Category: Java

Okey so this is the fun part that I promised to write about :D. I managed to cook up a use-case to demonstrate RDF querying and making use of the semantic data. The data that I am using for querying, is the rdf data sources available in the UK data.gov site. With some analysis I figured out that this task can be fundamentally archived using the combination of Mashup and Gadget Technologies. My choice of tools were WSO2 Mashup Server and WSO2 Gadget Server for their great flexibility and of cause for other obvious reasons :D. However the Mashup Server does not natively support RDF data retrieval, hence I had to do some work to get such functionality integrated. The great fact about the mashup server is its extensibility, the concept of host objects and the ability to write custom host objects and its pluggable nature comes handy in such cases. The high level architecture of what I am trying to achieve is as follows.

To implement the above architecture with the tools at hand I created a custom host object that can be plugged to the Mashup Server. When dealing with semantic web related tasks and RDF data handling HP’s Jena java library comes in handy. With the use of Jena-ARQ (for SPARQL) api I managed to get the host object working with few lines of code.

.....
            Dataset dataSet = DatasetFactory.create(sparqlObject.rdfDataSource);
            // Create a new query form a given user query
            String queryString = sparqlObject.spaqrlQuery;
            Query query = QueryFactory.create(queryString);
            QueryExecution qe = QueryExecutionFactory.create(query, dataSet);
            ResultSet results = qe.execSelect();
.....
           resultString = ResultSetFormatter.asXMLString(results);
..... OR.....
           ByteArrayOutputStream bos = new ByteArrayOutputStream();
           ResultSetFormatter.outputAsJSON(bos, results);

With the host object in place, the next task was to create a Mashup in-order to query the rdf data with a given source (EndPoint or data source). The javascript service (Mashup) is created to serve this purpose, where the consumer can specify the RDF endpoint or the data source with the SPARQL query and retrieve the dataset in XML or JSON.

.....
function RdfDocQueryService(rdfDataSource, rdfQuery, resultType) {
   var sparqlObj = new SparqlHostObject();
   sparqlObj.rdfDataSource = rdfDataSource;
   sparqlObj.spaqrlQuery = rdfQuery;
   sparqlObj.resultType = resultType;
   return new XML(sparqlObj.getDataFromRdfSource());
}

Finally to bind everything together, lets try querying some data. My example usecase is to use the query at N2 blog to retrieve traffic monitoring points in UK roads. The query to retrieve the data set as follows,

#List the uri, latitude and longitude for road traffic monitoring points on the M5
PREFIX road:
PREFIX rdf:
PREFIX geo:
PREFIX wgs84:
PREFIX xsd:
SELECT ?point ?lat ?long WHERE {
  ?x a road:Road.
  ?x road:number "A4"^^xsd:NCName.
  ?x geo:point ?point.
  ?point wgs84:lat ?lat.
  ?point wgs84:long ?long.
}

To visualize these points I have created a gadget with the aid of Google Maps api. This gadget can be hosted in the Gadget Server, where it can dynamically retrieve traffic monitoring points for each road in the UK and display them in the map as follows.