Linked Data

Web == One huge DB A seminar on Linked Data and Semantic Web

The Web of Linked Documents

Issues of Linked Documents

Loosely structured Data
Unstructured hyperlinks
Disconnected Data

What is Semantic Web? “ The first step is putting data on the Web in a form that machines can naturally understand, or converting it to that form. This creates what I call a Semantic Web – a web of data that can be processed directly or indirectly by machines.” ~ Tom Heath

The Web of Linked Data

The Semantic Web

This converts the whole web into a single database.
Designed for machines and humans.
Links between things-things and things-and its description.
Encourages reuse, reduces redundancy
Enables to add more value to data over the network.

Linked Data Technology Stack

URIs
HTTP
RDF (Resource Description Framework)
RDFS/OWL

URIs – Not Just for Web Pages

“A Uniform Resource Identifier (URI) provides a simple and extensible means for identifying a resource.” -- RFC 3986
Examples

http://dbpedia.org/resource/Tim_Berners-Lee
http://dbpedia.org/resource/Model_Engieering_College
http://sws.geonames.org/4671654/

HTTP

Data access mechanism
Using http:// URIs to identify things allows people to look these things up

RDF: Resource Description Framework

Data format for describing things and their interrelations.
Descibes relations based on triples; subject-predicate-object

The RDF Data Model

subject -> predicate -> object Tom -> worksFor -> Talis Talis -> basedIn -> Birmingham <uri> -> <uri> -> <uri> eg: <http://dbpedia.org/resource/Talis_Group> <http://xmlns.com/foaf/0.1/based_near> <http://sws.geonames.org/3333125/>

Data Merging with RDF

Consider a profile of Richard

This can be naturally merged

This is linked Data

Data discovery in RDF

Advantages of RDF

Information in different sources merge naturally
Clients can look up every URI in and RDF graph over the Web to get addictional info.
Easy description of relations with the help of RDFS

Linked Data principles

Use HTTP URIs
Provide useful information in RDF
Include RDF links to other URIs

Why publish Linked Data?

Ease of discovery
Ease of consumption

standards-based data sharing

Reduced redundancy
Added value

build ecosystems around your data/content

The Linking Open Data Project

Community project with W3C support
Take existing open data sets
Make them available on the Web in RDF
Interlink them with other data sets
Began early 2007

LOD Cloud as on November 2007

LOD Cloud as on March 2008

LOD Cloud as on March 2009

The latest LOD cloud as on July 2009

Setting RDF links

RDF links enable LD browsers and crawlers to discover additional data
The application domain determine the RDF properties used in predicate section.

eg: for describing people : foaf:knows, foaf:based_near
for showing another same data source: owl:sameAs

We can set the RDF links manually and generate links automatically using algorithms.

Generating RDF links manually

Identify the datasets you want to link to.
Search for the right URIs to link to using search interfaces if provided. Or using LD browsers
Use services like Uriqr to search for existing URIs and choose the most popular one suiting your need.

Auto generating RDF links

Used on Large datasets
Two major methods

Pattern Based algorithm

Searches the URIs on various data sets for similar patterns and link generate the RDF links

Property based algorithms

Searches additional informations of the resource at the resource end points rather than the URIs alone.

Eg: interlinking Dbpedia and Geonames.

Now how to put it on Linked Data

Serve as static files

Simplest way is to produce static RDF files and upload them to your webserver.

When creating FOAF profiles, and RDF vocabularies
RDF is generated by some other software.

Serving RDB on

The conventional DB is untouched
A mapping is done to produce the RDF files.
Tools like D2R server and OpenLink Virtuoso is used.

RDB to Linked Data

SPARQL

S PARQL P rotocol a nd R DF Q uery L anguage
Retrieves data from RDF
Similar to SQL syntax

Eg:

PREFIX foaf: < http://xmlns.org/foaf/0.1/ >

SELECT ?acquaintance FROM < http://w3.org/People/Berners-Lee/card > WHERE { < http://w3.org/People/Berners-Lee/card#i > foaf:knows ?acquaintance . FILTER isURI(?acquaintance) }

Linked Data Applications