RDF: Building Block for the Semantic Web Jim Ellenberger UCCS CS5260 Spring 2011

2 RDF - Jim Ellenberger - May, 2011 Semantic web What is it? A phrase coined by Tim Berners-Lee, inventor of the WWW, in a 2001 Scientific American Article Berners-Lee and others have described it as a major component of “Web 3.0” Wikipedia defines it well: A “web of data” that enables machines to understand the semantics, or meaning, of information on the WWW Extends the network of hyperlinked human-readable web pages by inserting machine-readable metadata Enables automated agents to access the Web more intelligently and perform tasks on behalf of users

3 RDF - Jim Ellenberger - May, 2011 Why do we need it? Traditional web technologies like HTML are focused on organizing, presenting and linking documents Can’t directly access the meaning of information on the Web Can’t provide consistent methods to aggregate and query information on the Web Semantic web technologies provide these missing components Information can be stored, aggregated and queried based on its meaning All of this can be automated, because the information is available in machine-readable formats

4 RDF - Jim Ellenberger - May, 2011 How is the semantic web implemented? There is a need to encode and manipulate knowledge on the web, but how can it be done? Technologies that describe and manipulate information based on meanings and relationships Resource Description Framework (RDF) Data interchange formats (RDF/XML, N3, Turtle, N-Triples) Notations (RDFS, OWL) Query languages (SPARQL) My focus: RDF Essentially, the building block for all semantic web technologies Originally specified W3C as a metadata language; it was extended to accommodate semantic web concepts See

5 RDF - Jim Ellenberger - May, 2011 RDF: general structure RDF is graph-based Not hierarchical like XML and other data description formats Single pieces of information are graph nodes and the relationships between them are graph edges Advantages of graph-based model Virtually any kind and number of relationships can be represented - no need to adhere to a hierarchy Diverse graphs can be combined as simply as defining a relationship between two nodes - no need for graphs to have compatible hieracrchies

6 RDF - Jim Ellenberger - May, 2011 RDF statements The basic unit of information in RDF is a statement or triple with three components Subject – thing the statement is about Predicate or property – a property or characteristic of the subject Object – the value of the property or characteristic Example, a statement about a camera: The D300 – subject of the statement is manufactured by – predicate Nikon – object of the predicate This triple encodes a single piece of information: The D300 is manufactured by Nikon

7 RDF - Jim Ellenberger - May, 2011 RDF URIs Subjects and objects that make up RDF statements are called resources In order to be useful web wide, resources and the predicates that link them need identifiers that are: Unique – to avoid confusion Universally accessible – to make useable web wide These identifiers are called URIs - Uniform Resource Identifiers The camera example in URIs: - subject - predicate - object

8 RDF - Jim Ellenberger - May, 2011 More abut URIs URIs are not URLs (but URLs are URIs) URLs represent things retrievable from the web URIs represent things identified on the web, which may or may not be retrievable Where do URIs come from? Use an existing URI if an appropriate one exists: If one doesn’t exist, make your own: If you create your own, it must be universally accessible and must return data to RDF clients

9 RDF - Jim Ellenberger - May, 2011 Camera example in graph form D300 Nikon ured_by

10 RDF - Jim Ellenberger - May, 2011 Camera example linked to other graphs D300 Nikon ured_by [URL: Review] [URL of Stock Price] [URL: review_of] [URL: stock_price_of]

11 RDF - Jim Ellenberger - May, 2011 What Does RDF Look Like in the Wild? RDF statements need to be serialized to be used on the WWW and processed by machines There are many formats used for this: RDF/XML Turtle N3 RDFa RDF/XML is probably the most common

12 RDF - Jim Ellenberger - May, 2011 RDF/XML Example RDF is not XML, but it can be encoded in XML The camera example, in RDF/XML: <rdf:RDF xmlns:rdf= xmlns:mypage=" XML Tags rdf:RDF - begin RDF document rdf:Description – begin description of subject(s) rdf:about – URI for the subject mypage:manufactured_by – the predicate rdf:resource – URI for the object

13 RDF - Jim Ellenberger - May, 2011 A real world example: OpenCalais OpenCalais is a web service that automatically generates semantic metadata in RDF/XML from text submitted to it This is a portion of OpenCalais’ output when “D300” is submitted: <rdf:RDF xmlns:rdf= xmlns:c=" Nikon D300 Digital Camera Essentially, the edited RDF code contains the triple electronics product (subject) name (predicate) Nikon D300 Digital Camera (object)

14 RDF - Jim Ellenberger - May, 2011 What else is happening? DBPedia project Publishes Wikipedia information in semantic web formats FOAF - Friend of a Friend project Uses RDF to describe relationships among people OpenPSI project Publishes UK government data in semantic web formats GoodRelations vocabulary A means to publish product info in semantic web formats

15 RDF - Jim Ellenberger - May, 2011 Important Issues The amount of information that could be encoded is staggering Encoding meaning isn’t always straightforward -- e.g., what does “young” mean? Not everyone wants their information freely available Information can be a commodity Information can be a trade secret Accuracy -- how do we deal with information that is inaccurate or deceptive Performance -- how will semantic web data stores perform compared to more traditional datasets?

16 RDF - Jim Ellenberger - May, 2011 Conclusion There is quite a bit more to RDF RDF has more capabilities than described here RDF has been expanded with other technologies to create still more capabilities There are also many related areas to explore How can RDF data be created? How can it be stored? How can it be served and retrieved? Once we retrieve RDF data, what should we do with it?