Linked Data Templates

AtomGraph sitemap ontology contains Linked Data Templates (LDTs) expressed in RDF/OWL. Linked Data Template defines a class of document resources that:

• their URI identifiers share the same URI pattern (match specified URI template)
• their RDF representation was generated from the same SPARQL query

One template can override and specialize another by extending (sub-classing) it and redefining its properties. Templates gp:Document and gp:Container are built-in templates, all other templates extend them.

An example of a Linked Data Template:

<#LabelResourcesContainer> a owl:Class, gp:Template ;
    rdfs:subClassOf gp:Container ;
    gp:path "/resources/labelled" ;
    gp:query <#DescribeLabelResources> ;
    gp:defaultLimit 20 ;
    gp:defaultOrderBy "label" ;
    rdfs:label "Labelled resource container" ;
    rdfs:isDefinedBy <#> .

In this example, resource's URI template is /resources/labelled, defined using gp:uriTemplate property. The template is mapped to the following SPARQL query in SPIN syntax using gp:query property:

<#DescribeLabelResources>    a       sp:Describe, sp:Query ;
      sp:text """
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX foaf: <http://xmlns.com/foaf/0.1/>
PREFIX sioc: <http://rdfs.org/sioc/ns#>
PREFIX dc:   <http://purl.org/dc/elements/1.1/>
PREFIX dct:  <http://purl.org/dc/terms/>

DESCRIBE ?this ?resource WHERE {
    {
        SELECT ?resource
        WHERE {
            ?resource rdfs:label|dct:title ?label .
            FILTER isURI(?resource) .
        }
    } .
}"""^^xsd:string .

During request processing in Processor, if a resource with relative URI /resources/labelled (relativized against webapp's base URI) is requested, the SPIN query will be built with gp:defaultLimit and gp:defaultOrderBy modifier values and executed:

PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX foaf: <http://xmlns.com/foaf/0.1/>
PREFIX sioc: <http://rdfs.org/sioc/ns#>
PREFIX dc:   <http://purl.org/dc/elements/1.1/>
PREFIX dct:  <http://purl.org/dc/terms/>

DESCRIBE ?this ?resource WHERE {
    {
        SELECT ?resource
        WHERE {
            ?resource rdfs:label|dct:title ?label .
            FILTER isURI(?resource) .
        }
    LIMIT 20
    ORDER BY ?label
    } .
}

For a more detailed explanation, see our paper Linked Data Templates: Ontology-driven approach to read-write Linked Data

URI templates

AtomGraph Processor is currently using JAX-RS @Path URI template syntax, which is based on Java-style regexp pattern. JAX-RS defines a priority algorithm which GP is using to select the best match even if the request URI is matching multiple templates. For example, more specific /resources/labelled takes precedence over the catch-all /{path: .*}.

Query templates

AtomGraph Processor is using TopBraid SPIN API to store and manage SPARQL queries as RDF. Queries are attached to resource classes as SPIN queries using the gp:query property. It expects a query resource, i.e. an instance of sp:Describe or sp:Construct. SPIN also allows using instances of SPIN Templates wherever a query is expected (they are known as "template calls").

Read more about SPIN - SPARQL Inferencing Notation.

Query bindings

Like in SPIN API, ?this variable has a special meaning in AtomGraph. When processsing SPARQL query, GP always sets values to the following variables:

?this

the URI of the request (excluding query string)

?baseUri

the base URI of the application

If you define a template with query DESCRIBE ?this, what will be executed is DESCRIBE <http://localhost:8080/> if request to http://localhost:8080 matches this template.

User interface templates

For XSLT stylesheet support, see AtomGraph Web-Client.