples from this snippet:

<> wdrs:describedby <http://ecw.example.org/powder1.xml>
<http://monarchy.example.org/divine_right.html>  wdrs:describedby <http://monarchy.example.org/powder2.xml>

A POWDER-aware Web browser may use the description provided in powder2.xml to decide to render the hyperlink in different ways. For example, if the document about Divine Right is not suitable for display on mobile devices, the browser may not include the hyperlink at all when the primary document is viewed on a handheld device but would do so on a desktop device.

4.2 Linking a Resource to a POWDER Processor to Acquire RDF

As described in Section 3, a POWDER processor may be set up as an online service through which RDF triples about a particular resource may be obtained. The simplest way to achieve this is to append the IRI of the POWDER processor with ?u=referer, adding the IRI of a POWDER data source if necessary, and include it in either an HTML or HTTP Link.

RDF-aware user agents, including those without any POWDER implementation, will be able to process the data received.

4.3 Linking POWDER documents

POWDER documents may refer to each other to facilitate easier discovery by user agents. The more element is a child element of the root element, may appear any number of times, and uses the src attribute to point to external POWDER documents. The data is retained in POWDER-S where it becomes an rdfs:seeAlso property with the POWDER-S document as its subject, as shown in Example 4-5 below.

1  <?xml version="1.0"?>
2  <powder xmlns="http://www.w3.org/2007/05/powder#" 
3        xmlns:ex="http://example.org/vocab#">

4    <more src="http://another.example.com/powder2.xml" />

5    <attribution>
6      <issuedby src="http://authority.example.org/company.rdf#me" />
7      <issued>2007-12-14T00:00:00</issued>
8    </attribution>

9    <dr>
10     <iriset>
11       <includehosts>example.com</includehosts>
12     </iriset>

13     <descriptorset>
14       <ex:color>red</ex:color>
15       <ex:shape>square</ex:shape>
16       <displaytext>Everything on example.org is red and square</displaytext>
17       <displayicon src="http://example.org/icon.png" />
18     </descriptorset>
19   </dr>

20 </powder>

1  <?xml version="1.0"?>
2  <rdf:RDF
3     xmlns:wdrs="http://www.w3.org/2007/05/powder-s#"
4     xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
5     xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
6     xmlns:owl="http://www.w3.org/2002/07/owl#"
7     xmlns:ex="http://example.org/vocab#">
8  
9     <owl:Ontology rdf:about="">
10      <wdrs:issuedby rdf:resource="http://authority.example.org/company.rdf#me" />
11      <wdrs:issued>2007-12-14T00:00:00</wdrs:issued>
12      <rdfs:seeAlso rdf:resource="http://another.example.com/powder2.xml" />
13    </owl:Ontology>
14  
15    <owl:Class rdf:nodeID="iriset_1">
16      <owl:equivalentClass>
17        <owl:Class>
18          <owl:intersectionOf rdf:parseType="Collection">
19            <owl:Restriction>
20              <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
21              <owl:hasValue rdf:datatype="http://www.w3.org/2001/XMLSchema#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(example\.com)(:([0-9]+))?\/</owl:hasValue>
22            </owl:Restriction>
23          </owl:intersectionOf>
24        </owl:Class>
25      </owl:equivalentClass>
26    </owl:Class>
27 
28    <owl:Class rdf:nodeID="descriptorset_1">
29      <wdrs:text>Everything on example.com is red and square</wdrs:text>
30      <wdrs:logo rdf:resource="http://example.org/icon.png" />
31      <rdfs:subClassOf>
32        <owl:Class>
33          <owl:intersectionOf rdf:parseType="Collection">
34            <owl:Restriction>
35              <owl:onProperty rdf:resource="http://example.org/vocab#color" />
36              <owl:hasValue>red</owl:hasValue>
37            </owl:Restriction>
38            <owl:Restriction>
39              <owl:onProperty rdf:resource="http://example.org/vocab#shape" />
40              <owl:hasValue>square</owl:hasValue>
41            </owl:Restriction>
42          </owl:intersectionOf>
43        </owl:Class>
44      </rdfs:subClassOf>
45    </owl:Class>
46  
47    <owl:Class rdf:nodeID="iriset_1">
48      <rdfs:subClassOf rdf:nodeID="descriptorset_1"/>
49    </owl:Class>
50  
51 </rdf:RDF>

Example 4-5 is a repeat of Examples 2-1 and 2-3 to show the more element.

4.4 Requesting a DR from a Repository

There are no normative rules on how to publish large numbers of Description Resources. However, individuals or organizations that create DRs are encouraged to make them available both as POWDER and POWDER-S documents. The latter would best be exposed through a SPARQL endpoint. It is noteworthy that if a POWDER document is edited, it SHOULD be made available at a new IRI. As a practical step, that IRI is generally best discovered through an HTTP 302 redirect from an IRI published as the 'latest location.'

The publisher is, of course, free to set up whatever system they feel appropriate, but as there are no normative rules it is essential to provide adequate documentation, perhaps including sample queries that yield results that can be processed efficiently.

Examples are provided in the Primer.

5 Trust

Trust is a critical aspect of Description Resources; however, trust is very much a matter of opinion. The level of trust demanded of a given DR will depend on what the description says and to what use it will be put. For example, an individual user finding a DR that declares a Web site to offer children's birthday party ideas can make his/her own assessment of its quality and usefulness. In contrast, a multi-million dollar business will need very strong assurance that a DR declaring a Web site to be medically accurate and freely available is trustworthy before including it in a portal of high quality license-free, healthcare materials. For this reason, we do not define a single trust mechanism that must be used. Rather, the following sections describe a variety of different methods of adding trust to DRs, some of which may be used in combination. Where applicable, we define vocabulary terms designed to aid the building of trust.

These measures add to POWDER's inherent trust model, namely:

• All (valid) POWDER documents are attributed so that it is always clear who one should ask 'did you really say that?'
• The document structure, whether in POWDER or POWDER-S, does not allow for the semantics to be amended or expanded upon by publishing other data elsewhere on the Web (unless the author specifically wants to make his/her descriptions available for use by others).

5.1 Discovering the Trust Mechanism: the authenticate Property

Any method provided by a DR author for adding trust to their descriptions needs to be discoverable. To this end, we define the authenticate property with a domain of dcterms:Agent and foaf:Agent. That is, it's part of the description of an entity that creates POWDER documents, not of the POWDER documents themselves. The range of authenticate is simply rdfs:Resource and so its value can be machine-processable, such as a WSDL document, or a human-readable document describing any authentication services that the DR author offers or steps he/she recommends. The following snippet of RDF shows a foaf:Organization class (a subclass of foaf:Agent) such as might be pointed to by the src attribute of an issuedby element. It includes a pointer to a document that describes how to authenticate the Exemplary Description Authority's POWDER documents.

<foaf:Organization rdf:ID="me">
  <foaf:homepage rdf:resource="http://authority.example.org/" />
  <foaf:name>The Exemplary Description Authority</foaf:name>
  <foaf:nick>EDA</foaf:nick>
  <authenticate rdf:resource="http://authority.example.org/how_to_authenticate.html" />
</foaf:Organization>

5.2 Certification using POWDER

Trust in Description Resources may readily be enhanced through third-party certification. That is, an entity can publish data that is identifiable through its origin and/or a digital signature that states that the named entity certifies that the claims and assertions made in an identified POWDER document are correct. The level of trust that can be placed in the data then becomes equivalent to the level of trust held in the certification body. Such certificates may themselves by expressed in a DR as exemplified below.

1  <?xml version="1.0"?>
2  <powder xmlns="http://www.w3.org/2007/05/powder#">

3    <attribution>
4      <issuedby src="http://authority.example.org/company.rdf#me" />
5      <issued>2007-12-14T00:00:00</issued>
6      <validfrom>2008-01-01T00:00:00</validfrom>
7      <validuntil>2008-12-31T23:59:59</validuntil>
8    </attribution>

9    <dr>
10     <iriset>
11       <includeresources>http://www.example.com/powder.xml</includeresources>
12     </iriset>
  
13     <descriptorset>
14       <sha1sum>j6lwx3rvEPO0vKtMup4NbeVu8nk=</sha1sum>
15       <certified>true</certified>
16       <displaytext>authority.example.org certifies that claims made by example.com are true. Valid throughout 2008.</displaytext>
17       <displayicon src="http://authority.example.org/icon.png" />
18     </descriptorset>
19   </dr>
10 </powder>

This example introduces a number of features.

It is the nature of certificates that they have a period of validity. Trustmarks are typically awarded for a period of a year, for example. To support this, POWDER defines two elements: validfrom and validuntil in lines 6 and 7. These elements are of type xsd:date and have the following operational semantics:

A POWDER document is temporally valid unless:

• there is a validfrom element present and today's date precedes it
• there is a validuntil date and today's date is after the date given.

Temporal validity is independent of trust; that is, a POWDER document that is temporally invalid may be trusted by a user (or user agent). Similarly, a temporally valid document may not be trusted. However, from a policy perspective, the publisher of any Description Resource should be prepared to stand by their claims for the entirety of any stated validity period.

If an HTTP cache header is set, the valid until dates are only applicable within the expiry period. Thus a processor MAY NOT continue to use the DR beyond its cache period until a fresh copy of the DR has been retrieved.

There are no formal semantics associated with validfrom and validuntil.

There are two further features shown in Example 5-1 that have not been discussed in this document previously.

In line 14, the descriptor set includes the sha1sum element which contains a SHA-1 Sum of the (one) described resource. Although not formalized in the semantics of POWDER, this element provides processors with an integrity checking mechanism. The publisher of Example 5-1 is providing a very precise description of the document at http://www.example.com/powder.xml. User agents are therefore empowered to calculate a SHA-1 hash of the described resource and use that to decide whether or not to trust the data. The sha1sum descriptor takes a single hash as its value and may only appear once in a descriptor set. It is therefore only useful when describing a single resource, as is the case in Example 5-1.

Since the expression of certificates in this way is at the heart of several POWDER use cases [USECASES], we also define the certified element which has a data type of xsd:boolean.

The inverse relationship is also supported by the certifiedby element. That is, the attribution element in http://www.example.com/powder.xml may include a pointer to this certificate by giving its IRI as the value of the src attribute of the certifiedby element. Only complete POWDER documents may be certified, not individual DRs.

Example 5-1 is shown in its Semantic POWDER form below.

1  <?xml version="1.0"?>
2  <rdf:RDF
3     xmlns:wdrs="http://www.w3.org/2007/05/powder-s#"
4     xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
5     xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
6     xmlns:owl="http://www.w3.org/2002/07/owl#"
7     xmlns:ex="http://example.org/vocab#">
8  
9     <owl:Ontology rdf:about="">
10      <wdrs:issuedby rdf:resource="http://authority.example.org/company.rdf#me" />
11      <wdrs:issued>2007-12-14T00:00:00</wdrs:issued>
12      <wdrs:validfrom>2008-01-01T00:00:00</wdrs:validfrom>
13      <wdrs:validuntil>2008-12-31T23:59:59</wdrs:validuntil>
14    </owl:Ontology>
15  
16    <owl:Class rdf:nodeID="iriset_1">
17      <owl:equivalentClass>
18        <owl:Class>
19          <owl:intersectionOf rdf:parseType="Collection">
20            <owl:Restriction>
21              <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
22              <owl:hasValue rdf:datatype="http://www.w3.org/2001/XMLSchema#string">^(http\:\/\/www\.example\.com\/powder\.xml)$</owl:hasValue>
23            </owl:Restriction>
24          </owl:intersectionOf>
25        </owl:Class>
26      </owl:equivalentClass>
27    </owl:Class>
28   
29    <owl:Class rdf:nodeID="descriptorset_1">
30      <wdrs:text>authority.example.org certifies that claims made by example.com are true. Valid throughout 2008.</wdrs:text>
31      <wdrs:logo rdf:resource="http://authority.example.org/icon.png" />
32      <rdfs:subClassOf>
33        <owl:Class>
34          <owl:intersectionOf rdf:parseType="Collection">
35            <owl:Restriction>
36              <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#certified" />
37              <owl:hasValue>true</owl:hasValue>
38            </owl:Restriction>
39            <owl:Restriction>
40              <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#sha1sum" />
41              <owl:hasValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</owl:hasValue>
42            </owl:Restriction>
43          </owl:intersectionOf>
44        </owl:Class>
45      </rdfs:subClassOf>
46    </owl:Class>
47  
48    <owl:Class rdf:nodeID="iriset_1">
49      <rdfs:subClassOf rdf:nodeID="descriptorset_1"/>
50    </owl:Class>
51  
52 </rdf:RDF>

5.2.1 Full Interpretation of Example 5-1

Example 5-1 is a POWDER document that contains a single DR that describes a single resource - another POWDER document at http://www.example.com/powder.xml. The same resource could be described using core Semantic Web technologies, but POWDER offers a number of additional features so that, in effect, Example 5-1 can certify that http://www.example.com/powder.xml is true.

First of all, like all POWDER documents, Example 5-1 is attributed to an identified entity. That entity's description is likely to include the authenticate property described in Section 5.1, giving information on how its DRs may be authenticated.

Secondly, POWDER supports time-limited assertions (albeit through informal semantics). Operationally, Example 5-1 is only valid for the 2008 calendar year.

Thirdly, the DR includes text and graphics that can be displayed to the end user, providing a readily accessible human-readable representation of the machine-processable data.

Fourthly, by providing a SHA1 hash of the described resource, the publisher of the certificate is making it clear exactly what it is certifying and that if the described document is tampered with (and therefore leads to a different hash) the description will no longer be accurate and should probably be disregarded.

5.3 Supporting Evidence: The supportedby Property

The certification mechanism detailed in the previous section entails a direct relationship between the content provider (or at least the DR author) and a certification body. The supportedby property is similar in that it points to a third party from which data is available that will confirm the claims and assertions made in the DR; however, there need not be a direct relationship between the parties concerned. The DR author may discover independently that a third party shares their view and wishes to adduce further evidence to support their own description by pointing to the external body. One scenario in which this might obtain is described in section 2.1.7 of the Use Cases document [USECASES]. There, a DR is used to declare a Web site to be suitable for children with external support for the claim coming from a company that offers a Web site classification service.

In common with authenticate and certifiedby, supportedby has a range of rdfs:Resource, and this may be either a machine-processable or human-readable document. supportedby can be an element within the attribution element or an individual DR. In the latter case, supportedby is specified as a child of element descriptorset.

As an example, the mobileOK scheme allows content providers to claim conformance to a set of Best Practices designed to ensure at least a functional experience on mobile devices. Such a claim may be supported by reference to the mobileOK checker. In the example below, the owners of example.com are making a claim that all the resources on their Web site conform to mobileOK Basic. As supporting evidence, users are welcome to put any resource through the W3C mobileOK checker.

1  <?xml version="1.0"?>
2  <powder xmlns="http://www.w3.org/2007/05/powder#">

3    <attribution>
4      <issuedby src="http://www.example.com/company.rdf#me" />
5      <issued>2008-06-25T00:00:00</issued>
6      <supportedby src="http://validator.w3.org/mobile/" />
7    </attribution>

8    <dr>
9      <iriset>
10       <includehosts>example.com</includehosts>
11     </iriset>

12     <descriptorset>
13       <typeof src="http://www.w3.org/2008/06/mobileOK#conformant" />
14       <displaytext>The example.com Web site conforms to mobileOK</displaytext>
15       <displayicon src="http://www.w3.org/2005/11/MWI-Icons/mobileOK.png" />
16     </descriptorset>
17   </dr>

18 </powder>

1  <?xml version="1.0"?>
2  <rdf:RDF
3    xmlns:wdrs="http://www.w3.org/2007/05/powder-s#"
4    xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
5    xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
6    xmlns:owl="http://www.w3.org/2002/07/owl#"
7    xmlns:dcterms="http://purl.org/dc/terms/"
8    xmlns:foaf="http://xmlns.com/foaf/0.1/"
9    xmlns:ex="http://example.org/vocab#">
  
10   <owl:Ontology rdf:about="">
11     <wdrs:issuedby rdf:resource="http://www.example.com/company.rdf#me" />
12     <wdrs:issued>2008-06-25T00:00:00</wdrs:issued>
13     <wdrs:supportedby rdf:resource="http://validator.w3.org/mobile/" />
14   </owl:Ontology>
 
15   <owl:Class rdf:nodeID="iriset_1">
16     <owl:equivalentClass>
17       <owl:Class>
18         <owl:intersectionOf rdf:parseType="Collection">
19           <owl:Restriction>
20             <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
21             <owl:hasValue rdf:datatype="http://www.w3.org/2001/XMLSchema#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(example\.com)(:([0-9]+))?\/</owl:hasValue>
22           </owl:Restriction>
23         </owl:intersectionOf>
24       </owl:Class>
25     </owl:equivalentClass>
26   </owl:Class>
 
27   <owl:Class rdf:nodeID="descriptorset_1">
28     <wdrs:text>The example.com Web site conforms to mobileOK</wdrs:text>
29     <wdrs:logo rdf:resource="http://www.w3.org/2005/11/MWI-Icons/mobileOK.png" />
30     <rdfs:subClassOf>
31       <owl:Class>
32         <owl:intersectionOf rdf:parseType="Collection">
33         </owl:intersectionOf>
34       </owl:Class>
35     </rdfs:subClassOf>
36     <rdfs:subClassOf rdf:resource="http://www.w3.org/2008/06/mobileOK#conformant" />
37   </owl:Class>
 
38   <owl:Class rdf:nodeID="iriset_1">
39     <rdfs:subClassOf rdf:nodeID="descriptorset_1"/>
40   </owl:Class>
 
41 </rdf:RDF>

5.4 Trusted Source

Description Resources exist independently of the content they describe and are dereferenced from an IRI of their own. This is usually discovered by following a link from the described content. If the Description Resource is hosted on a trusted server, typically one operated by a trustmark scheme or certification body, then the trustworthiness of the DR is equal to the trustworthiness of the server and its operator.

Imagine good.example.com has a commercial relationship with powder.example.org. The operators of good.example.com go through a review process run by powder.example.org and are then entitled to include a tag like the one below in the code of their Web pages.

<link rel="describedby" href="http://powder.example.org/dr46" type="application/powder+xml" />

The document at powder.example.org remains entirely under the control of the reviewer, who is free to change or remove it at any time.

N.B. Any such change must take account of any valid until date given in the document and the fact that a POWDER-aware user agent may cache DRs until that date occurs. The Primer has more to say about this.

5.5 Machine Learning

It is a fundamental aspect of RDF that descriptive terms are explicitly defined by IRI. Therefore if a particular descriptive vocabulary becomes widely used, it will mean the same thing wherever it is found. This is important when deploying content analysis software that uses machine-learning techniques. Once the software has been trained to recognize content matching descriptions made using a particular vocabulary, where DRs are encountered using that same vocabulary, the newly discovered content can be analyzed and a probability of its accuracy be calculated.

5.6 Trust Summary

The methods cited here do not comprise an exhaustive list. Other techniques, such as XML Signature [XMLSIG] and Web of Trust [WOT], may be equally applicable. As noted in the introduction, trust is a human judgment that can only be made by weighing the likelihood that the data is true against the consequences of it being false. This judgment is highly dependant on the circumstances under which the need to extend trust arises. It is clear, however, that Description Resources are unlikely to be trusted in isolation and that both their publishers and consumers will only benefit from their existence if one or more techniques for enhancing trust are employed.

6 References

6.1 Normative References

FORMAL

Protocol for Web Description Resources (POWDER): Formal Semantics 2008, S. Konstantopoulos, P. Archer. This document is at http://www.w3.org/TR/powder-formal/

GRDDL

Gleaning Resource Descriptions from Dialects of Languages (GRDDL) W3C Recommendation 11 September 2007. D. Connolly. This document is at http://www.w3.org/TR/grddl/

GROUP

Protocol for Web Description Resources (POWDER): Grouping of Resources, A. Perego, P. Archer. This document is at http://www.w3.org/TR/powder-grouping/

XML dateTime

XML Schema Part 2: Datatypes Second Edition W3C Recommendation 28 October 2004. P. Biron, A. Malhotra (Eds). This document is at http://www.w3.org/TR/2004/REC-xmlschema-2-20041028/#dateTime

RFC4287

The Atom Syndication Format M. Nottingham,R. Sayre (Eds) RFC 4287, December 2005. This document is at http://www.ietf.org/rfc/rfc4287.txt

AREG

Atom Link Relations This document is at http://www.iana.org/assignments/link-relations/link-relations.xhtml

RFC3987

Internationalized Resource Identifiers (IRIs), M. Duerst, M. Suignard. RFC January 2005. This document is at http://www.ietf.org/rfc/rfc3987.txt

MIME

RFC 2046: Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types N. Freed, N. Borenstein. This document is at http://tools.ietf.org/html/rfc2046

OWL

OWL Web Ontology Language Guide M. K. Smith, C. Welty, D. L. McGuinness. W3C Recommendation 10 February 2004. This document is at http://www.w3.org/TR/2004/REC-owl-guide-20040210/

RFC2119

Key words for use in RFCs to Indicate Requirement Levels, S. Bradner. IETF, March 1997. This document is at http://www.ietf.org/rfc/rfc2119.

RFC2616

Hypertext Transfer Protocol -- HTTP/1.1, R. Fielding et al, June 1999. This document is at http://www.ietf.org/rfc/rfc2616.txt

XF

XForms 1.0 (Third Edition), W3C Recommendation 29 October 2007. J. Boyer. This document is at http://www.w3.org/TR/xforms/#serialize-urlencode

RFC3023

XML Media Types, M. Murata, S. St.Laurent, D. Kohn. RFC January 2001. This document is at http://www.ietf.org/rfc/rfc3023.txt

RFC3986

Uniform Resource Identifier (URI): Generic Syntax, T. Berners-Lee, R. Fielding, L. Masinter. RFC January 2005. This document is at http://www.ietf.org/rfc/rfc3986.txt

[XQXP]

XQuery 1.0 and XPath 2.0 Functions and Operators, A. Malhotra, J. Melton, N. Walsh. W3C Recommendation, 23 January 2007. This document is at http://www.w3.org/TR/xpath-functions/

6.2 Informative References

DC

Dublin Core Metadata Initiative

FOAF

FOAF Vocabulary Specification 0.9 Namespace Document 24 May 2007, D. Brickley, L. Miller. This document is at http://xmlns.com/foaf/spec/

HLINK

Link Relations and HTTP Header Linking M. Nottingham. Internet Draft 17, April 2009. This document is at http://tools.ietf.org/html/draft-nottingham-http-link-header-05

MOBILEOK

W3C mobileOK Scheme 1.0 J. Rabin. This document is at http://www.w3.org/TR/mobileOK/

MOBILEOK CHECKER

W3C mobileOK Basic Checker D. Hazaël-Massieux, F Daoust. This tool is at http://validator.w3.org/mobile/

PRIMER

Protocol for Web Description Resources (POWDER): Primer 2008, K. Scheppe. This document is at http://www.w3.org/TR/powder-primer/

RDFA

RDFa Primer 1.0 Embedding RDF in XHTML, B. Adida, M. Birbeck. This document is at http://www.w3.org/TR/xhtml-rdfa-primer/

SPARQL

SPARQL Query Language for RDF E. Prud'hommeaux, A. Seaborne. This document is at http://www.w3.org/TR/rdf-sparql-query/

TESTS

Protocol for Web Description Resources (POWDER): Test Suite 2008, A. Kukurikos. This document is at http://www.w3.org/TR/powder-test/

USECASES

POWDER: Use Cases and Requirements W3C Working Group Note 31 October 2007, P. Archer. This document is at http://www.w3.org/TR/powder-use-cases/

XGR

W3C Incubator Group Report P. Archer, J. Rabin, 20 February 2007. This document is at http://www.w3.org/2005/Incubator/wcl/XGR-wcl/

XMLSIG

XML-Signature Syntax and Processing, Donald. Eastlake, J. Reagle, D. Solo (Eds). W3C Recommendation 12 February 2002. This document is at http://www.w3.org/TR/xmldsig-core/

WCL-XG

W3C Content Label Incubator Group February 2006 - February 2007

WDR

Protocol for Web Description Resources (POWDER): Web Description Resources XML Schema (WDR), A. Perego, K. Smith. This document is at http://www.w3.org/2007/05/powder

WDRB

Protocol for Web Description Resources (POWDER): Web Description Resources "Base" XML Schema (WDRB), A. Perego, K. Smith. This document is at http://www.w3.org/2007/05/powder-base

WDRD

Protocol for Web Description Resources (POWDER): Web Description Resources Datatypes (WDRD), A Perego, K. Smith. This document is at http://www.w3.org/TR/powder-xsd

WDRS

Protocol for Web Description Resources (POWDER): POWDER-S Vocabulary (WDRS), P. Archer. This document is at http://www.w3.org/2007/05/powder-s

WOT

Web Of Trust RDF Ontology, D. Brickley. This document is at http://xmlns.com/wot/0.1/

WSDL

See, for example, Web Services Description Language (WSDL) Version 2.0 Part 0: Primer D. Booth, C. K. Liu, W3C Recommendation 26 June 2007. This document is at http://www.w3.org/TR/wsdl20-primer/

ITS

Internationalization Tag Set (ITS) Version 1.0. W3C Recommendation 03 April 2007, C. Lieske, F. Sasaki. This document is at http://www.w3.org/TR/its/.

ITS-Rules

POWDER Internationalization Tage Set Rules File This document is at http://www.w3.org/2007/powder/powder_itsrules.xml

7 Acknowledgements

The editors duly acknowledge the earlier work in this area carried out by the Web Content Label Incubator Activity, especially the contribution made by Jo Rabin. David Booth and Jeremy Carroll of HP contributed substantially to the operational/semantic model for POWDER, with further contributions from Dan Brickley and support from all members of the POWDER Working Group.

8 Change Log

8.1 Changes since Proposed Recommendation

• XML Schema namespace corrected
• Sentence added to clarify the Regular Expression Syntax used
• Sentence describing white space processing rules amended to refer to, and be consistent with, XML attribute value normalization.
• Change log reduced to refer only to changes since PR

Appendix A: Summary of POWDER Elements & Properties

A.1 XML Elements (in the wdr namespace)

A.2 RDF/OWL Classes and Properties (in the wdrs namespace)

Appendix B: POWDER Internet Media Type and Macintosh File Type

The following Media Type has been submitted to the IESG for review, approval, and registration with IANA

contact:

Phil Archer or Matt Womer

See also:

How to Register a Media Type for a W3C Specification

Internet Media Type registration, consistency of use
TAG Finding 3 June 2002 (Revised 4 September 2002)

The Internet Media Type / MIME Type for POWDER is "application/powder+xml".

It is recommended that POWDER files use the file extension of .xml (all lowercase) on all platforms.

It is recommended that POWDER files stored on Macintosh HFS file systems be given a file type of "TEXT".

Type name:

application

Subtype name:

powder+xml

Required parameters:

None

Optional parameters:

None

Encoding considerations:

Identical to those of "application/xml" as specified in [RFC3023], section 3.2.

Security considerations:

POWDER is used to make assertions, sometimes socially sensitive, about web resources. Consumers of POWDER should be aware of the source and chain of custody of this data. Security considerations for URIs (Section 7 of RFC 3986 [RFC3986]) and IRIs (Section 8 of RFC 3987 [RFC3987]) apply to the extent that describing resources in POWDER may prompt consumers to retrieve those resources.

Interoperability considerations:

There are no known interoperability issues.

Published specification:

http://www.w3.org/TR/powder-dr/

Applications which use this media type:

No known applications currently use this media type.

Additional information:

Magic number(s):

As specified for "application/xml" in RFC 3023 [RFC3023], section 3.2.

File extension(s):

.xml

Fragment identifiers:

Identical to that of "application/xml" as described in RFC 3023 [RFC3023], section 5.

Base URI:

As specified in RFC 3023 [RFC3023], section 6.

Macintosh file type code(s):

"TEXT"

Person & email address to contact for further information:

public-powderwg@w3.org

Intended usage:

COMMON

Restrictions on usage:

None

Author/Change controller:

The POWDER specification is a work product of the World Wide Web Consortium's Protocol for Web Description Resources (POWDER) Working Group. The W3C has change control over these specifications.

Appendix C: POWDER-S Internet Media Type and Macintosh File Type

The following Media Type has been submitted to the IESG for review, approval, and registration with IANA

contact:

Phil Archer or Matt Womer

See also:

How to Register a Media Type for a W3C Specification

Internet Media Type registration, consistency of use
TAG Finding 3 June 2002 (Revised 4 September 2002)

The Internet Media Type / MIME Type for POWDER-S is "application/powder-s+xml".

It is recommended that POWDER-S files use the file extension of .rdf (all lowercase) on all platforms.

It is recommended that POWDER-S files stored on Macintosh HFS file systems be given a file type of "TEXT".

Type name:

application

Subtype name:

powder-s+xml

Required parameters:

None

Optional parameters:

None

Encoding considerations:

Identical to those of "application/xml" as specified in [RFC3023], section 3.2.

Security considerations:

POWDER-S is used to make assertions, sometimes socially sensitive, about web resources. Consumers of POWDER should-S be aware of the source and chain of custody of this data. Security considerations for URIs (Section 7 of RFC 3986 [RFC3986]) and IRIs (Section 8 of RFC 3987 [RFC3987]) apply to the extent that describing resources in POWDER-S may prompt consumers to retrieve those resources.

Interoperability considerations:

There are no known interoperability issues.

Published specification:

http://www.w3.org/TR/powder-dr/

Applications which use this media type:

No known applications currently use this media type.

Additional information:

Magic number(s):

As specified for "application/xml" in RFC 3023 [RFC3023], section 3.2.

File extension(s):

.rdf

Fragment identifiers:

Identical to that of "application/xml" as described in RFC 3023 [RFC3023], section 5.

Base URI:

As specified in RFC 3023 [RFC3023], section 6.

Macintosh file type code(s):

"TEXT"

Person & email address to contact for further information:

public-powderwg@w3.org

Intended usage:

COMMON

Restrictions on usage:

None

Author/Change controller:

The POWDER-S specification is a work product of the World Wide Web Consortium's Protocol for Web Description Resources (POWDER) Working Group. The W3C has change control over these specifications.

Appendix D: describedby Link Relationship

The following Link Relationship has been submitted to IANA for review, approval, and inclusion in the Atom Link Relations registry

Attribute Value:

describedby

Description:

The relationship A 'describedby' B asserts that resource B provides a description of resource A. There are no constraints on the format or representation of either A or B, neither are there any further constraints on either resource.

Expected display characteristics

None

Security considerations:

Descriptions of resources may be socially sensitive, may require processing to be understood and may or may not not be accurate. Consumers of descriptive resources should be aware of the source and chain of custody of the data. Security considerations for URIs (Section 7 of RFC 3986) and IRIs (Section 8 of RFC 3987 ) apply to the extent that describing resources may affect consumers' decisions about how or whether to retrieve those resources.

Documentation:

http://www.w3.org/TR/powder-dr/#assoc-linking