Transcript semanticWeb
cs236607 1 The Need “Most of the Web's content today is designed for humans to read, not for computer programs to manipulate meaningfully.” Berners-Lee, T, Hendler, J & Lassila, O ‘The semantic web’, Scientific American, May 2001 cs236607 2 Semantic Processing We want to be able to pose complex search tasks that use the semantics of pieces of information, e.g. I want to purchase a DVD of “Dore the Explorer” at a price lower than 10$. Is such a CD available at amazon.com? cs236607 3 Current search agents are not suitable for such task cs236607 4 Current Solution Use “intelligent” agents The Semantic-Web Approach Content is machine-understandable by being bound to some formal description of itself (i.e. metadata) cs236607 5 Goals Web of data - provides common data representation framework to facilitate integrating multiple sources to draw new conclusions Increase the utility of information by connecting it to its definitions and to its context More efficient information access and analysis cs236607 6 Applications Agents searching Web and retrieving valuable information to the end user Web services publishing their information Programs running to merge data of different web services and create new results from them cs236607 7 Ontologies & Inference Engines “For the semantic web to function, computers must have access to structured collections of information and sets of inference rules that they can use to conduct automated reasoning.” Berners-Lee, T, Hendler, J & Lassila, O ‘The semantic web’, Scientific American, May 2001 cs236607 8 The Four Building Blocks 1. 2. 3. 4. XML RDF Ontologies Agents cs236607 9 XML “XML allows users to add arbitrary structure to their documents but says nothing about what the structures mean” cs236607 10 RDF –Resource Description Framework Meaning encoded in sets of ‘triples’: entities have properties which have values Entities, properties and values all have distinct URIs “imagine that we have access to a variety of databases with information about people, including their addresses. If we want to find people living in a specific zip code, we need to know which fields in each database represent names and which represent zip codes. RDF can specify that "(field 5 in database A) (is a field of type) (zip code)," using URIs rather than phrases for each term.” Berners-Lee, T, Hendler, J & Lassila, O ‘The semantic web’, Scientific American, May 2001a cs236607 11 Ontologies Database A and Database B may use different fields to contain ‘zip code’ Ontologies sort this out Ontology = ‘a document or file that formally defines the relations among terms’ Ontologies for the web normally have A taxonomy A set of inference rules cs236607 12 Agents “Agent based computing appears to be the appropriate paradigm to work in a complex world with multiple ontologies, fragments and multiple inferencing engines.” Stork, Hans-Georg and Mastroddi, Franco, Semantic Web Technologies - a New Action Line in the European Commission’s IST Programme, 2001 cs236607 13 The Power of Agents - Integration “The real power of the Semantic Web will be realized when people create many programs that collect Web content from diverse sources, process the information and exchange the results with other programs. The effectiveness of such software agents will increase exponentially as more machine-readable Web content and automated services (including other agents) become available.” Berners-Lee, T, Hendler, J & Lassila, O ‘The semantic web’, Scientific American, May 2001 cs236607 14 ‘Ambient Intelligence’ “In the next step, the Semantic Web will break out of the virtual realm and extend into our physical world. URIs can point to anything, including physical entities, which means we can use the RDF language to describe devices such as cell phones and TVs.” Berners-Lee, T, Hendler, J & Lassila, O ‘The semantic web’, Scientific American, May 2001 cs236607 15 cs236607 16 What is RDF? A part of the semantic-Web activity RDF is a general-purpose language for representing information on the Web Specifically, objects and relationships Designed to allow computer applications to process data based on its semantics Rather than displaying data to humans (as opposed to RSS) An RDF document is actually a labeled graph that is represented in XML The specific language is called RDF/XML W3C recommendation (Feb. 2004) 17 RDF Data The basic element: Triple (labeled edge) Statement Subject RDF document: edgelabeled graph predicate Object Person#845 address #1002 street Herzel city Haifa postalCode 6941 18 The XML Syntax of RDF John Smith page.html John’s Home Page <?xml version=“1.0”?> <rdf:RDF xmlns:rdf=“http://www.w3.org/TR/WD-rdf-syntax#” xmlns:dc=“http://purl.org/metadata/dublin_core#”> <rdf:Description about=“page.html”> <dc:Creator>John Smith</dc:Creator> <dc:Title>John’s Home Page</dc:Title> </rdf:Description> </rdf:RDF> 19 Structured Values page.html dc:Title John’s Home Page dc:Creator Name John Smith Email [email protected] ... <Description about=“page.html”> <dc:Creator> <Description> <corp:Name>John Smith</corp:Name> <corp:Email>[email protected]</corp:Email> </Description> </dc:Creator> <dc:Title>John’s Home Page</dc:Title> </Description> </RDF> 20 Dublin Core A set of fifteen basic properties for describing generalized Web resources The “obvious” mapping of Dublin Core properties into RDF properties has not yet been approved by the Dublin Core initiative, but is generally a good example 21 Dublin Core “Title”: the name given to the resource “Creator”: the person or organization primarily responsible for the resource “Subject”: what the resource is about “Description”: a description of the content “Publisher”: the person or organization responsible for making the resource available “Contributor”: someone who has provided content to the resource other than the creator “Date”: date of creation or publication 22 Dublin Core “Type”: type of resource, such as home page, technical report, novel, photograph… “Format”: data format of the resource “Identifier”: URL, ISBN number, … “Source”: another resource that this resource is derived from “Language”: the language of the content “Relation”: another resource and its relationship to this one “Coverage”: the portion of time or space described by this resource (atlases, histories, etc.) “Rights”: the intellectual property rights adhering to this resource, or a pointer to them 23 Advanced RDF Containers: bags, sequences, alternatives aboutEach, aboutEachPrefix Reification (higher order statements) Namespaces and Vocabularies 24 Creating RDF documents Manually from HTML or “user domain XML” With special assisting tools – like Protégé, Reggie, DC- dot, RDF for XML Ideally – with some automated procedure from HTML/XML documents Can we use XSLT there? 25 cs236607 26 RDF Schema Not an XML Schema! A “companion” specification for RDF spec Class, Type, subClassOf, domain, range Misc: label, comment, isDefinedBy,etc. 27 Example <?xml version="1.0"?> <rdf:RDF xmlns:rdf= "http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#" xml:base= "http://www.animals.fake/animals#"> <rdfs:Class rdf:ID="animal" /> <rdfs:Class rdf:ID="horse"> <rdfs:subClassOf rdf:resource="#animal"/> </rdfs:Class> </rdf:RDF> cs236607 28 cs236607 29 RDF Schema is Limited We cannot express facts such as Two classes are disjoint Build a class that is the union of two classes Cardinality restriction Scope of properties Provide relationships between properties, such as transitive, unique, inverse cs236607 30 OWL A Web ontology language that is more expressive than RDF and RDF Schema Written in XML on top of RDF Using OWL we want to provide exact descriptions of items and the relationships between them Basically, built upon Description Logics cs236607 31