SPARQL 1.1 Federated Query Extension

The SERVICE keyword instructs a federated query processor to invoke a portion of a SPARQL query against a remote SPARQL endpoint. This section presents examples of how to use the SERVICE keyword. The following sections define the syntax and semantics of this extension.

Simple query to a remote SPARQL endpoint

This example shows how to query a remote SPARQL endpoint and join the returned data with the data from the local RDF Dataset. Consider a query to find the names of the people we know. Data about the names of various people is available at the http://people.example.org/sparql endpoint:

  @prefix foaf:  <http://xmlns.com/foaf/0.1/> .
  @prefix : <http://example.org/> .

  :people15  foaf:name     "Alice" .
  :people16  foaf:name     "Bob" .
  :people17  foaf:name     "Charles" .
  :people18  foaf:name     "Daisy" .

and one wants to combine with a local FOAF file

http://example.org/myfoaf.rdf that contains the single triple:

<http://example.org/myfoaf/I> <http://xmlns.com/foaf/0.1/knows>  <http://example.org/people15> . 

Query:

PREFIX foaf:   <http://xmlns.com/foaf/0.1/>
SELECT ?name
FROM <http://example.org/myfoaf.rdf>
WHERE
{
  <http://example.org/myfoaf/I> foaf:knows ?person .
  SERVICE <http://people.example.org/sparql> { 
    ?person foaf:name ?name . } 
}

This query, on the data above, has one solution:

Query Result:

name
"Alice"

SPARQL query with OPTIONAL to two remote SPARQL endpoints

Imagine we want to query people and optionally obtain their interests and the names of people they know. Imagine for instance, two endpoints containing data about people:

Data in the default graph at remote SPARQL endpoint: http://people.example.org/sparql

  @prefix foaf:  <http://xmlns.com/foaf/0.1/> .
  @prefix : <http://example.org/> .

  :people15  foaf:name     "Alice" .
  :people16  foaf:name     "Bob" .
  :people17  foaf:name     "Charles" .
  :people17  foaf:interest     <http://www.w3.org/2001/sw/rdb2rdf/> .

and data in the default graph the remote SPARQL endpoint: http://people2.example.org/sparql

  @prefix foaf:  <http://xmlns.com/foaf/0.1/> .
  @prefix : <http://example.org/> .

  :people15  foaf:knows    :people18 . 
  :people18  foaf:name     "Mike" .
  :people17  foaf:knows    :people19 . 
  :people19  foaf:name     "Daisy" .

Query:

PREFIX foaf:   <http://xmlns.com/foaf/0.1/>
SELECT ?person ?interest ?known
WHERE
{
  SERVICE <http://people.example.org/sparql> { 
    ?person foaf:name ?name .  
    OPTIONAL { 
      ?person foaf:interest ?interest .
      SERVICE <http://people2.example.org/sparql> { 
        ?person foaf:knows ?known . } }
  }    
}

This query, on the data above, has three solutions:

Query Result:

person	interest	known
"Alice"
"Bob"
"Charles"	<http://www.w3.org/2001/sw/rdb2rdf/>	<http://example.org/people19>

Notice that in the query above there is a nested SERVICE in the OPTIONAL clause. This query requires the SPARQL query service at http://people.example.org/sparql to support basic federated query.

Service Execution Failure

The execution of a SERVICE pattern may fail due to several reasons: the remote service may be down, the service IRI may not be dereferenceable, or the endpoint may return an error to the query. Normally, under such circumstances the invoked query containing a SERVICE pattern fails as a whole. Queries may explicitly allow failed SERVICE requests with the use of the SILENT keyword. The SILENT keyword indicates that errors encountered while accessing a remote SPARQL endpoint should be ignored while processing the query. The failed SERVICE clause is treated as if it had a result of a single solution with no bindings.

In the following query the SILENT keyword is present. If the remote SPARQL endpoint is not available because the SPARQL endpoint does not exist, it is down or it is not accessible the query will return a solution sequence of one empty solution mapping. If the SILENT keyword is not present, the query will stop and return the error.

Data in <http://people.example.org/sparql> endpoint:

  <http://example.org/people15>  <http://xmlns.com/foaf/0.1/name>     "Charles" .

Query:

PREFIX foaf:   <http://xmlns.com/foaf/0.1/>
SELECT ?name
WHERE
{
  SERVICE SILENT <http://people.example.org/sparql> { 
    <http://example.org/people15> foaf:name ?name . }
}

Query result if an error occurs while querying the remote SPARQL endpoint:

name

Interplay of SERVICE and VALUES (Informative)

SPARQL 1.1 Query includes the VALUES clause (VALUES), which can be used to provide an unordered solution sequence that is joined with the results of the query evaluation. Implementers of SPARQL 1.1 Federated Query may use the VALUES clause to constrain the results received from a remote endpoint based on solution bindings from evaluating other parts of the query.

The following example shows how SERVICE and VALUES can work together. Suppose a query that asks for all instances of foaf:Person in the default graph and also their known people in the remote endpoint http://example.org/sparql:

Data in the default graph:

  @prefix foaf:  <http://xmlns.com/foaf/0.1/> .
  @prefix : <http://example.org/> .

  :a a foaf:Person  ;
     foaf:name     "Alan" ;
     foaf:mbox;     "alan@example.org" .
  :b a foaf:Person  ;
     foaf:name     "Bob" ;
     foaf:mbox     "bob@example.org" .

and data in the default graph the remote SPARQL endpoint http://example.org/sparql:

  @prefix foaf:  <http://xmlns.com/foaf/0.1/> .
  @prefix : <http://example.org/> .

  :a  foaf:knows     :b . 
  :b  foaf:knows     :c .
  :c  foaf:knows     :a .
  :a  foaf:interest  "SPARQL 1.1 Basic Federated Query" . 
  :b  foaf:interest  "SPARQL 1.1 Query" .
  :c  foaf:interest  "RDB2RDF Direct mapping" .

Query:

PREFIX foaf:   <http://xmlns.com/foaf/0.1/>
SELECT ?s
{
  ?s a foaf:Person .
  SERVICE <http://example.org/sparql> {?s foaf:knows ?o }
}

When the original query is executed naively, with an unconstrained service call the endpoint may return more results than necessary. It may also happen that the SPARQL endpoint will not return all of them. Many existing SPARQL endpoints have restrictions in the number of results they return and may miss the ones matching subjects ?s from the local default graph. Thus, an implementation of a query planner for federated queries may decide to decompose the query into two queries instead, where first the bindings from the local default graph are evaluated:

Query:

PREFIX : <http://example.org/>
PREFIX foaf:   <http://xmlns.com/foaf/0.1/>
SELECT ?s
{
  ?s a foaf:Person
} 

This query, on the data above, has two solutions:

Query Result:

s
<http://example.org/a>
<http://example.org/b>

Next, dispatch to the remote endpoint <http://example.org/sparql> a constrained query with the solutions for ?s:

PREFIX foaf:   <http://xmlns.com/foaf/0.1/>
PREFIX : <http://example.org/>
SELECT * {?s foaf:knows ?o } VALUES (?s) { (:a) (:b) }

The query process involving SERVICE limits the data returned to the data it needs for the overall query:

Query:

PREFIX foaf:   <http://xmlns.com/foaf/0.1/>
SELECT ?s ?o
{
  ?s a foaf:Person
  SERVICE <http://example.org/sparql> {?s foaf:knows ?o }
} 

This query, on the data above using VALUES, has the expected two solutions to the overall query:

Query Result:

s	o
<http://example.org/a>	<http://example.org/b>
<http://example.org/b>	<http://example.org/c>