SEMANTIC WEB 03 RDF DATA MODEL RESOURCE DESCRIPTION FRAMEWORK IMRAN IHSAN ASSISTANT PROFESSOR, AIR UNIVERSITY, ISLAMABAD

Transcription

1 SEMANTIC WEB 03 RDF DATA MODEL RESOURCE DESCRIPTION FRAMEWORK IMRAN IHSAN ASSISTANT PROFESSOR, AIR UNIVERSITY, ISLAMABAD

2 MOTIVATION How do you encode the piece of knowledge: or <theory> "The theory of relativity was discovered by Albert Einstein." <name>theory of Relativity</name> <discoverer>albert Einstein</discoverer> </theory> <person> <name>albert Einstein</name> <discovered>theory of Relativity</discovered> </person> or <person name="albert Einstein"> <discovered>theory of Relativity</discovered> </person> There is no unique way (in XML) to represent knowledge. Information represented in such ways is not easy to integrate. (Why?) RDF helps to solve this problem. 2

3 GOALS Understand the RDF data model, including URI and IRI concepts Triples Resources Literals Blank nodes Lists 3

4 RDF OVERVIEW RDF = Resource Description Framework W3C Recommendation since 1998 Version 1.1 since 2014 RDF is a data model Originally used for metadata for web resources, then generalized Encodes structured information Universal, machine readable exchange format Data structured in graphs Vertices, edges 4

5 PARTS OF THE RDF GRAPH URIs Used to reference resources unambiguously Literals Describe data values with no clear identity like "100 km/h" Blank nodes Facilitate existential quantification for an individual with certain properties without naming it 5

6 EXAMPLE OF AN RDF GRAPH 6

7 RDF TRIPLE COMPONENTS OF AN RDF TRIPLE Modeled using linguistic categories (but not always consistent) Allowed assignments: Subject: URI or blank node Predicate: URI (a.k.a. property) Object: URI, blank node or literal Node and edge labels should be unambiguous, so that the original graph is reconstructable from triple list 7

8 URI URI = Uniform Resource Identifier Used to create globally unique names for resources Every object with a clear identity can be a resource Books, places, organizations... In books domain the ISBN serves the same purpose 8

9 URI SYNTAX Extension of the URL concept Not every URI denotes a web document, but the URL is often used as URI for web documents Starts with URL schema, which is separated from the rest by ":" examples: http, ftp, mailto, file Typically hierarchical structure [scheme:][//authority][path][?query][#fragment] 9

10 SELF-DEFINED URIS Necessary if resource has no URI yet or URI is not known Use HTTP URIs of own website to avoid naming collisions Facilitates creation of documentation of URI at this location Example: Separation of URI for a resource (a real-world thing) and its documentation (e.g. an HTML page) with the help of URI references (with # -attached fragments) or content negotiation Example: URI for Shakespeare's "Othello": bad (why?): good: 10

11 IRI IRI = Internationalized Resource Identifier Generalization of URI concept IRI can contain Unicode Example: 사막 11

12 LITERALS Used to model data values Representation as strings Interpretation through datatype Literals without datatype are treated as strings Literals may never be the origin of a node of an RDF graph Edges may never be labeled with literals 12

13 TURTLE SYNTAX Language to serialize RDF Triples to strings Turtle Terse RDF Triple Language URIs in angle brackets: < Literals in quotes " "^^xsd:float Triples are subject-predicate-object sentences terminated with a dot. < < "Leipzig"@de. Whitespace and line breaks are ignored outside of identifiers Status: W3C Recommendation, 13

14 TURTLE ABBREVIATIONS In Turtle one can use abbreviations abbr ':' <URI>. dbr: < One can transform < < dbr: rdfs: < dbr:leipzig rdfs:label 14

15 TURTLE ABBREVIATIONS Triples with the same subject can be grouped geo: dbr:leipzig dbp:hasmayor dbr:burkhard_jung ; rdfs:label "Leipzig"@de ; geo:lat " "^^xsd:float ; geo:long " "^^xsd:float. Even triples with the same subject and predicate can be grouped dbp:. dbr:leipzig dbp:locatedin dbr:saxony, dbr:germany; dbp:hasmayor dbr:burkhard_jung. 15

16 LITERALS II DATATYPES Example: xsd:decimal 16

17 DATATYPES IN RDF So far: literals are untyped, treated as strings: "02" < "100" < "11" < "2" Typing allows better, in other words, semantic interpretation of values Datatypes get identified by URIs and are freely choosable Typically usage of XML Schema Datatypes (XSD) Syntax: "data value"^^<datatype-uri> rdf:html and rdf:xmlliteral are the only predefined datatypes in RDF Used for HTML and XML fragments 17

18 EXAMPLE Graph: dbr: geo: xsd: < dbr:leipzig geo:lat " "^^xsd:float ; geo:long " "^^xsd:float. 18

19 LANGUAGE DECLARATION Influences only untyped literals rdfs: < rdfs:label "Леи In RDF 1.0 the following literals were all different, but implementations typically treated them the xsd: rdfs: dbr: < dbr:leipzig rdfs:label "Leipzig", "Leipzig"^^xsd:string. As of RDF 1.1 "Leipzig" is a shorthand for "Leipzig"^^xsd:string. 19

20 N-ARY RELATIONS I Cooking with RDF "For the preparation of mango chutney you need 450g of green mango, a teaspoon of cayenne pepper..." 1st attempt to model this recipe: Not satisfying: Ingredients and amounts coded as strings Search for recipes which contain green mango not easily ex: < ex:chutney ex:hasingredient "450g green mango", "1tsp Cayenne pepper". 20

21 N-ARY RELATIONS II Cooking with RDF "For the preparation of mango chutney you need 450g of green mango, a teaspoon of cayenne pepper..." 2nd attempt to model this recipe: Even worse: No unambiguous association between ingredient and amount ex: < ex:chutney ex:ingredient ex:greenmango; ex:amount "450g" ; ex:ingredient ex:cayennepepper; ex:amount "1tsp". 21

22 N-ARY RELATIONS III Problem: it is a real trivalent, or ternary relationship (see e.g. databases) Recipe Ingredient Amount Mango Chutney green Mango 450g Mango Chutney Cayenne pepper 1 tsp Directly not possible to express in RDF Solution: introduction of helper nodes 22

23 N-ARY RELATIONS IV Helper nodes in RDF: As graph: In Turtle ex: < ex:chutney ex:hasingredient ex:chutneyingredient1. ex:chutneyingredient1 ex:ingredient ex:greenmango; ex:amount "450g". 23

24 BLANK NODES Blank nodes can be used for resources which don't need to be named Can be read as existential statements As graph: In Turtle ex: < ex:chutney ex:hasingredient _:id1. _:id1 ex:ingredient ex:greenmango; ex:amount "450g". # can be shortened: ex:chutney ex:hasingredient [ ex:ingredient ex:greenmango; ex:amount "450g" ]. 24

25 LISTS General data structures for enumerating arbitrarily many resources Distinction between Container: adding new elements possible ordered and unordered container types Collections: ordered list; adding new elements impossible Can be modeled with previously presented tools, so no additional expressiveness 25

26 TYPES OF CONTAINER The list root node is assigned one of the following rdf:types: rdf:seq Interpretation as ordered list, sequence rdf:bag Interpretation as unordered set Order coded in RDF not relevant rdf:alt Set of alternatives Usually only one list element relevant 26

27 COLLECTIONS Idea: recursive partition of list into a head element and (possibly empty) rest list Turtle Syntax (Shortened Notation with ex: < ex:aksw ex:groupleaders (ex:sören ex:jens ex:axel). 27

28 SUMMARY extensively supported standard for storing and exchanging data enables almost syntax-independent representation of distributed information in a graph based data model pure RDF is very individual oriented almost no possibility to represent schema 28

29 MINI PROJECT II 1. Create a small knowledge base in Turtle describing a domain (your family)! 2. Write an RDF resource description describing yourself in Turtle with labels in two different languages, your birthday and age! 3. Draw an RDF graph for representing a recipe for cup cakes! 4. Create an RDF list of European countries! 29