From the Web to the Semantic Web: RDF and RDF Schema

Transcription

1 From the Web to the Semantic Web: RDF and RDF Schema Languages for web Master s Degree Course in Computer Engineering - (A.Y. 2016/2017)

2 The Semantic Web [Berners-Lee et al., Scientific American, 2001] The Semantic Web is the extension of the World Wide Web that enables people to share content beyond the boundaries of applications and website a standardized way of expressing the relationships between web pages allow machines to understand the meaning of hyperlinked information retrieve information from the web (using the web spiders from RDF files) Core Specifications RDF: used for expressing knowledge in a decentralized world and it is the foundation of the Semantic Web OWL (Ontology Web Language): extends the expressibility of RDF with an additional layer of semantics 2 of 31

3 Same name, different meanings 3 of 31

4 Different names, same meaning 4 of 31

5 Semantic Web Layer Cake Unrealized Web technologies Crypto: ensure and verify that SW statements are coming from trusted source Trust: verify derived statement Proof: provide explanations about the answers given by automated agents Standardized Semantic Web technologies RDF: represent data about resources RDFS: provide basic vocabulary for RDF OWL: extend RDFS by adding more advanced constructs SPARQL: RDF querying language to retrieve information for SW applications RIF: describe relations that cannot be directly described using description logic used in OWL Hypertext Web technologies SW resources uniquely identified by IRI (URI) XML: create documents composed of structured data XML Namespaces: use markups from more sources 5 of 31

6 What is RDF? Resource Description Framework (RDF) is a framework for representing information in the Web. add machine-readable information to Web pages provide a standards-compliant way for exchanging data abstract syntax (data model) defined to link all RDF-based languages and specifications abstract syntax based on three key data structures RDF statement: a subject-predicate-object triple Subject Predicate Object RDF graph: a set of RDF triples RDF dataset: a collections of RDF graphs 6 of 31

7 RDF Statement express a relationship between two resources: subject and object predicate represents the nature of their relationship relationship (RDF property) is phrased in a directional way (from subject to object) resources identified by IRI (can appear in all three positions of a triple) literals are basic values (not IRIs, may only appear in the object position) triples also represented as a connected graph 7 of 31

8 RDF Serializations save RDF datasets (e.g. as file) share dataset over the Web Different serialization formats for RDF graphs RDF/XML (XML syntax for RDF) Turtle family of RDF languages (N-Triples, Turtle, TriG and N-Quads) JSON-LD (JSON-based RDF syntax) RDFa (for HTML and XML embedding) 8 of 31

9 RDF/XML Syntax originally developed in the late 1990s RDF triples specified within an XML element rdf:rdf rdf:description tag used to define sets of triples having as subject the IRI specified by the about attribute verbose syntax <?xml version="1.0"?> <rdf:rdf xmlns:rdf=" xmlns:dbpedia-owl=" xmlns:dbpedia=" <rdf:description rdf:about=dbpedia:the_matrix> <dbpedia-owl:starring rdf:resource=dbpedia:keanu_reeves/> </rdf:description> </rdf:rdf> 9 of 31

10 N-Triples Syntax simple line-based, plain-text way for serializing RDF graphs each line represents a triple, full IRIs are enclosed in angle brackets (<>) dot at the end of the line signals the end of the triple < < < < < < Subject Predicate Object 10 of 31

11 Turtle (Terse RDF Triple Language) extension of N-Triples, introduces a number of syntactic shortcuts support for namespace prefixes, lists and shorthands for datatyped literals PREFIX dbpedia : < PREFIX dbpedia-owl : < good trade-off between ease of writing, parsing and readability Same (subject, predicate), different objects comma notation dbpedia:the_matrix dbpedia-owl:starring Subject Predicate dbpedia:keanu_reeves, Object #1 dbpedia:carrie-ann_moss. Object #2 Same subjects, different (predicate, object) pairs semicolon notation dbpedia:the_matrix dbpedia-owl:starring Subjects dbpedia:keanu_reeves, Pred/Obj #1 dbpedia:carrie-ann_moss; dbpedia-owl:director Pred/Obj #2 dbpedia:the_wachowskis. 11 of 31

12 TriG and N-Quads for multiple-graphs TriG: extension of Turtle enabling the specification of multiple graphs in the form of an RDF dataset GRAPH < { <bob#me> a foaf:person ; foaf:knows <alice#me> ; schema:birthdate " "^^xsd:date ; foaf:topic_interest wd:q } GRAPH < Special:EntityData/Q12418> { wd:q12418 dcterms:title "Mona Lisa" ; dcterms:creator < } < dcterms:subject wd:q N-Quads: extension to N-Triples, add a fourth element to a line, capturing the graph IRI of the triple <subject> <predicate> <object> <graph-iri> 12 of 31

13 JSON-LD Syntax a JSON syntax for RDF graphs and datasets used to transform JSON documents to RDF with minimal JSON document describing how the document can be mapped to an RDF IRI identifying the resource corresponding to the current JSON object { } "@context": "example-context.json", "@id": " "@type": "Person", "birthdate": " ", "knows": " "interest": { "@id": " "title": "Mona Lisa", "subject_of": " "creator": " } { } "@context": { "foaf": " "Person": "foaf:person", "interest": "foaf:topic_interest", "knows": { "@id": "foaf:knows", "@type": "@id" }, "birthdate": { "@id": " "@type": " }, "dcterms": " "title": "dcterms:title", "creator": { "@id": "dcterms:creator", "@type": "@id" }, "subject_of": { "@reverse": "dcterms:subject", "@type": "@id" } } 13 of 31

14 RDFa Syntax used to embed RDF data within HTML and XML documents search engines can aggregate this data (when crawling the Web) and enrich search results 4 special RDFa attributes to enable specification of RDF triples within HTML: resource, property, typeof, prefix <div resource=" typeof="foaf:person"> <p> Bob knows <a property="foaf:knows" href=" and was born on the <time property="schema:birthdate" datatype="xsd:date"> </time>. </p> <p> Bob is interested in <span property="foaf:topic_interest" resource=" Mona Lisa</span>. </p> </div> 14 of 31

15 Basic Constructs rdf:type (a resource is an instance of a class) dbpedia:the_matrix rdf:type dbpedia-owl:film. dbpedia:the_matrix a dbpedia-owl:film. rdf:property (a resource is an RDF property) dbpedia-owl:starring a rdf:property. dbpedia-owl:director a rdf:property. RDF containers used to describe group of resources rdf:bag list of unordered values rdf:seq list of ordered values rdf:alt list of alternative values 15 of 31

16 RDF and RDFS References Resource Description Framework (RDF) W3C Recommendation, 10 February 2004, RDF 1.1 Primer W3C Working Group Note 24 June 2014, RDF 1.1 XML Syntax W3C Recommendation, 25 February 2014, N-Triples - A line-based syntax for an RDF graph W3C Recommendation, 25 February 2014, Turtle (Terse RDF Triple Language) W3C Recommendation, 25 February 2014, JSON-LD 1.0 (A JSON-based Serialization for Linked Data) W3C Recommendation, 16 January 2014, Dublin Core Metadata Initiative RDF Schema 1.1 W3C Recommendation, 25 February 2014, 16 of 31