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title: ARQ - Basic Federated SPARQL Query

There are already ways to access remote RDF data. The simplest is to read a document which is an RDF graph and query it. Another way is with the SPARQL protocol which allows a query to be sent to a remote service endpoint and the results sent back (in RDF, or an XML-based results format or even a JSON one).

SERVICE is a feature of SPARQL 1.1 that allows an executing query to make a SPARQL protocol to another SPARQL endpoint.

Syntax

PREFIX : <http://example/>
PREFIX  dc:     <http://purl.org/dc/elements/1.1/>

SELECT ?a
FROM <mybooks.rdf>
{
  ?b dc:title ?title .
  SERVICE <http://sparql.org/books>
     { ?s dc:title ?title . ?s dc:creator ?a }
}

Algebra

There is an operator in the algebra.

(prefix ((dc: <http://purl.org/dc/elements/1.1/>))
  (project (?a)
    (join
      (BGP [triple ?b dc:title ?title])
      (service <http://sparql.org/books>
          (BGP
            [triple ?s dc:title ?title]
            [triple ?s dc:creator ?a]
          ))
      )))

Performance Considerations

This feature is a basic building block to allow remote access in the middle of a query, not a general solution to the issues in distributed query evaluation. The algebra operation is executed without regard to how selective the pattern is. So the order of the query will affect the speed of execution. Because it involves HTTP operations, asking the query in the right order matters a lot. Don't ask for the whole of a bookstore just to find a book whose title comes from a local RDF file - ask the bookshop a query with the title already bound from earlier in the query.

Controlling SERVICE requests.

The SERVICE operation in a SPARQL query may be configured via the Context. The values for configuration can be set in the global context (accessed via ARQ.getContext()) or in the per-query execution context.

The prefix srv: is the IRI <http://jena.hpl.hp.com/Service#>.

Symbol	Usage	Default
srv:queryTimeout	Set timeouts	none
srv:queryCompression	Enable use of deflation and GZip	true
srv:queryClient	Enable use of a specific client	none
srv:serviceContext	Per-endpoint configuration	none

srv:queryTimeout

As documented above.

srv:queryCompression

Sets the flag for use of deflation and GZip.

Boolean: True indicates that gzip compressed data is acceptable.

srv:queryClient

Enable use of a specific client

Provides a slot for a specific HttpClient for use with a specific SERVICE

srv:serviceContext

Provides a mechanism to override system context settings on a per URI basis.

The value is a Map<String,Context> where the map key is the URI of the service endpoint, and the Context is a set of values to override the default values.

If a context is provided for the URI, the system context is copied and the context for the URI is used to set specific values. This ensures that any URI specific settings will be used.