Transcript Document
The SEMANTIC Web Dr.P.Kefalas Computer Science Dept The World Wide Web now I need to have my hair dyed at the closest hairdresser sometime on Friday Where is the nearest hairdresser? What is the telephone number? How can I fix an appointment? WWW Why this process cannot be automated? Web is based on HTML documents that are: Machine readable, and Human interpretable So, all the processes should be devised by humans, and (in the best case scenario) performed by humans through the web. How can this process be automated? Web should be structured in such way that documents are: Machine readable, and Machine interpretable. “By augmenting Web pages with data targeted at computers and by adding documents solely for computers, we will transform the Web into the Semantic Web” (Tim Berners-Lee,2001) How can this process be automated? “Computers will find the meaning of semantic data by following hyperlinks to definitions of key terms and rules for reasoning about them logically. The resulting infrastructure will spur the development of automated Web services such as highly functional agents” (Tim Berners-Lee,2001) The World Wide Web as it should be I need to have my hair dyed at the closest hairdresser sometime on Friday The Semantic Web OK agent Your appointment is set for Friday 7 of March at 18:00, at HairStyle 15 Tsimiski Street. What is an agent? Someone or Something that “acts on behalf”. An Agent is a software computer system that has following properties Autonomy: agents operate without intervention Social Ability: agents interact with each other Reactivity: agents perceive their environment and act upon it, thus responding to changes of the environment Pro-activeness: agents exhibit goal-directed behaviour and take initiative to achieve them (Wooldrigde & Jennings, 95) Agents on Semantic Web Agents on the Semantic Web need to: Be able to collect web content from diverse sources Process the information Reason about the collected contents Exchange the results with other agents Therefore, agents should be able to understand the meaning of the Web contents, i.e. Web contents must incorporate semantics. Towards Semantic Web Step 1: XML XML: eXtensible Markup Language XML is accepted as THE emerging standard for data interchange on the Web. XML allows authors to create their own markup (e.g. <AUTHOR>), which seems to carry some semantics. However, from a computational perspective tags like <AUTHOR> carries as much semantics as a tag like <H1>. A computer simply does not know, what an author is and how the concept author is related to e.g. a concept person. (www.w3c.org) XML is defined only at syntactic level, so you cannot rely on machines to unambiguously determine the correct meaning of tags. Extensible means that it only provides a data format, not an actual vocabulary. XML is a meta-language. Markup means that certain sequences of characters contain information indicating the role of the content Tags are those words between pointy brackets. An element is the main XML entity: an opening and closing tag. They need to be properly nested. Example: XML <customer-details id="AcPharm39156"> <name>Acme Pharmaceuticals Co.</name> <address country="US"> <street>7301 Smokey Boulevard</street> <city>Smallville</city> <state>Indiana</state> <postal>94571</postal> </address> </customer-details> Towards Semantic Web Step 2: RDF RDF: Resource Description Framework RDF uses XML as an interchange syntax. The RDF specifications provide a lightweight ontology system to support the exchange of knowledge on the Web. RDF defines a syntactical convention and a simple data model for representing data’s machine-processable semantics. RDF description uses: object-attribute-value triples that correspond to subject-verb-object of an elementary sentence. Towards Semantic Web Step 3: URI URI: Universal Resource Identifier Subject-verb-object are identified by a URI, just as used in a link on a Web page. URIs ensure that concepts are not just words in a document but are tied to a unique definition that anyone can find on the Web. URLs are the most common type of URI Sentence: Ora Lassila is the creator of the resource http://www.w3.org/Home/Lassila. Example: RDF and URI Subject http://www.w3.org/Home/Lassila Predicate Creator Object "Ora Lassila" FIGURE: RDF as a semantic network In XML: <?xml version="1.0"?> <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntaxns#" xmlns:s="http://description.org/schema/"> <rdf:Description about="http://www.w3.org/Home/Lassila"> <s:Creator>Ora Lassila</s:Creator> </rdf:Description> </rdf:RDF> Towards Semantic Web Step 4: Ontologies An ontology is a specification of objects, concepts, and relations in the area of interest (Gruber, 1993) An ontology: Describe a taxonomy of classes Must describe the relationships among terms Provide a set of inference rules An agent must represent its knowledge in the vocabulary of a specified ontology. So that, all agents that share the same ontology for knowledge representation have an understanding of the “words” in the agent communication language. What have you got on “models”? Agent 1 Software Ontology Agent 2 She is talking about software DAML: DARPA Agent Markup Language OIL: Ontology Inference Layer DAML+OIL The DAML language is being developed as an extension to XML and RDF. The latest release of the language (DAML+OIL) provides a rich set of constructs with which to create ontologies and to markup information so that it is machine readable and understandable. DAML+OIL provides: Rules for describing further constraints Rules for describing further relationships among resources Domain and range restrictions Union, disjunction, inverse and transitive rules Example: DAML in XML <daml:Class rdf:about="#Animal"> <rdfs:comment> Animals have exactly two parents, i.e.: If x is an animal, then it has exactly 2 parents. </rdfs:comment> <rdfs:subClassOf> <daml:Restriction daml:cardinality="2"> <daml:onProperty rdf:resource="#hasParent"/> </daml:Restriction> </rdfs:subClassOf> </daml:Class> <daml:Class rdf:about="http://www.daml.org/2001/03/daml+oilex.daml#Animal"> Semantic Web Layers Unicode and URI layer make sure that we use international character sets and provide means for identifying objects. The XML layer make sure that we can integrate the semantic web definitions with the other XML based standards. With RDF it is possible to make statements about objects with URIs and define vocabularies that can be referred to by URIs He ontology layer can define relations between the different concepts With digital signatures we can prove the identity of who is on the other end of the digital communication. The logic layer enables the writing of rules The proof layer executes the rules The trust layer evaluates whether to trust the given proof Conclusion To date, the Web has developed most rapidly as a medium of documents for people rather than for data and information that can be processed automatically. The Semantic Web aims to make this happen. Links to Sources T.Berners-Lee et al, The Semantic Web, Scientific American, 284 (5), May 2001 www.w3.org/2001/sw www.SemanticWeb.org www.daml.org Communications of ACM and IEEE Intelligent Systems and Applications: Special Issues on “Semantic Web”