RDFa: putting RDF on the Web

Digital Enterprise Research Institute www.deri.ie

RDFa:
putting RDF on the Web

Benjamin.Heitmann@deri.org

Chapter 1
 Copyright 2007 Digital Enterprise Research Institute. All rights reserved. www.deri.org

Overview

 Part 1: Motivation
 Use case for embedding RDF on XHTML pages
 Bonus: Explaining “what do I do?” at a Christmas party
 Part 2: The foundation: RDF in a nutshell
 data model
 formal semantics
 Part 3: RDFa with examples
 the RDFa attributes
 visible and invisible embedding
 handle with care: CURIEs and implicit blank nodes
 tools for consuming and publishing RDFa

2 of 28 Benjamin Heitmann

About me

 Research interest:
bringing the Semantic Web to the IT mainstream by
applying software engineering methodologies
 currently looking for a PhD Thesis topic
 Master thesis topic:
Transitioning web application frameworks
towards the Semantic Web
 worked with Eyal Oren (ActiveRDF) and Max Völkel
(RDF2Go, RDFReactor)
 Student research topic: simplifying RDF semantics



Motivation

Chapter 4

Publishing structured content

 Current Web: written for humans (mostly)
 Interesting information for machine agents exists
 Example: music events in Galway (date, performer, venue)
 Problem:
 web page mark-up does not explicitly encode information
 scraping of web pages is expensive:
 webpage may change without warning
 human intervention is necessary

5 of 28 Part 1: Motivation Benjamin Heitmann

A potential provider of event data


A potential consumer of event data


Possible solution

 Embed machine readable data on the same web page
 Benefits:
 write and publish once
 readableby humans and machine agents
 easy maintenance for publisher
 easy consuming of data after discovery
 Two approaches:
 Microformats:
– fixed vocabulary, not extendable and customisable
 RDFa
– all the benefits of RDF: flexible and customisable
– all the overhead of RDF: data model and formal semantics


A side note: Explaining “what do you
do?” at a party

 Quicker use case
 suitable for explaining “what do you actually do?”
 like, lets say, for the Christmas holidays

 What can Google do today?
 List Restaurants in Dublin
 Can Google also give you this information?
 Restaurants in Dublin, open on Thursday late, serving
Pepperoni Pizza for under 15 Euro
 RDFa (and the Semantic Web) could make this
possible!



The foundation: RDF in a nutshell

Chapter 10

Show of hands

 Who can answer these questions:

1.What is the RDF data model?

2.What is the formal semantics of RDF?

11 of 28 Part 2: RDF Foundation Benjamin Heitmann

RDF data model: A graph

 Graph with nodes and
directed arcs
 Graph consists of triples
 Each triple has a
subject, predicate and
object
 Contrast with other data
models:
 SQL: tables and relations
 XML: tree of nodes with
attributes


RDF data model: node types

1.Uniform Resource
Blank
Identifier (URI): URI Literal
Node
 basically like in the
browser location field
 globally unique Subject X X
2.Literal
 like a string
 can optionally have either
a data type or a language
Predicate X
tag
3.Blank Node
 place holder Object X X X
 only locally unique


An example with all three types


RDF formal semantics

 uses “model theory” to provide
“a globally coherent notion of meaning”
 provides basis for inference rules
 specifies the semantics of RDF
 Fundamental property:
open world semantics and monotonic reasoning
 Facts: I go to work from Monday to Friday.
 Question: Will I go to work on the weekend?
 Closed world semantics: no facts about the weekend ->
answer: no.
 Open world semantics: answer not possible
 Monotonic reasoning: adding new data always possible


Model theory example

 Three triples:
 <ex:a> <ex:b> <ex:c> .
 <ex:c> <ex:a> <ex:a> .
 <ex:c> <ex:b> <ex:a> .

 True with this
interpretation:


What to remember about RDF

 RDF is the foundation of
the Semantic Web
 defines the data model
 provides base layer
semantics
 other standards extend
these semantics (like RDF
Schema and OWL)
 domain ontologies
provide domain specific
semantics (like FOAF) on
top of RDF Schema



RDFa with examples

Chapter 18

What is RDFa?

 syntax for embedding an RDF graph in an XHTML
document
 uses XHTML attributes for expressing RDF properties
 properties about
 same page
 or external URI
 properties can reuse visible page content or be
invisible
 XHTML documents with RDFa are backwards
compatible
 GRDDL transformation to extract RDF exists

19 of 28 Part 3: RDFa Benjamin Heitmann

RDFa Example: Graph


RDFa Example: Source

<div xmlns:foaf=quot;http://xmlns.com/foaf/0.1/quot; about=quot;#mequot; rel=quot;foaf:knowsquot;>
<ul>
<li typeof=quot;foaf:Personquot;>
<a property=quot;foaf:namequot; rel=quot;foaf:homepagequot; href=quot;http://example.com/bobquot;>Bob</a>
</li>
<a property=quot;foaf:namequot; rel=quot;foaf:homepagequot; href=quot;http://example.com/evequot;>Eve</a>
</li>
<a property=quot;foaf:namequot; rel=quot;foaf:homepagequot; href=quot;http://example.com/manuquot;>Manu</a>
</li>
</ul>
</div>


What just happened?

 RDFa works a little bit like RDF/XML
 xmlns:foaf declares the FOAF namespace
 about=”#me” defines the subject of a triple
 rel=”foaf:knows” defines the predicate
 typeof=”foaf:Person” defines a resource type
 typeof without explicit URLs leads to blank nodes
 property=”foaf:name” uses the literal “Bob”
 rel=”foaf:homepage” uses the href

 That’s 12 triples for just 8 XML nodes


CURIEs versus URIs

 Notall of the 10 RDFa attributes can use URIs
 CURIE (Compact URI )
 example: foaf:Person
 curie := [ [ prefix ] ':' ] reference
 prefix is either a defined name space or the default
namespace
 URI (like http://dbpedia.org/resource/London)
 SafeCURIE
 CURIE in square brackets, example: [wiki:Biome]
 prevents ambiguities between URIs and CURIEs
 Idea: Subject and Object can be external, use URIs.
Predicate should be internal, so use CURIE.


Attributes for subjects

@about : URI or SafeCURIE
define a subject of a triple
@src : URI
a not clickable resource object, like a picture or multimedia
object


Attributes for predicates

@rel : CURIEs
express relationship between resources
@rev : CURIEs
express reverse relationship between resources
@property : CURIEs
express relationship between a subject and a literal


Attributes for objects

@resource : URI or SafeCURIE
an invisible resource object, that is not clickable
@href : URI
a resource object that is clickable like a link
@content : string
invisible literal object for a triple
@datatype : XML data for a literal
@typeof : specify class of a subject


Another RDFa example
<html xmlns=quot;http://www.w3.org/1999/xhtmlquot;
xmlns:cal=quot;http://www.w3.org/2002/12/cal/ical#quot;
xmlns:xsd=quot;http://www.w3.org/2001/XMLSchemaquot; >
<head><title>Jo's Friends and Family Blog</title></head>
<body>
<p typeof=quot;cal:Veventquot;>
I'm holding
<span property=quot;cal:summaryquot;>
one last summer Barbecue
</span>,
on
<span property=quot;cal:dtstartquot; content=quot;2007-09-16T16:00:00-05:00quot;
datatype=quot;xsd:dateTimequot;>
September 16th at 4pm
</span>.
</p>
</body>
</html>


Publishing and consuming RDFa

 Publishing RDFa
 use any CMS that has custom templates
 Drupal
 Wordpress (use pods plugin for wp 2.7)
 Consuming RDFa
 use GRDDL to convert any web page with RDFa to RDF/XML
 Operator Plug-in for Firefox

 Take a look at http://rdfa.info/wiki/Tools for more


Summary

 RDF provides the data model and formal semantics
for the Semantic Web
 RDFa embeds the data model in XHTML pages
 very flexible, can express any RDF graph
 lotsof nested tags
 can reuse existing visible content
 or can be completely invisible
 beware of the difference between URIs and CURIEs
 watch out for implicit blank nodes
 generic tools for publishing and consuming exist


RDFa: putting RDF on the Web

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