Extensions in ARQ

There are several ways to extend ARQ:

Value Functions - additional operations in FILTERS
Graph matching extensions - addition graph matching operations
DESCRIBE handlers
New query engines for performance in special circumstances
Support for finding blank nodes

Any system that implements the Jena Graph API or Jena Model API can be queried.

Functions

SPARQL allows custom functions in filter expressions so that queries can be used on domain-specific data. SPARQL defines a function by URI (or qname) in FILTER expressions. ARQ provides a function library and supports application-provided functions. There is a mechanism for dynamically loading.

Function Library

Function name	Description
`jfn:langeq(literal, lang)`	Compare the language tag of the literal with a language tag. Comparison reflects language tag rules so "foo"@en matches "EN-gb".
`jfn:now()`	Current time. Actually, the fixed time the query started.
`jfn:bnode(?x)`	Return the blank node label if ?x is a blank node.

More functions will be added. Contributions welcome.

Function Registry

@@add details - see javadoc for now.

Dynamic Functions

URIs for functions in the (fake) URI scheme java: are dynamically loaded. The class name forms the scheme specific part of the URI.

The ARQ function library uses this mechanism. The namespace of the ARQ function library is <java:com.hp.hpl.jena.query.function.library.> (note the trailing dot).

PREFIX jfn: <java:com.hp.hpl.jena.query.function.library.>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>

SELECT ?v
{  ?x rdfs:label ?v .
    FILTER jfn:langeq(?v, "en" )
}

Matcher Extensions

ARQ also provides the ability to write custom graph matching extensions. This not part of SPARQL; the application will need to use the ARQ query language (a superset of SPARQL). Unlike a value function, a matcher extension can set variables and generate multiple solutions.

EXT q:name(?a, ?b, ?c)

@@ to be written. Extension is loaded like functions. Likely to change.

DESCRIBE handlers

The DESCRIBE result form in SPARQL does not define an exact form of RDF to return. Instead, it allows the server or query processor to return what it considers to be an appropriate description of the resources located. This description will be specific to the domain, data modelling or application.

ARQ comes with one built-in handler which calculates the blank node closure of resources found. While suitable for many situations, it is not general (for example, a FOAF file usually consists of all blank nodes). ARQ allows the application to replace or add handlers for producing DESCRIBE result forms.

Application-specific handlers can be added to the DescribeHandlerRegistry. The handler will be called for each resource (not literals) identified by the DESCRIBE query.

Blank Node Labels

URIs from with scheme name "_" (which is illegal) are created as blank node labels for directly accessing a blank node in the queried graph or dataset. This are constant terms in the query - not unnamed variables. Do not confuse these with the standard qname notation for blank nodes in queries. This is not portable - use with case.

<_:1234-5678-90>    # A blank node in the data

_:b0                # A blank node in the query - a variable

ARQ Documentation Page