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Virtuoso SPARQL Query Service

Introduction

The Virtuoso SPARQL query service implements the SPARQL Protocol for RDF (W3C Working Draft 25 January 2006) providing SPARQL query processing for RDF data available on the open internet.

The query processor extends the standard protocol to provide support for multiple output formats. At present this uses additional query parameters.

Service Endpoint

Virtuoso reserves the path '/sparql/' and a synonym path '/SPARQL/' for SPARQL service.

In the current implementation, Virtuoso defines virtual directories for HTTP requests that come to the port specified as 'ServerPort' in the '[HTTPServer]' section of Virtuoso configuration file and refer to one of these two path strings. So if the Virtuoso installation on host example.com listens for HTTP requests on port 8080 then client applications should use the 'service endpoint' string equal to 'http://example.com:8080/sparql/'.

Both GET and POST requests are supported by both server and client. The server recognizes the 'Accept: ...' line of request header to find MIME types preferred by the connected client and adjust the output mode of the response. The client chooses between GET and POST automatically, using the length of query text as a criterion.

If the SPARQL endpoint is accessed without any url and query entered, will be shown a page with interactive query form.

Request Methods

Method Supported? Notes
GET Yes short queries are sent in GET mode
POST Yes Queries longer than 1900 bytes are POST-ed
DELETE No
PUT No

Functions

The SPARQL client can be invoked by three similar functions.

Function Notes
DB.DBA.SPARQL_REXEC behaves like DBA.SPARQL_EVAL, but executes the query on the specified server. The procedure does not return anything. Instead, it creates a result set.
DB.DBA.SPARQL REXEC_TO_ARRAY behaves like DBA.SPARQL_EXEC_TO_ARRAY (), but executes the query on the specified server. The function return a vector of rows, where every row is represented by a vector of values of fields.
DB.DBA.SPARQL_REXEC_WITH_META has no local 'SPARQL_EVAL' analog. It produces not only an array of result rows but also array of metadata about result set in a format used by the exec () function.

Request Parameters

Parameter Notes Required? Occurrence
service service URI such as 'http://demo.openlinksw.com/sparql/' Yes
query text of the query Yes
dflt_graph default graph URI (string or NULL) No
named_graphs vector of named graphs (or NULL to not override what's specified in the query No
req_hdr additional HTTP header lines that should be passed to the service; 'Host: ...' is most popular No
maxrows limit on numbers of rows that should be returned (actual size of result set may differ) No
bnode_dict dictionary of known blank node names, or NULL for usual loading No

Response Codes

Response Format

All standard MIME types of SPARQL Protocol are supported.

Mimetype Supported for result of Query
'application/sparql-results+xml' SELECT
'application/sparql-results+xml' ASK
'application/rdf+xml' CONSTRUCT
'application/rdf+xml' DESCRIBE
'text/rdf+n3' CONSTRUCT
'text/rdf+n3' DESCRIBE

If the HTTP header returned by the remote server does not contain 'Content-Type' line, the MIME type may be guessed from the text of the returned body. Error messages returned from the service are returned as XML documents, using the mime type application/xml. The documents consist of a single element containing an error message.

Additional Response Formats -- SELECT

Use the format parameter to select one of the following alternate output formats:

Format Value Description Mimetype
HTML HTML document containing query summary and tabular results text/html
JSON JSON serialization of results. Conforms to the draft specification Serializing SPARQL Query Results in JSON application/sparql-results+json
Javascript Javascript serialization of results. Generates an HTML table with the CSS class sparql. The table contains a column indicating row number and additional columns for each query variable. Each query solution contributes one row of the table. Unbound variables are indicated with a non-breaking space in the appropriate table cells. application/javascript
XML application/sparql-results+xml
TURTLE text/rdf+n3

Examples

There is also a live Virtuoso Demo Server instance available at all times for experimentation and demonstrations. It includes a collection of saved queries demonstrating Virtuoso's implementation of the DAWG's SPARQL test-suite; a collection of SPARQL query language use-cases that enables interactive and simplified testing of a RDF Triple Store's SPARQL implementation.

Implementation Notes

This service has been implemented using OpenLink Virtuoso.

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