Linked data: accessibility

The following post is an excerpt from my thesis entitled Linked open data for public sector information.
Linked data requires using dereferenceable HTTP URIs that serve as open access points to data. Resolution of linked data URIs may be either implemented by serving static files or by generating resource representations on the fly.
Linked data may be published in static files in one of the RDF serializations described in the previous post about RDF. This approach is used mainly for serving RDF vocabularies and ontologies, transfer of datasets for local batch processing, or for files with embedded RDF. Serving static files is easy to implement, however, their content is fixed and difficult to manipulate and update. To take advantage of the flexible nature of linked data on-demand, dynamically generated RDF representations may be served instead. One option for this approach is to use wrappers for dynamic data extraction from non-RDF data sources. For example, D2R Server allows to expose relational databases as RDF through a pre-defined mapping.
However, to reap the full benefits of RDF a triple store should be used to store the data. Triple store is a database optimized for storage and retrieval of RDF data. To publish data from a triple store SPARQL endpoints are used as the interfaces users interact with. The endpoints expose an interface defined by the SPARQL Protocol for RDF, which allows to query or manipulate data and serves the query results in XML via HTTP. In order to comply with linked data principles publishers should use front-end applications that implement dereferencing and content negotiation. A common way how to expose RDF as linked data is through lightweight SPARQL wrappers that dereference URIs to concise bounded descriptions [1] of the requested resources, the descriptions of which they retrieve via SPARQL queries. Example implementations of linked data front-ends include Pubby or Graphite.
To ease the transition to the use of linked data for web developers specification of Linked Data API was created. Linked Data API is a framework for more user-friendly APIs interacting with linked data in a way that follows the guidelines of REST and uses simple data formats, such as JSON. Among the example implementations of this framework are Puelia and Elda.

References

1. STICKLER, Patrick. CBD: concise bounded description [online]. W3C Member Submission. June 3^rd, 2004 [cit. 2012-04-23]. Available from WWW: http://www.w3.org/Submission/CBD/