Linked Data

What is Linked Data?

Linked Data is about using the Web to connect data.

The Wikipedia page for Linked Data is http://en.wikipedia.org/wiki/Linked_Data

A great summary of specifications related to linked data is available at http://linkeddata-specs.info/

How is this Connected to the OpenUp Platform?

One of the underlying data formats within the OpenUp Platform is RDF (see below). We store the RDF in an RDF store and make this available to query using SPARQL (see below). The results from SPARQL queries can be returned in various formats (see Content Negotiation below).

Resource Description Framework (RDF)

A very good place to start reading about RDF is the W3 primer available at http://www.w3.org/TR/2004/REC-rdf-primer-20040210/

RDF is described on the W3 website at http://www.w3.org/RDF/

The Wikipedia page for RDF is http://en.wikipedia.org/wiki/Resource_Description_Framework

voiD

voiD is a vocabulary and a set of instructions that enables the discovery and usage of linked datasets. The OpenUp platform can store voiD descriptions, either separately on as part of other data.

More detail on voiD can be found at http://vocab.deri.ie/void/guide

SPARQL

SPARQL is a query language for RDF. It is analagous with using SQL to query a relational database.

The W3 SPARQL specification can be found at http://www.w3.org/TR/rdf-sparql-query/

The Wikipedia page for SPARQL is http://en.wikipedia.org/wiki/SPARQL

Results Formats

Depending up on the query type the results can be returned in one of the following formats:

Plain text
SPARQL-XML (http://www.w3.org/TR/rdf-sparql-XMLres/)
JSON (http://www.json.org/)
RDF/XML (for CONSTRUCT queries)
Turtle (http://en.wikipedia.org/wiki/Turtle_(syntax))

How Do You Query an Endpoint?

To actually query one of the SPARQL endpoints you simply need to do one of the following requests.

HTTP Method	Parameter Name	Purpose	Notes
GET	query	Contains the text of the SPARQL query	The text needs to be encoded using application/x-www-form-urlencoded format
	output	Contains the details on the required return format	Valid values are: text, sparql, json, rdfxml, turtle
POST	query	Contains the text of the SPARQL query	The text needs to be encoded using application/x-www-form-urlencoded format
	output	Contains the details on the required return format	Valid values are: text, sparql, json, rdfxml, turtle

Content Negotiation

The system utilises content negotiation by default. Content negotiation allows an application to request the format of the results it needs as part of the HTTP request.

Information on content negotiation can be found at http://en.wikipedia.org/wiki/Content_negotiation

The available formats are given in the table below:

Required Format	Accept Header MIME Type
SPARQL/XML	application/sparql-results+xml
RDF/XML	application/rdf+xml
JSON	application/sparql-results+json
Turtle	application/turtle

Details on SPARQL/XML are available at http://www.w3.org/TR/2007/CR-rdf-sparql-XMLres-20070925/

Details on JSON results are available at http://www.w3.org/TR/rdf-sparql-json-res/

How SPARQL Compliant is the OpenUp System?

The OpenUp platform implements all of SPARQL 1.0.