Exposing RDF Data as a Data Service

This tutorial will guide you on how to expose an RDF datasource as a data service using WSO2 Data Services Server. To demonstrate this feature, we will use a sample RDF file that is shipped with WSO2 DSS by default. Given below are the details of this datasource: 

The RDF file (Movies.rdf), stored in the <DSS_HOME>/samples/resources/ folder contains data about some popular movies. Each movie data has the following sub elements: "title", "director", "year", "genre" and "actor".

See the following topics for instructions:

Start the Create New Data Service wizard

Now, let's start creating the data service from scratch:

1. Log in to the management console of DSS using the following URL on your browser:  "https://10.100.5.65:9443/carbon/".

2. Click Create under the Data Service menu to open the Create Data Service window.

3. Enter RDF as the data service name as shown below. Leave the default values for the other fields.
   

4. Click Next to go to the Datasources screen.
   

Add and RDF file as the datasource

You can add an RDF file as the datasource as explained below.

1. Click Add New Datasource and enter values as shown below.
   
2. Follow the instructions below to fill the datasource details.
   • Datasource ID: Enter RDF as the ID for the datasource. 
   • Datasource Type: Select RDF as the datasource type from the list. You can then specify the location of your RDF file: ./samples/resources/Movies.rdf
   • RDF File Location: Enter the path to the RDF file. In this tutorial we are using a sample RDF file that is shipped with your product. The file location is ./samples/resources/Movies.rdf.
3. Save the datasource.
4. Click Next to go to the Queries screen.

Define a query for the datasource

Now let's start writing a query for getting data from the Movies.rdf file. The query will specify the data that should be fetched by this query, and the format that should be used to display data when the query is invoked.

1. Click Add New Query to open the Add New Query screen.
2. Enter the following values:
   • Query ID: Enter GetMoviesbyGenre as the query ID.
   • Datasource: Select the datasource for which you are going to write a query. Select the RDF datasource that you created previously.
     

   • SPARQL: In this field, enter the sparql query describing the data that should be retrieved from the RDF datasource. We will use the following query:
     

     PREFIX cd: <http://www.popular.movies/cd#>
      
     SELECT ?title ?director ?year ?genre ?actor 
     WHERE {   
           ?movie cd:title ?title. 
           ?movie cd:director ?director. 
           ?movie cd:year ?year. 
           ?movie cd:genre ?genre. 
           ?movie cd:actor ?actor.
     }

Create input mapping

The input mapping section is used to specify the type of input that should be given in order to get the output result.

1. Click Add New Input Mapping to start creating the input mapping.
2. We will create an input mapping for 'genre', which will allow us to query for data based on the genre of the movie. Enter the values as shown below.
   
3. The new input mapping will be listed as shown below.
   
   

Create output mapping

Now let's specify output mappings to specify how the data fetched from the query should be displayed. We will create output mappings for all the types of data in the RDF file:  title, director, year, genre and actor. Follow the steps given below.

1. In the Output type field, specify the format in which the query results should be presented. You can select XML, JSON or RDF. We will use XML for this tutorial.
2. In the Grouped by element field, specify a grouping for all the output mappings. This will be the XML element that will group the query result. Enter Movies in this field. 
3. In the Row Name field, specify the XML element that should group each individual result. Enter Movie in this field.
4. Click Add New Output Mapping to start creating the output mapping for the title column. Enter values as shown below:
   
5. Click Add to save the output mapping. You will now have one output mapping listed for the GetMoviesbyGenre query as shown below.
   

6. Now, add the following output mappings:

   Mapping Type	Element Name	Datasource Type	Datasource Column Name	Parameter Type	Schema Type
   Element	title	Column	title	SCALAR	string
   Element	director	Column	director	SCALAR	string
   Element	year	Column	year	SCALAR	string
   Element	genre	Column	genre	SCALAR	string
   Element	actor	Column	actor	SCALAR	string

Define an operation to invoke the query

Follow the steps given below.

1. Once you have defined the query, click Next to go to the Operations screen.
   
2. Click Add New Operation and enter the following values:
   • Operation Name: Enter GetMoviesbyGenre as the operation name.
   • Query ID: Select the GetMoviesbyGenre query in this field.
   
3. Save the operation. 

Finish creating the data service 

Once you have defined the operation, click Finish to complete the data service creation process. You will now be taken to the Deployed Services screen, which shows all the data services deployed on the server.

Invoking your data service 

You can try the data service you created by using the TryIt tool that is in your product by default. 

1. Go to the Deployed Services screen.
2. Click the Try this service link for the Excel data service. The TryIt Tool will open with the RDF service.
   
3. Enter Drama as the genre: <xs:Genre xmlns:xs="http://ws.wso2.org/dataservice">Drama</xs:Genre>
4. Select the GetMoviesbyGenre operation and click Send.

5. The following XML response will be published on the TryIt tool:

   Movies xmlns="http://ws.wso2.org/dataservice">
      <Movie>
         <title>Ratatouille</title>
         <director>Jan Pinkava</director>
         <year>2007</year>
         <genre>Drama</genre>
         <actor>Teddy Newton</actor>
      </Movie>
      <Movie>
         <title>Catch Me If You Can</title>
         <director>Steven Spielberg</director>
         <year>2002</year>
         <genre>Drama</genre>
         <actor>Antoine Drolet Dumoulin</actor>
      </Movie>
      <Movie>
         <title>The Spectacular Now</title>
         <director>James Ponsoldt</director>
         <year>2013</year>
         <genre>Drama</genre>
         <actor>Andre Royo</actor>
      </Movie>
      <Movie>
         <title>Adventureland</title>
         <director>Greg Mottola</director>
         <year>2009</year>
         <genre>Drama</genre>
         <actor>Ryan Mcfarland</actor>
      </Movie>
      <Movie>
         <title>Begin Again</title>
         <director>John Carney</director>
         <year>2014</year>
         <genre>Drama</genre>
         <actor>Eric Burton</actor>
      </Movie>
      <Movie>
         <title>Flipped</title>
         <director>Rob Reiner</director>
         <year>2010</year>
         <genre>Drama</genre>
         <actor>Michele Messmer</actor>
      </Movie>
      <Movie>
         <title>Beginners</title>
         <director>Mike Mills</director>
         <year>2011</year>
         <genre>Drama</genre>
         <actor>Seth T Walker</actor>
      </Movie>
      <Movie>
         <title>Bridget Jones's Diary</title>
         <director>Cilla Ware</director>
         <year>2001</year>
         <genre>Drama</genre>
         <actor>John Clegg</actor>
      </Movie>
   </Movies>