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Semantic web

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Semantic Web

Semantic Web

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    Semantic web Semantic web Presentation Transcript

    • Semantic Web
    • Overview What is Semantic Web Semantic Web Vision Semantic Web Layers RDF, RDFS, OWL Tools GATE Applications
    • What is Semantic Web? Semantic means that the meaning of data can be discovered by computers "The Semantic Web is an extension of the current web in which information is given well-defined meaning, better enabling computers and people to work in cooperation." - Tim Berners-Lee
    • Definition The Semantic Web is a project to create a universal medium for information exchange by putting documents with computer-processable meaning (semantic) on the World Wide Web The Semantic Web extends the Web through the use of standards, markup languages and related processing tools
    • The aims of Semantic Web Indexing and retrieving information Annotation The Web as a interoperable database Machine retrieval of data Web based services Discovery of services Intelligent software agents
    • Semantic Web Vision Oriented toward machine-readable resources rather than human-readable Requires resources to be described so that machines know what they mean  Description in terms of metadata Use of logic interpretation for inference
    • Semantic Web Layers
    • Semantic Web Layers XML (Extensible Markup Language)- The language framework that is used to define nearly all new languages that are used to interchange data over the Web XML Schema -A language used to define the structure of specific XML language
    • Semantic Web Layers RDF (Resource Description Framework)- a language used to describe all sort of information and meta data RDF Schema-A framework that provides a means to specify basic vocabularies for specific RDF application language to use
    • Semantic Web Layers Ontology- defines vocabularies and establish the usage of words and terms in context of specific vocabulary Logic and Proof –is used to establish the consistency and correctness of data sets and to infer conclusion that aren’t explicitly stated
    • Semantic Web agents Metadata will be used to identify and extract information from Web sources. Ontologies will be used to assist in Web searches, to interpret retrieved information, and to communicate with other agents. Logic will be used for processing retrieved information and for drawing conclusions.
    • RDF
    • RDF• “Resource Description Framework”• RDF is a data model • Originally for describing metadata for web pages • Structured information • Universal, machine-readable data exchange model • Syntax uses XML for serialization• Statements can be modeled with • Resources: an element, a URI, a literal • Properties: directed relation between two resources • Statements: triples of two resources linked by property
    • RDF• Generally triple can be viewed as a graph • both “ object: and “ subject” are the graph nodes • “properties are the edges• XML syntax is only the tools for practical usage instead of graph• Components • URIs – for referencing resources • Literals – data values • Empty nodes (blank nodes) – talking about something which doesn’t have a name
    • RDF Example • Subject: URIs and empty nodes • Predicate: URIs ( also called properties) • Object: URIs and empty nodes and literals
    • XML syntax for RDF Example
    • RDF Example
    • RDF XML Code Example1. <?xml version="1.0"?>2. <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"3. xmlns:dc="http://purl.org/dc/elements/1.1/"4. xmlns:exterms="http://www.example.org/terms/">5. <rdf:Description rdf:about="http://www.example.org/index.html">6. <exterms:creation-date>August 16, 1999</exterms:creation-date>7. <dc:language>en</dc:language>8. <dc:creator rdf:resource="http://www.example.org/staffid/85740"/>9. </rdf:Description>10. </rdf:RDF>
    • A simple example “The book has the title War and Peace” Graphical RDF Statement has the title War and The book The book Peace RDF in a XML document <?xml version="1.0"?> <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:dc="http://purl.org/dc/element/1.1"> <rdf:Description rdf:about="http://amazon.com/books"> <dc:title> War and Peace</dc:title> </rdf:Description></rdf:RDF>
    • Ontology We can express ontology as: Ontology =<taxonomy, inference rules>And we can express a taxonomy as: Taxonomy <{classes}, {relations}> Ontology Languages (RDFS, OWL) has formal foundations that allow us to infer additional (implicit) statements
    • RDF Schema Intended to structure RDF resources RDFS  Set theory – rdfs:Class  Relation – rdf:Property, rdfs:domain, rdfs:range  Hierarchy – rdfs:subClassOf, rdfs:subPropertyOf  Built-in Datatype – xsd:string, xsd:dataTime
    • RDF & RDFS RDF is graphical formalism ( + XML syntax + semantics)  for representing metadata  for describing the semantics of information in a machine- accessible way RDFS extends RDF with “schema vocabulary”, e.g.:  Class, Property  type, subClassOf, subPropertyOf  range, domain
    • Limitations of RDF/RDFS No standard for expressing primitive data types such as integer, etc. All data types in RDF/RDFS are treated as strings. No standard for expressing relations of properties (unique, transitive, inverse etc.) No standard for expressing whether enumerations are closed. No standard to express equivalence, disjointedness etc. among properties
    • OIL and DAML RDFRDFS define a framework, however they have limitations. There is a need for new semantic web languages with following requirements  They should be compatible with (XML, RDF/RDFS)  They should have enough expressive power to fill in the gaps in RDFS  They should provide automated reasoning support Ontology Inference Layer (OIL) and DARPA Agent Markup Language (DAML) are two important efforts developed to fulfill these requirements. Their combined efforts formed DAML+OIL declarative semantic language.
    • OIL and DAML DAML+OIL is built on top of RDFS.  It uses RDFS syntax.  It has richer ways to express primitive data types. DAML+OIL allows other relationships (inverse and transitivity) to be directly expressed. DAML+OIL provides well defined semantics, This provides followings:  Meaning of DAML+OIL statements can be formally specified.  Machine understanding and automated reasoning can be supported.  More expressive power can be provided.
    • ExampleExample: T. Rex is not herbivore and not a currently living species. This statement can be expressed in DAML+OIL, but not in RDF/RDFS since RDF/RDFS cannot express disjointedness. DAML+OIL provides automated reasoning by providing such expressive power.  For instance, a software agent can find out the “list of all the carnivores that won’t be any threat today” by processing the DAML+OIL data representation of the example above.  RDF/RDFS does not express “is not” relationships and exclusions.
    • OWL
    • Web Ontology Language = OWL OWL is an extra layer, a bit like RDFS  own namespace, own terms  it relies on RDF Schemas It is a separate recommendation  actually… there is a 2004 version of OWL (“OWL 1”)  and there is an update (“OWL 2”) published in 2009
    • OWL- Web Ontology Language OWL is a vocabulary extension of the RDF and is derived from the DAML+OIL Web Ontology Language. OWL  Description Logic  Class, Thing, Nothing  DatatypeProperty, ObjectProperty, AnnotationProperty,…  Class  oneOf, disjointWith, unionOf, complementOf, intersectionOf …  Restriction, onProperty, cardinality, hasValue…  Property  inverseOf , TransitiveProperty , SymmetricProperty  FunctionalProperty, InverseFunctionalProperty  Equality– equivalentClass , sameAs , differentFrom…  Ontology annotation – Ontology, imports, versionInfo
    • Term equivalences For classes:  owl:equivalentClass: two classes have the same individuals  owl:disjointWith: no individuals in common For properties:  owl:equivalentProperty  remember the a:author vs. f:auteur?  owl:propertyDisjointWith
    • Term equivalences For individuals:  owl:sameAs: two URIs refer to the same concept (“individual”)  owl:differentFrom: negation of owl:sameAs
    • Example owl:equivalentProperty a:author f:auteur owl:equivalentClass a:Novel f:Roman
    • Property characterization In OWL, one can characterize the behavior of properties (symmetric, transitive, functional, reflexive, inverse functional…) One property can be defined as the “inverse” of another
    • What this means is…  If the following holds in our triples::email rdf:type owl:InverseFunctionalProperty.<A> :email "mailto:a@b.c".<B> :email "mailto:a@b.c".
    • What this means is…  If the following holds in our triples::email rdf:type owl:InverseFunctionalProperty.<A> :email "mailto:a@b.c".<B> :email "mailto:a@b.c". then, processed through OWL, the following holds, too:<A> owl:sameAs <B>.
    • Keys “if two persons have the same emails and the same homepages then they are identical” Identification is based on the identical values of two properties The rule applies to persons only
    • Previous rule in OWL:Person rdf:type owl:Class; owl:hasKey (:email :homepage) .
    • What it means is…If:<A> rdf:type :Person ; :email "mailto:a@b.c"; :homepage "http://www.ex.org".<B> rdf:type :Person ; :email "mailto:a@b.c"; :homepage "http://www.ex.org".then, processed through OWL, the following holds,too:<A> owl:sameAs <B>.
    • Classes in OWL In RDFS, you can subclass existing classes… that’s all In OWL, you can construct classes from existing ones:  enumerate its content  through intersection, union, complement  etc
    • Enumerate class content:Currency rdf:type owl:Class; owl:oneOf (:€ :£ :$). I.e., the class consists of exactly of those individuals and nothing else
    • Union of classes:Novel rdf:type owl:Class.:Short_Story rdf:type owl:Class.:Poetry rdf:type owl:Class.:Literature rdf:type owl:Class; owl:unionOf (:Novel :Short_Story :Poetry).  Other possibilities: owl:complementOf, owl:intersectionOf, …
    • For example… If::Novel rdf:type owl:Class.:Short_Story rdf:type owl:Class.:Poetry rdf:type owl:Class.:Literature rdf:type owl:Class; owl:unionOf (:Novel :Short_Story :Poetry).<myWork> rdf:type :Novel . then the following holds, too:<myWork> rdf:type :Literature .
    • What we have so far… The OWL features listed so far are already fairly powerful E.g., various databases can be linked via owl:sameAs, functional or inverse functional properties, etc. Many inferred relationship can be found using a traditional rule engine
    • The most used Semantic WebTools RDF Gateway- it runs both a Web application server and database design to handle RDF content Jena -Java API for RDF Smore: Semantic Markup, Ontology and RDF Editor Drive - a C# API. It parses and validate RDF documents.
    • General Architecture for Text Engineering (GATE)
    • What is GATE?An architecture A macro-level organisational picture for LE software systems.A framework For programmers, GATE is an object-oriented class library thatimplements the architecture.A development environment For language engineers, computational linguists et al, GATE is agraphical development environment bundled with a set of tools for doinge.g. Information Extraction.Some free components... ...and wrappers for otherpeoples componentsTools for: evaluation; visualise/edit; persistence; IR; IE; dialogue; ontologies; etc. 46(21)
    • Where did GATE come from?A number of researchers realised in the early- mid-1990s (e.g. in TIPSTER):• Increasing trend towards multi-site collaborative projects• Role of engineering in scalable, reusable, and portable HLT solutions• Support for large data, in multiple media, languages, formats, and locations• Lower the cost of creation of new language processing components• Promote quantitative evaluation metrics via tools and a level playing fieldHistory:• 1996 – 2002: GATE version 1, proof of concept• March 2002: version 2, rewritten in Java, component based, more users• Fall 2003: new development cycle 47(21)
    • Applications Swoogle DBpedia Flickr PhotoStuff
    • Swoogle• Swoogle is a crawler based indexing and retrieval system for Semantic Web• Swoogle crawls and discovers documents written in RDF,OWL• Swoogle classifies a Semantic Web Document(SWD) as – • Semantic Web Ontology (SWO) – Defines new terms • Semantic Web Databases (SWDB) – Makes assertions about individuals
    • Reference & Resources http://www.w3.org/TR/rdf-primer/ http://www.w3.org/DesignIssues/Semantic.html http://www.w3.org/TR/rdf-syntax-grammar/ http://www.authorstream.com/Presentation/sneha.ch http://www.w3.org/TR/owl-semantics/