INTERNET TECHNOLOGIES THE SEMANTIC WEB ΑΝΑΓΝΩΣΤΟΠΟΥΛΟΣ ΙΩΑΝΝΗΣ INTERNET TECHNOLOGIES History of the Semantic Web • Web was “invented” by Tim Berners-Lee (amongst others), a physicist working at CERN • TBL’s original vision of the Web was much more ambitious than the reality of the existing (syntactic) Web: “... a goal of the Web was that, if the interaction between person and hypertext could be so intuitive that the machine-readable information space gave an accurate representation of the state of people's thoughts, interactions, and work patterns, then machine analysis could become a very powerful management tool, seeing patterns in our work and facilitating our working together through the typical problems which beset the management of large organizations.” • TBL (and others) have since been working towards realising this vision, which has become known as the Semantic Web 1 INTERNET TECHNOLOGIES Where we are Today: the Syntactic Web INTERNET TECHNOLOGIES Difficulties in the Syntactic Web… • Complex queries involving background knowledge – Find information about “animals that use sonar but are not either bats or dolphins” • Locating information in data repositories – Travel enquiries – Prices of goods and services – Results of human genome experiments • Assigning complex tasks to web “agents” – Book me a holiday next weekend somewhere warm, not too far away, and where they speak French or English 2 INTERNET TECHNOLOGIES What is the Problem? • Consider a typical web page: • Markup consists of: – rendering information (e.g., font size and colour) – Hyper-links to related content • Semantic content is accessible to humans but not (easily) to computers… INTERNET TECHNOLOGIES Need to Add “Semantics” • External agreement on meaning of annotations – E.g., Dublin Core http://www.lub.lu.se/cgi-bin/nmdc.pl • Agree on the meaning of a set of annotation tags – Problems with this approach • Inflexible • Limited number of things can be expressed • Use Ontologies to specify meaning of annotations – Ontologies provide a vocabulary of terms – New terms can be formed by combining existing ones 3 INTERNET TECHNOLOGIES Structure of an Ontology Ontologies typically have two distinct components: • Names for important concepts in a knowledge domain – Elephant is a concept whose members are a kind of animal – Herbivore is a concept whose members are exactly those animals who eat only plants or parts of plants – Adult_Elephant is a concept whose members are exactly those elephants whose age is greater than 20 years • Background knowledge/constraints on the domain – Adult_Elephants weigh at least 2,000 kg – All Elephants are either African_Elephants or Indian_Elephants – No individual can be both a Herbivore and a Carnivore INTERNET TECHNOLOGIES Thesaurus vs. Ontology Controlled Vocabulary Terms: Metal working machinery, equipment and supplies, metal-cutting machinery, metal-turning Ontology equipment, metal-milling equipment, milling insert, turning insert, etc. Concepts Relations: use, used-for, broader-term, narrower- Logical-Conceptual term, related-term Semantics (Strong) Thesaurus Real (& Possible) Terms World Referents Term ‘Semantic’ Relations: Semantics Logical Concepts Entities: Metal working machinery, equipment and z Equivalent = (Weak) supplies, metal-cutting machinery, metal-turning Semantic Relations: equipment, metal-milling equipment, milling insert, z Used For (Synonym) turning insert, etc. z Subclass Of UF Relations: subclass-of; instance-of; part-of; has- geometry; performs, used-on;etc. Properties: geometry; material; length; operation; z Part Of z Broader Term BT UN/SPSC-code; ISO-code; etc. Values: 1; 2; 3; “2.5 inches”; “85-degree-diamond”; z Arbitrary Relations z Narrower Term NT “231716”; “boring”; “drilling”; etc. Axioms/Rules: If milling-insert(X) & operation(Y) & z Meta-Properties on material(Z)=HG_Steel & performs(X, Y, Z), then z Related Term RT has-geometry(X, 85-degree-diamond). Relations 4 INTERNET TECHNOLOGIES Ontology Spectrum: One View strong semantics Modal Logic First Order Logic Logical Theory Is Disjoint Subclass of Description Logic with transitivity DAML+OIL, OWL property UML Conceptual Model Is Subclass of RDF/S Semantic Interoperability XTM Extended ER Thesaurus Has Narrower Meaning Than ER DB Schemas, XML Schema Structural Interoperability Taxonomy Is Sub-Classification of Relational Model, XML Syntactic Interoperability weak semantics INTERNET TECHNOLOGIES Ontology Representation Levels Level Example Constructs Knowledge Class, Relation, Instance, Representation (KR) Function, Attribute, Property, Constraint, Axiom, Language (Ontology Rule Language) Level: Meta Level to the Ontology Concept Level Ontology Concept Person, Location, Event, (OC) Level: Parent, Hammer, River, Object Level to the KR FinancialTransaction, Language Level, BuyingAHouse, Automobile, Meta Level to the TravelPlanning, etc. Instance Level Ontology Instance Harry X. Landsford III, Ralph (OI) Level: Waldo Emerson, Person560234, Object Level to the PurchaseOrderTransactionEve Ontology Concept nt6117090, 1995-96 V-6 Ford Level Taurus 244/4.0 Aerostar Automatic with Block Casting # 95TM-AB and Head Casting 95TM 5 INTERNET TECHNOLOGIES A Semantic Web — First Steps Make web resources more accessible to automated processes • Extend existing markup with semantic markup – Metadata annotations that describe content/function of web accessible resources • A prerequisite is a standard web ontology language – Need to agree common syntax before we can share semantics – Syntactic web based on standards such as HTTP and HTML INTERNET TECHNOLOGIES What Problems Do Ontologies & the Semantic Web Help Solve? • Heterogeneous database problem – Different organizational units, Service Needers/Providers have radically different databases – Different syntactically: what’s the format? – Different structurally: how are they structured? – Different semantically: what do they mean? – They all speak different languages (access, description, schemas, meaning) • Relevant document retrieval/question-answering problem – What is the meaning of your query? – What is the meaning of documents that would satisfy your query? – Can you obtain only meaningful, relevant documents? 6 INTERNET TECHNOLOGIES RDF and RDFS • RDF stands for Resource Description Framework • It is a W3C candidate recommendation (http://www.w3.org/RDF) • 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 INTERNET TECHNOLOGIES Building Blocks in RDF Semantic Web Metadata URI Data about data – labeling and structuring Universal Resource Identifier – information in a document an universal and unique name for any resource http://www.something.com/one 7 INTERNET TECHNOLOGIES The RDF statements • Statements are <subject, predicate, object> triples: <Ian,hasColleague,Uli> • Can be represented as a graph: hasColleague Ian Uli • Statements describe properties of resources • A resource is any object that can be pointed to by a URI: – a document, a picture, a paragraph on the Web; – a book in the library, a real person – isbn://5031-4444-3333 – … • Properties themselves are also resources (URIs) INTERNET TECHNOLOGIES URIs • URI = Uniform Resource Identifier • URLs (Uniform Resource Locators) are a particular type of URI, used for resources that can be accessed on the WWW (e.g., web pages) • In RDF, URIs typically look like “normal” URLs, often with fragment identifiers to point at specific parts of a document: – http://www.somedomain.com/some/path/to/file#fragmentID 8 INTERNET TECHNOLOGIES Linking Statements • Each triple (Subject, Predicate, Object) represents an RDF statement • Subject Æ Resource (R) • Predicate Æ Property (P) • Object Æ Resource (R) Object(R) Subject(R) hasColleague Ian John hasColleague hasHomePage(P) hasColleague(P) Carole Subject(R) http://www.cs.mam.ac.uk/~johnsmith Object(L) INTERNET TECHNOLOGIES What RDF/S provide? • RDFS enables you to make simple, generic statements about your Web object classes, properties • RDF enables you to make specific statements about your Web object instances (of those classes, properties) • A set of RDF statements can be viewed in 3 ways: – A set of triples: consider them as rows/tuples in a database – A directed graph: consider them as a complex, navigatable data structure 9 INTERNET TECHNOLOGIES The RDF Data Model Simple but powerful data model for the description of resources and the creation of metadata Consists of three core concepts: ¾Resource ¾Property ¾Statement + Class (in RDF Schema) Similar to other modeling approaches (e.g. object-oriented modeling), but property-centric, not class-centric INTERNET TECHNOLOGIES RDF Syntax • RDF has an XML syntax that has a specific meaning: • Every Description element describes a resource • Every attribute or nested element inside a Description is a property of that Resource • We can refer to resources by using URIs <Description about="some.uri/person/ian_horrocks"> <hasColleague resource="some.uri/person/john_smith"/> </Description> <Description about="some.uri/person/john_smith"> <hasHomePage>http://www.cs.mam.ac.uk/~johnsmith</hasHomePage> </Description> <Description about="some.uri/person/carole_goble"> <hasColleague resource="some.uri/person/john_smith"/> </Description> 10 INTERNET TECHNOLOGIES RDF Schema (RDFS) RDF Schema allows you to define vocabulary terms and the relations