Title: ARQ - Querying Remote SPARQL Services

SPARQL is a
[query language](http://www.w3.org/TR/sparql11-query/) and a
[remote access protocol](http://www.w3.org/2001/sw/DataAccess/proto-wd/).
The remote access protocol can be used with plain HTTP or over
[SOAP](http://www.w3.org/TR/soap12-part0/).

See [Joseki](http://www.joseki.org/) for an implementation of an
RDF publishing server, using the SPARQL protocol (HTTP and SOAP).
Joseki uses ARQ to provide SPARQL query access to Jena models,
including Jena persistent models.

ARQ includes a query engine capable of using the HTTP version. A
version using SOAP is include in Joseki.

## From your application

The `QueryExecutionFactory` has methods for creating a
`QueryExecution` object for remote use.
`QueryExecutionFactory.sparqlService`

These methods build a query execution object that uses the query
engine in `com.hp.hpl.jena.sparql.engine.http`.

The remote request is made when the `execSelect`, `execConstruct`,
`execDescribe` or `execAsk` method is called.

The results are held locally after remote execution and can be
processed as usual.

## From the command line

The [`arq.sparql` command](cmds.html#arq.sparql) can issue remote
query requests using the `--service` argument:

    java -cp ... arq.query --service 'http://host/service' 'SELECT ?s WHERE {?s [] []}'

This takes a URL that is the service location.

The query given is parsed locally to check for syntax errors before
sending.

## Firewalls and Proxies

Don't forget to set the proxy for Java if you are accessing a
public server from behind a blocking firewall. Most home firewalls
do not block outgoing requests; many corporate firewalls do block
outgoing requests.

If, to use your web browser, you need to set a proxy, you need to
do so for a Java program.

Simple examples include:

    -DsocksProxyHost=YourSocksServer

    -DsocksProxyHost=YourSocksServer -DsocksProxyPort=port

    -Dhttp.proxyHost=WebProxy -Dhttp.proxyPort=Port

This can be done in the application
*if it is done before any network connection are made*:

       System.setProperty("socksProxyHost", "socks.corp.com");

Consult the Java documentation for more details. Searching the web
is also very helpful.


[ARQ documentation index](index.html)