Harvesting RDF Statements from XLinks

1.1 Terminology

[Definition: The key words must, must not , required, shall, shall not , should, should not , recommended, may, and optional in this specification are to be interpreted as described in [IETF RFC 2119]. ]

Some special terms are defined here in order to clarify their relationship to similar terms used in the technologies on which the mapping is based. Refer to [XLink] and [RDF] for definitions of other technical terms used here.

[Definition: harvesting]

The process of generating RDF statements from XLink elements.

[Definition: Resource]

A "resource" is anything identified by a URI.

[Definition: Participating resource]

A resource that has been identified in a link to serve as a potential starting or ending point of traversal.

1.2 Notation and Document Conventions

The xlink: and rdf: prefixes are used throughout to stand for the declaration of the XLink and RDF namespaces, respectively, on elements in whose scope the so-marked element or attribute appears (on the same element or on some ancestor element), whether or not a namespace declaration is present in the example. The use of specific namespace prefixes is an editorial convienience; as dictated by the Names in XML Recommendation [XML-Names], any prefix may be used as long as the URI it maps to is the correct one.

3.1 Synthesizing XPointers

RDF is based on the use of URIs for identifying resources. In XLink, the linking element itself (in the case of a simple link) or a subelement of the linking element (in the case of an extended link) often serves as one of the participating resources in the link. This requires that we be able to define URI references that identify those linking elements. Those URI references must follow the XPointer specification.

Any legal XPointer that identifies the proper element is allowed. However, in order that different implementations harvest equivalent RDF statements from an XLink, the procedure in this section should be used when synthesizing XPointers for such linking elements. The general approach recommended is for the synthesized XPointer to do element-wise navigation down the tree to reach the linking element. The navigation begins at the nearest identified point in the tree.

More formally, the base of the synthesized URI reference shall be specified as defined in [XML Base].

The fragment identifier of the synthesized URI reference shall be delimited from the URI by the '#' character, as required by RFC 2396[RFC 2396]. The fragment identifier of the synthesized URI reference shall be an XPointer[XPTR].

The XPointer should follow the production:

Recommended Syntax for Synthesized XPointers

     XPointer    ::= Name
                   | ChildSeq

     ChildSeq    ::=  '/' [1-9] [0-9]* ('/' [1-9] [0-9]*)*
                   |  Name ('/' [1-9] [0-9]*)+ 

The initial locator term of the XPointer should be an ID reference to the nearest ancestor of the linking element, including the linking element itself, that bears an attribute of type ID. If no such attribute exists on any ancestor of the linking element, the '/' character should be the first linking term, indicating that navigation proceeds from the document element.

As an example, consider a document that contains the following simple link:

In heavy trading, <org
  xlink:type='simple'
  xlink:href="http://www.foo.com/"
  xml:base="http://www.bar.com/report1"
  ID="com231"
>Foo Manufacturing</org> closed sharply lower...

The synthesized XPointer for this linking element is:

http://www.bar.com/report1#com231

3.2 Generating RDF Predicates

Unless stated otherwise, RDF statements are generated to represent the information for the arcs in a link. The value of the xlink:arcrole attribute, if one is given on an "arc"-type element or "simple"-type element, must be mapped to the predicate of the RDF statement. Note that the value of the xlink:arcrole attribute is already required, by the XLink specification, to be a URI reference.

If no xlink:arcrole attribute is specified, harvesting software should not generate an RDF statement. That is certainly the safest course. However, many XML files may not have arcrole attributes. Implementations that wish to attempt to harvest RDF statements from such files may map the element type of the linking element to the predicate of the RDF statement, as long as the element type is namespace qualified. This ensures that an an absolute URI reference can be constructed from the namespace URI and the local part. In this case the namespace name and the local part are concatenated to synthesize the absolute URI reference for the predicate. Implementations should examine the namespace URI to test if it ends in one of the URI characters '#', '?', or '/'. If it does then the namespace URI and the local part shall be concatenated using the simple approach documented in [RDF]. If the namespace URI does not end in such a character, implementations may create a URI reference by inserting a '#' character. Note however that such URI references must not be exchanged with external parties, as they are not guaranteed to actually exist.

3.3 Simple Linking Elements

If a simple link's xlink:arcrole attribute has the value "http://www.w3.org/1999/xlink/properties/linkbase", the link shall be harvested according to section 3.5 Linkbases. Otherwise the mapping defined in this section shall be used.

All simple links define zero or one traversal arcs. No traversal arc is specified if the xlink:href attribute is not specified. Therefore, harvesting software shall not generate an RDF statements if there is no xlink:href attribute in the link.

The starting resource of the simple link shall be mapped to the subject of the RDF statement. Note that the starting resource of a simple link is the linking element itself. Therefore, the harvesting software must create a URI reference that identifies the linking element, as defined in section 3.1 Synthesizing XPointers.

The predicate of the RDF statement is obtained from the simple Link as defined in 3.2 Generating RDF Predicates.

The ending resource of the simple link shall be mapped to the object of the RDF statement. Note that the ending resource of a simple link is always a URI reference, provided as the value of the xlink:href attribute.

If an xlink:role attribute is specified on the simple link, it shall result in an additional statement being added to the model. The subject of the statement is the ending resource of the simple link, its predicate is "rdf:type", and its object is the resource identified by the role attribute.

If an implementation wishes to use facilities defined in the RDF Schema specification [RDFSchema], it may add a second statement to the RDF model when an xlink:role attribute is specified. The subject of the second statement is the resource identified by the role attribute, its predicate is "rdf:type", and its object is the resource "rdfs:Class". The second statement should only be added to the model if an equivalent statement is not already part of the model.

An example of a simple linking element is:

... In a <x:extRef
  xlink:type="simple"
  xlink:href="http://www.foo.com/papers/crops.txt"
  xlink:arcrole="http://links.org/namespace/cite"
  xlink:role="http://links.org/namespace/screed"
>recent paper</x:extRef>, Dr. Taylor assumes that ...

Mapping that link according to this specification (and assuming it was the fourth child element within the third child element of the document) results in the RDF model shown below:

If the xlink:role attribute had not been specified, then the result would have been the RDF model shown below:

3.4 Extended XML Links

We first describe the rules for harvesting the components of an extended link (arcs, locators, and resources). Then we describe the rules for the extended link as a whole.

3.4.4 title-Type Element

XLink title-type elements have an XLink-defined meaning only if they appear as a child element within an extended, locator, or resource element.

If an XLink extended-, locator-, or resource-type element contains one or more title-type elements, one RDF statement shall be added to the model for each title element. The subject of the statement shall be either the value of the xlink:href attribute (in the case of a locator element) or a synthesized XPointer identifying the extended or resource element. The predicate of the statement shall be xlink:title. The object of the statement shall be a synthesized XPointer identifying the title element. (Identifying the title element, rather than just its content, allows attributes such as xml:lang to be captured along with the title.)

Note:

Implementations may add a second RDF statement to the model for each title-type element. The object of the second statement shall be a synthesized XPointer identifying the title element. The predicate of the second statement shall be rdf:value. The object of the second statement shall be the content of the title element. (If the title element contains mixed content, the object is a string containing XML markup. The implementation's facilities for dealing with situations where the rdf:parseType attribute has the value "literal" will be needed.)

As an example, consider the following fragment of an extended link:

<annotation xlink:type='extended' ID='genid22'> <caption xlink:type='title' ID='genid23'><i>Recent</i> comments</caption> <link xlink:type='arc' ...

The RDF statements harvested from the title are shown below:

Recent</i>">

3.5 Linkbases

A linkbase is an XML document which functions like a database of links. A linkbase arc is an XLink element (simple- or arc-type) whose xlink:arcrole attribute takes the value of "http://www.w3.org/1999/xlink/properties/linkbase". The ending resource of a linkbase arc is a linkbase.

When harvesting software encounters a linkbase arc, it shall not generate an RDF statement for the arc. It should traverse the arc to retrieve the linkbase, and harvest the links from the linkbase to add to the current model using the methods specified in this Note.