The RDFa DOM API

Table of Contents

• 1. Introduction
• 2. The RDFa DOM API
• 3. RDF Resource
  • 3.1 Methods
• 4. RDF Literal
  • 4.1 Attributes
• 5. RDF Node
• 6. URI Reference
  • 6.1 Attributes
• 7. BlankNode
  • 7.1 Attributes
• 8. RDFTriple
  • 8.1 Attributes
  • 8.2 Methods
• 9. RDF Triple List
  • 9.1 Attributes
  • 9.2 Methods
• 10. Iterating through RDF Triples
  • 10.1 Attributes
  • 10.2 Methods
• 11. Filtering RDF Triples
  • 11.1 Methods
• 12. Querying and Testing DOM nodes for RDF Triples
  • 12.1 Attributes
  • 12.2 Methods
  • 12.3 Methods
• A. Acknowledgements
• B. References
  • B.1 Normative references
  • B.2 Informative references

1. Introduction

RDFa [RDFA-SYNTAX] has seen substantial growth since it became an official W3C Recommendation in October 2008. It has seen wide adoption among search companies, e-commerce sites, governments, and content management systems. There are numerous interoperable implementations and growth is expected to continue to rise.

In an effort to ensure that browser-based applications are able to fully utilize RDFa, this specification outlines a set of routines that are capable of extracting RDFa from web pages for use in Javascript applications, as well as browser extensions. The RDFa DOM API allows a programmer to query the RDFa data expressed by a document and then make programmatic decisions based on the semantic data embedded in the page.

2. The RDFa DOM API

The RDFa DOM API strives to be a simple set of calls that a developer may use to retrieve triples contained in the document. It is an extension of the existing DOM specification [DOM-LEVEL-1]. The key design issue of the RDFa DOM API is built on using filters for retrieving RDF triples from DOM nodes or retrieving DOM nodes with RDFa content. Therefore it first defines a set of basic types. These types (i.e., RDF Resource, RDF Literal, RDF Node, URI Reference, and Blank node) extend the DOM API in order to program with RDF triples. For retrieving RDF triples from DOM nodes, the RDFa DOM API makes use of so called RDFTripleFilters. Inspired by NodeFilters in the DOM specification, RDFTripleFilters can be user defined and provide a high flexibility in filtering RDF triples for application programmers. For conveinience reasons, the RDFa DOM API provides a set of predefined RDFTripleFilters (i.e.,RDF_OBJECT_IS_LITERAL, RDF_OBJECT_IS_URI, RDF_PREDICATE_IS_TYPE) that cover the main use cases in retrieving RDF from RDFa. In order to retrieve DOM nodes that contain certain RDFa content, the RDFa DOM API extended the w3c:dom:Node interface with a lookup method called containsRDFa(). An RDFTripleFilters can be passed as optional parameter in order to test DOM nodes for certain RDF triples.

The Hierarchy of Basic Datatypes

The basic types of the RDFa DOM API are DOMString, and a hierarchy starting with RDF Resource with its descendants: RDF Literal, RDF Node, URI Reference, and Blank Node.

The hierarchy is defined as follows:

• RDF Resource
  • RDF Literal
  • RDF Node
    • URI Reference
    • Blank Node

3. RDF Resource

interface RDFResource {
    RDFLiteral createLiteral (in DOMString value, in optional DOMString language);

    RDFLiteral createLiteral (in DOMString value, in URI type);

    URI        createURI (in DOMString value);
    URI        createURI (in DOMString namespace, in DOMString suffix);

    BlankNode  createBlankNode (in DOMString id);
};

4. RDF Literal

An RDF Literal represents lexical values being defined in RDFa data. In RDF, literals may be attached with language information about the given text given as language tag, or a datatype given as URI reference. The language tag is an identifier for a certain language as defined by [RFC-3066]. The datatype's URI reference defines the datatype of the text value, e.g., xsd:DataTime or xsd:boolean. According to the RDF specification, for a given RDF Literal, either language or type information can be given. If the type is set, the RDF Literal conforms to the RDF specification of Typed Literal in [RDF-CONCEPTS]. Otherwise it conforms to be a Plain Literal.

interface RDFLiteral : RDFResource {

    readonly attribute DOMString value;
    readonly attribute DOMString language;
    readonly attribute URI       type;

};

5. RDF Node

interface RDFNode : RDFResource {
};

6. URI Reference

interface URI : RDFNode {
    readonly attribute DOMString value;

    readonly attribute DOMString namespace;
};

7. BlankNode

interface BlankNode : RDFNode {
    readonly attribute DOMString id;
};

8. RDFTriple

In the RDFa DOM API, RDFTriple defines the data structure to represent an RDF triples as specified in [RDF-CONCEPTS]. The RDFa DOM API is intended to extract RDFTriple objects from the source document.

interface RDFTriple {
    readonly attribute RDFNode     subject;

    readonly attribute URI         predicate;
    readonly attribute RDFResource object;
    readonly attribute URI         context;

    RDFTriple createTriple (in RDFNode? subject, in URI? predicate, in RDFResource? object);

};

9. RDF Triple List

The RDFTripleList is a plain list of RDFTriple objects.

interface RDFTripleList {
    readonly attribute unsigned long length;

    RDFTriple get (in unsigned long index);
};

10. Iterating through RDF Triples

The RDFTripleIterator is an Iterator through RDFa content inside Nodes of the DOM tree.

interface RDFTripleIterator {
    readonly attribute Node            root;

    readonly attribute RDFTripleFilter filter;
    RDFTriple         previousRDFTriple ();
    RDFTriple         nextRDFTriple ();

    RDFTripleIterator createRDFTripleIterator (in Node root, in RDFTripleFilter filter);

};

11. Filtering RDF Triples

The RDFa DOM API provides this RDF filter for testing RDFTriple for certain conditions. These conditions can be implemented by application developers. An RDFTripleFilter may be created with a default RDF triple used as dynamic filter pattern. E.g., if an RDFTripleFilter is specified to filter all RDF triples with rdfs:label as predicate, the developer might want to use this filter to search for labels of instances of rdf:type foaf:Person and foaf:Organisation. Therefore he creates the RDF triple patterns (null, rdf:type, foaf:Person) and (null, rdf:type, foaf:Organisation) and passes them to his predefined filter instead of writing additonal two filters. The RDFa DOM API predefines a bunch of static RDFTripleFilter objects for common RDF queries:

• RDF_PREDICATE_IS_TYPE
• RDF_OBJECT_IS_LITERAL
• RDF_OBJECT_IS_URI

interface RDFTripleFilter {
    boolean acceptRDFTriple (in RDFTriple triple, in optional RDFTriple pattern);

};

12. Querying and Testing DOM nodes for RDF Triples

This is an extension of w3c:org:dom::Document. It adds one function to retrieve RDF data from web pages.

interface Document {
    readonly attribute RDFTripleFilter RDF_PREDICATE_IS_TYPE;

    readonly attribute RDFTripleFilter RDF_OBJECT_IS_LITERAL;
    readonly attribute RDFTripleFilter RDF_OBJECT_IS_URI;
    RDFTripleList getRDFTriples (in optional RDFTripleFilter filter);

};

This is an extension of DOM::Node specification. It add two methods, one for testing if RDFa content can be extracted from a Node, the other for extracting this content as RDFTripleList.

interface Node {
    boolean containsRDFa (in optional RDFTripleFilter filter);
};