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.


PREFIX : <http://example/>
PREFIX  dc:     <>

FROM <mybooks.rdf>
  ?b dc:title ?title .
     { ?s dc:title ?title . ?s dc:creator ?a }


There is an operator in the algebra.

(prefix ((dc: <>))
  (project (?a)
      (BGP [triple ?b dc:title ?title])
      (service <>
            [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 arq: is <>.

Symbol Java Constant Default
arq:httpServiceAllowed ARQ.httpServiceAllowed true
arq:httpQueryClient ARQ.httpQueryClient System default.
arq:httpServiceSendMode `ARQ.httpServiceSendMode unset


This setting can be used to disable execution of any SERVICE request in query. Set to “false” to prohibit SERVICE requests.


The HttpClient object to use for SERVICE execution.


The HTTP operation to use. The value is a string or a QuerySendMode object.

String settings are:

Setting Effect
“POST” Use HTTP POST. Same as “asPost”.
“GET” Use HTTP GET unconditionally. Same as “asGetAlways”.
“asGetAlways” Use HTTP GET.
“asGetWithLimitBody” Use HTTP GET upto a size limit (usually 2kbytes).
“asGetWithLimitForm” Use HTTP GET upto a size limit (usually 2kbytes), and use a HTML form for the query.
“asPostForm” Use HTTP POST and use an HTML form for the query.
“asPost” Use HTTP POST.

Old Context setting

Old settings are honored where possible but should not be used:

The prefix srv: is the IRI <>.

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


As documented above.


Sets the flag for use of deflation and GZip.

Boolean: True indicates that gzip compressed data is acceptable.


Enable use of a specific client

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


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.