Introduction to theSemantic Web.ppt
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1、Introduction to the Semantic Web,Questions,What is the Semantic Web? Why do we want it? How will we do it? Who will do it? When will it be done?,“XML is Lisps bastard nephew, with uglier syntax and no semantics. Yet XML is poised to enable the creation of a Web of data that dwarfs anything since the
2、 Library at Alexandria.”- Philip Wadler, Et tu XML? The fall of the relational empire, VLDB, Rome, September 2001.,“The web has made people smarter. We need to understand how to use it to make machines smarter, too.”- Michael I. Jordan, paraphrased from a talk at AAAI, July 2002 by Michael Jordan (U
3、C Berkeley),“The Semantic Web will globalize KR, just as the WWW globalize hypertext” - Tim Berners-Lee,“The multi-agent systems paradigm and the web both emerged around 1990. One has succeeded beyond imagination and the other has not yet made it out of the lab.”- Anonymous, 2001,IOHO,The web is lik
4、e a universal acid, eating through and consuming everything it touches. Web principles and technologies are equally good for wireless/pervasive computing The semantic web is our first serious attempt to provide semantics for XML sublanguages It will provide mechanisms for people and machines (agents
5、, programs, web services) to come together. In all kinds of networked environments: wired, wireless, ad hoc, wearable, etc.,Origins,Tim Berners-Lees original 1989 WWW proposal described a web of relationships among named objects unifying many info. management tasks. Capsule historyGuhas MCF (94) XML
6、+MCF=RDF (96)RDF+OO=RDFS (99)RDFS+KR=DAML+OIL (00)W3Cs SW activity (01)W3Cs OWL (03),http:/www.w3.org/History/1989/proposal.html,W3Cs Semantic Web Goals,Focus on machine consumption: “The Semantic Web is an extension of the current web in which information is given well-defined meaning, better enabl
7、ing computers and people to work in cooperation.“ - Berners-Lee, Hendler and Lassila, The Semantic Web, Scientific American, 2001,TBLs semantic web vision,Semantic web stack 2006,Why is this hard?,after Frank van Harmelen and Jim Hendler,What a web page looks like to a machine,after Frank van Harmel
8、en and Jim Hendler,OK, so HTML is not helpful,Maybe we can tell the machine what the different parts of the text represent?,title,time,speaker,location,abstract,biosketch,host,XML to the rescue?,XML fans propose creating a XML tag set to use for each application.For talks, we can choose , , etc.,aft
9、er Frank van Harmelen and Jim Hendler,XML machine accessible meaning,But, to your machine, the tags still look like this.The tag names carry no meaning.XML DTDs and Schemas have little or no semantics.,after Frank van Harmelen and Jim Hendler,XML Schema helps,XML Schemas provide a simple mechanism t
10、o define shared vocabularies.,XML Schema file,after Frank van Harmelen and Jim Hendler,XML Schema file 42,XML Schema file 1,But there are many schemas,after Frank van Harmelen and Jim Hendler,Theres no way to relate schema,Either manually or automatically. XML Schema is weak on semantics.,An Ontolog
11、y level is needed,We need a way to define ontologies in XML So we can relate them So machines can understand (to some degree) their meaning,XML Ontology 256,imports,imports,=,Ontologies add Structure Constraints mappings,Today and tomorrow,Simple ontologies like FOAF & DC in use today Weve crawled m
12、ore than 3M FOAF RDF files We hope to be able to make effective use ontologies like Cyc in the coming decade There are skeptics Its a great research topic The SW community has a roadmap and some experimental languages Industry is still holding back They are being conservative We need more experiment
13、ation and exploration,Semantic Web Languages,Semantic web languages today,Today there are, IOHO, two semantic web languages RDF Resource Description Framework http:/www.w3.org/RDF/ DAML+OIL Darpa Agent Markup Language http:/www.daml.org/ (deprecated) OWL Ontology Web Language http:/www.w3.org/2001/s
14、w/ Topic maps (http:/topicmaps.org/) are another species, not based on RDF Microformats, Common Logic, etc. offer other possibilities,Two Semantic Web Notions,The semantic web The idea of a web of machine understandable information Agnostic about the technology used to support it May involve more AI
15、 (e.g., NLP) Human end users in the center The Semantic Web The current vision of a semantic web as defined by the W3C community: a web of data Using W3C supported standards (i.e., RDF, OWL, SPARQL, XML, RIF, etc. By machines for machines with human oriented applications on top.,RDF is the first SW
16、language,XML Encoding,Graph,stmt(docInst, rdf_type, Document) stmt(personInst, rdf_type, Person) stmt(inroomInst, rdf_type, InRoom) stmt(personInst, holding, docInst) stmt(inroomInst, person, personInst),Triples,RDF Data Model,Good for Machine Processing,Good For Human Viewing,Good For Reasoning,RDF
17、 is a simple language for building graph based representations,The RDF Data Model,An RDF document is an unordered collection of statements, each with a subject, predicate and object (aka triples) A triple can be thought of as a labelled arc in a graph Statements describe properties of web resources
18、A resource is any object that can be pointed to by a URI: a document, a picture, a paragraph on the Web, E.g., http:/umbc.edu/finin/cv.html a book in the library, a real person (?) isbn:/5031-4444-3333 Properties themselves are also resources (URIs),URIs are a foundation,URI = Uniform Resource Ident
19、ifier “The generic set of all names/addresses that are short strings that refer to resources“ URLs (Uniform Resource Locators) are a subset of URIs, used for resources that can be accessed on the web URIs look like “normal” URLs, often with fragment identifiers to point to a document part: http:/ UR
20、Is are unambiguous, unlike natural language terms the web provides a global namespace We assume references to the same URI are to the same thing,What does a URI mean?,Sometimes URIs denote a web resource http:/umbc.edu/finin/finin.jpg denotes a file We can use RDF to make assertions about the resour
21、ce, e.g., its an image and depicts a person with name Tim Finin, Sometimes concepts in the external world E.g., http:/umbc.edu/ denotes a particular University located in Baltimore This is done by social convention Cool URIs dont change http:/www.w3.org/Provider/Style/URI,The RDF Graph,An RDF docume
22、nt is an unordered collection of triples The subject of one triple can be the object of another So the result is a directed, labelled graph A triples object can also be a literal, e.g., a string.,Simple RDF Example,http:/umbc.edu/ finin/talks/idm02/,“Intelligent Information Systems on the Web and in
23、 the Aether”,http:/umbc.edu/,dc:Title,dc:Creator,bib:Aff,“Tim Finin”,“fininumbc.edu”,bib:name,bib:email,XML encoding for RDF,Intelligent Information Systems on the Web and in the AetherTim Fininfininumbc.edu,N triple representation,RDF can be encoded as a set of triples. .“Intelligent Information Sy
24、stems on the Web and in the Aether“ . _:j10949 “Tim Finin“ . _:j10949 “fininumbc.edu“ . _:j10949 . _:j10949 ._:j10949 .Note the gensym for the anonymous node _:j10949,Triple Notes,RDF triples have one of two forms:Triples are also easily mapped into logicbecoming: (,) With type(,) becoming () Exampl
25、e: subclass(man,person) sex(man,male) domain(sex,animal) man(adam) age(adam,100) Triples are easily stored and managed in DBMS Flat nature of a triple a good match for relational DBs,; Note: were not ; showing the actual ; URIs for clarity,N3 notation for RDF,N3 is a compact notation for RDF that is
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