The Resource Description Framework (RDF)

RDF is…

• …to the Web of Data what HTML is to the Web of Documents
• …(relatively) simple: everything is just triples
• …a data model: it is not a file format!
• …a logical formalism: it has a formal semantics
• …more than XML or JSON: XML and JSON have a tree-based model, RDF has a graph-based model
• …a Web standard: a W3C recommendation

A lingua franca for the Semantic Web

• Like HTML for documents, there can be many models that would achieve a Web of Data and a Semantic Web
• PDF documents can be linked and visualised in Web browsers…
• …but HTML makes it easy to read and write documents (whereas one cannot edit PDFs in a text editor)
• Design decisions govern HTML and RDF
• The decisions may not be the best for your application, but they set a common norm

RDF basics

• The core features of RDF are:
  • identify things (resources)
  • Express relations between things (properties)
• Additional important features are:
  • Assign data values to things (literals)
  • Organise things in categories (i.e., classes or types)
  • Add simple knowledge about categories and relations
• There are other, less important features (discussed later)

Identify things

• RDF is used to describe resources
• A resource may be anything (a real or imaginary entity, abstract or concrete)
• To describe a resource, it must be named or identified
• On the Web, the identification mechanism must be uniform at Web scale: an identifier must identify the same thing everywhere on the Web
• RDF uses Internationalized Resource Identifiers or IRIs (RFC 3987)

Internationalized Resource Identifiers

• IRIs generalise URIs (Uniform Resource Identifiers, RFC 3986) by allowing any UNICODE characters
• IRIs and URIs identify things but may be used as locators (i.e., as URLs) at the same time
• Examples:
  • urn:ietf:rfc:3987
  • svn://yadiyada.foo.bar/
  • mailto:antoine.zimmermann@emse.fr
  • ftp://ftp.liris.fr/#meta
  • http://en.wikipedia.org/wiki/User:Wikiuser100
  • http://w3id.org/people/az/me
• Note: to shorten notations, we use namespace prefixes, e.g., rdf: is for http://www.w3.org/1999/02/22-rdf-syntax-ns#

How to choose an IRI for something?

• If possible, reuse an existing IRI from an authoritative source, e.g.:
  • from a national library for books (library of congress, BNF, BNL, DNB)
  • from a government website for a ministry
• If not, make your own IRI:
  • use HTTP IRIs
  • use a namespace under your control
  • Cool URIs don’t change
  • Refer to the guide on Cool URIs for the Semantic Web

Relate things

• RDF can express binary relations between things, such as “Laura loves Helmut”, “Steven works for Google Inc.”, “EMSE is located in Saint-Étienne”
• This is written as a triple:
  (subject, predicate, object)
• where subject and object identify the resources in the relationship, and predicate identifies the relation
• The predicate in an RDF triple is always an IRI

Example:
(http://example.org/data/Laura, subject
http://social.relations.com/loves, predicate
http://exmple.org/data/Helmut) object

RDF triples

• Compact syntax:
  • use namespace prefixes
  • writesubjectpredicateobjectsideby side, separatedby spaces
• E.g.: ex:Laura rel:loves ex:Helmut
• Graph representation of a triple:
  
9

Data values

• As everything else, a data value (number, string, date) is a resource
• A specific data value can be identified with a literal, a character string that represents the value
• Every literal is typed such that its string representation can be interpreted as the correct value
• E.g., "42" represents the number fourty two if this is of type decimal integer, but represents sixty six if it is an hexadecimal integer

RDF literals

• An RDF literal has 2 or 3 components which are:
  • A lexical form which is a UNICODE string
  • A datatype IRI that can be any IRI
  • When the datatype IRI is rdf:langString, there is a language tag which is a BCP 47 tag
• Usually, weuse standard datatypeIRIsfromthe xsd: namespace (XML Schema Datatypes) and the rdf: namespace
• We will write literals "lexical form"^^datatypeIRI and when it is an rdf:langString, "lexical form"@langTag
• Examples:
  • "42"^^xsd:integer
  • "THX 1138""^^xsd:string
  • "chat"@fr,"chat"@en
  • "<p>The <em>beautiful</em> literal!</p>"^^rdf:HTML
  11

RDF graphs

• An RDF graph is a set of RDF triples
• RDF graphs can be drawn as directed, edge-labelled multi-graphs
  

Unidentified resources

• RDF can describe entities that are known to exist but whose identity is unknown (or is irrelevant/unimportant)
• E.g., a book has at least an author, but they may not be known
• The existence of a thing can be indicated in the subject or object position of a triple with a blank node
• E.g. “something is in my bag”
  

Putting RDF graphs into files

• An RDF graph (a set of RDF triples) can be serialised to multiple syntaxes (similarly to tabular data that can be written to CSV, Excel, HTML tables, SQL...)
• In this course, we will mostly use Turtle
• There is an XML-based syntax: RDF/XML
• There is a JSON-based syntax: JSON-LD (more on this one in a future session)
• There is an easy to parse, line-based triple syntax: N-Triples
• There is a syntax to embed RDF in HTML and XML documents: RDFa
• Let us learn to read and write Turtle in an online editor. Go to: https://perfectkb.github.io/yate/

The Turtle syntax (1)

• Full IRIs:

  <http://www.example.com/test#this>

• A simple triple:

  <http://www.example.com/test#this>
  		<http://relations.example.com/in>
  				<http://www.example.com/test#box> .

• Abbreviated IRIs (declare prefixes at the beginning of the file):

  # This is a comment
  @prefix ex: <http://www.example.com/test#> . # end dot!
  PREFIX rel: <http://relations.example.com/> # alternative notation (no dot!)
  ex:this rel:in ex:box . # dot ends statement

The Turtle syntax (2)

• Literals:

  ex:this rel:date "2019-09-13"^^xsd:date . # normal literal
  ex:this rel:name "this"@en . # language-tagged literal
  ex:this rel:code "TX32" . # xsd:string can be omitted
  ex:this rel:number 42 . # xsd:integer (no quotes)
  ex:this rel:sizeInMeters 3.75 . # xsd:decimal (use a dot)
  ex:this rel:isGood true . # xsd:boolean
  ex:this rel:isBorring false . # xsd:boolean

• Blanknodes:

  [] rel:in ex:box .
  _:b1  rel:in ex:box . # a blank node identifier...
  ex:me rel:likes _:b1 . # ...allows to reuse the same blank node

The Turtle syntax (3)

• Repeat the same subject and predicate:

  ex:box rel:contains ex:this .
  ex:box rel:contains ex:that .
  # can be written
  ex:box rel:contains ex:this, ex:that . # comma

• Repeat subject:

  ex:this rel:date "2019-09-13"^^xsd:date;
  	rel:name "this"@en; # new lines are optional
  	rel:code "TX32";
  	rel:nextTo ex:that, ex:thoot, ex:thus .

The Turtle syntax (4)

• More on blank nodes:

  # assume prefixes are declared
  ex:johnDoe rel:worksFor [
  		a ex:University; # the IRI rdf:type can be replaced by 'a'
  		rel:name "Berkley";
  	    rel:locatedIn ex:California
  	] .

• is the same as:

  PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
  # other prefixes must be declared
  ex:johnDoe rel:worksFor _:bnode .
  _:bnode	rdf:type ex:University . # 'a' and 'rdf:type' represents the same IRI
  _:bnode	rel:name "Berkley" .
  _:bnode	rel:locatedIn ex:California .

The Turtle syntax (5)

• Declaring a base IRI:

  @base <http://example.com/base/> . # ends with dot
  BASE <http://example.com/base/> # alternative syntax (no dot!)
  # prefixes must be declared
  <bob> a vocab:Person; # relative IRI
  		rel:knows <claire> .
  BASE <http://example.com/base2#> # base can be redefined
  <bob> rel:knows <http://example.com/base/bob> . # different bobs

• is the same as:

  <http://example.com/base/bob> a vocab:Person;
  	rel:knows <http://example.com/base/claire> .
  <http://example.com/base2#bob>
  	rel:knows <http://example.com/base/bob> .