CSE 428 Semantic Web Topics Introduction Jeff Heflin Lehigh University

The Semantic Web u “Semantic” + “Web” –Semantic: "of or relating to meaning in language" (Def. Webster’s) –Web: The World Wide Web u Official Definition –The Semantic Web is not a separate Web but an extension of the current one, in which information is given well-defined meaning, better enabling computers and people to work in cooperation. (Berners-Lee et al., Scientific American, May 2001)

Why Study Semantic Web? u Open source Semantic Web tools –from IBM, Hewlett-Packard, Nokia, etc. u Commercial software vendors –Oracle 11g RDBMS supports RDF and much of OWL –Adobe’s products use RDF to provide metadata for documents, photos –Semantic Web specific companies: TopQuadrant, Aduna Software, etc. u ~65 million Semantic Web documents (as of October 2009) –Yahoo SearchMonkey uses RDF to present richer search results –Google now indexes RDFa (a means for embedding RDF in web pages) u Semantic Web enabled sites –Data.gov: much of U.S. government’s open data is available in RDF –Newsweek: annotates articles with RDFa –BBC Music: exports RDF playlists, RDF for all artists –Harper’s Magazine: connects articles to events on a timeline –DBPedia: a Semantic Web version of Wikipedia

4 of 30 Semantic Web Standards u RDF(S) (1999, revised 2004) –essentially semantic networks with URIs –XML serialization syntax u OWL (2004) –extends RDF with more semantic primitives –based on description logics (DLs) –has a model theoretic semantics World Wide Web Consortium (W3C) Recommendations u:Chair John Smith rdf:type g:name g:Person g:name rdfs:Classrdf:Property rdf:type rdfs:subclassOf rdfs:domain A Band is a subset of the groups which only have Musicians as members

Semantic Web Layer Cake 5 of 30 From W3C (March 2007)

6 of 30 The End

7 of 30 Motivation u How to answer these queries? –find all bills dealing with “energy” that have been sponsored by Congressmen from states with populations less than 1 million –retrieve a list of all papers authored by NSF- funded individuals who are located in states in the Midwest –find a potential collaborator by identifying a friend-of-a-friend who has written an article on a related topic

8 of 30 The Semantic Web u Definition –The Semantic Web is not a separate Web but an extension of the current one, in which information is given well-defined meaning, better enabling computers and people to work in cooperation. (Berners-Lee et al., Scientific American, May 2001) u Ontology –a key component of the Semantic Web –ontologies define the semantics of the terms used in semi- structured web pages »identify context, provide shared definitions »has a formal syntax and unambiguous semantics –inference algorithms can compute what logically follows

9 of 30 A Web of Ontologies Foaf DBLP Congress Citeseer AIGPNSF Awards extends S3S3 S7S7 commits to The answer to a user’s query might require the combination of data from S 1, S2, S3, and S 4. Region S1S1 S2S2 Dublin Core S5S5 extends S4S4 S6S6 commits to extends

10 of 30 URIs and Namespaces u URI –Uniform Resource Identifier –includes URLs –but also anything that you can design an identification scheme for –helps to prevent collision of names –all the “symbols” in RDF are either URIs or Literals u Namespace –a mechanism for abbreviating URIs –by assigning a prefix for a URI fragment

11 of 30 Description Logic (DL) u form of knowledge representation –useful for formally defining classes –studied extensively in 1990’s –mature reasoning software »e.g., FaCT, RACER, Pellet u benefits –optimized computation of subsumption »calculate implicit subClassOf relations –ontology integration »if two ontologies are related via class expressions then subsumption can be used to compute an integrated ontology

12 of 30 OWL Class Constructors example taken from Ian Horrocks

13 of 30 OWL Axioms example taken from Ian Horrocks

14 of 30 Common Criticisms u Who will create all of the RDF/OWL data? u How do you integrate heterogeneous ontologies? u How can you handle spam / deceit / misinformation? u How can a system based on formal logic achieve Web scale?

15 of 30 Semantic Web Challenges u The Web is distributed –many sources, varying authority –inconsistency u The Web is dynamic –representational needs may change u The Web is enormous –systems must scale well u The Web is an open-world

16 of 30 Semantic Web Timeline Mar SHOE 0.90 (simple frames in HTML) Jan – SHOE 1.0 (frames + Horn logic) Feb – XML (semi- structured data for Web) Sep – Berners-Lee’s Semantic Web Roadmap Feb – RDF (semantic nets in XML) Mar – DAML+OIL (expressive DL in RDF) May 2001 – Berners-Lee et al. Scientific American article June – 1 st Int’l Semantic Web Conference Feb – OWL (W3C Rec.)