Describing Federations

This page explains how to describe data sources (federation members) and their access interfaces using the HeFQUIN Federation Description Vocabulary. These descriptions are used by the HeFQUIN query engine to understand how to access and integrate the data sources that should be made available to a given engine instance.

1. Basic Concepts

• fd:FederationMember - Represents a federation member (data source) that can be queried.
• fd:serviceURI - A property to specify the URI via which a federation member is identified in SERVICE clauses.
• fd:Interface - Represents the access mechanism used to query a federation member.
• fd:interface - A property linking a federation member to its interface.

2. Interface Types

Use one of the concrete subclasses of fd:Interface to describe how to query the federation member:

• fd:SPARQLEndpointInterface - For federation members accessible via the SPARQL protocol
• fd:TPFInterface - For federation members accessible via Triple Pattern Fragments (TPF)
• fd:brTPFInterface - For federation members accessible via Bindings-Restricted TPF (brTPF)
• fd:RESTInterface - For federation members that are based on a REST endpoint
• fd:GraphQLEndpointInterface - For federation members that are based on a GraphQL API
• fd:BoltInterface - For federation members accessible via the Bolt protocol (e.g., for Neo4j)

3. Key Properties

• fd:endpointAddress - URI of a federation member that can be accessed via a single HTTP endpoint (e.g., SPARQL, REST, and GraphQL endpoints)
• fd:exampleFragmentAddress - Example URL of a fragment that can be used to access a specific subset of the dataset of a federation member (e.g., for TPF and brTPF interfaces)
• fd:vocabularyMappingsFile - Link to an RDF file aligning different vocabularies
• fd:mappingConfiguration - Resource pointing to a mapping configuration for translating non-RDF data models (e.g., GraphQL, LPG) into RDF
• fd:wrapper - Link to an fd:Wrapper that has RML triples maps listed via a fd:rmlTriplesMaps property

4. Example Descriptions

SPARQL Endpoint

@prefix fd: <http://www.example.org/se/liu/ida/hefquin/fd#> .
@prefix ex: <http://example.org/> .

ex:dbpediaSPARQL
    fd:serviceURI <http://dbpedia.org/sparql> ;
    fd:interface [
        a fd:SPARQLEndpointInterface ;
        fd:endpointAddress <http://dbpedia.org/sparql>
    ] .

TPF Server

@prefix fd: <http://www.example.org/se/liu/ida/hefquin/fd#> .
@prefix ex: <http://example.org/> .

ex:dbpediaTPF
    fd:serviceURI <http://fragments.dbpedia.org/2016-04/en> ;
    fd:interface [
        a fd:TPFInterface ;
        fd:exampleFragmentAddress <http://fragments.dbpedia.org/2016-04/en>
    ] .

RML-Wrapped REST Endpoint

@prefix fd: <http://www.example.org/se/liu/ida/hefquin/fd#> .
@prefix rml: <http://w3id.org/rml/> .
@prefix ex: <http://example.org/> .

ex:openMeteo
    fd:serviceURI <https://api.open-meteo.com/v1/forecast> ;
    fd:interface _:openMeteoRestEndpoint ;
    fd:wrapper _:openMeteoWrapper .

_:openMeteoRestEndpoint a fd:RESTInterface ;
      fd:endpointAddress <https://api.open-meteo.com/v1/forecast> ;
      fd:queryParameters ( [ fd:paramName "latitude" ; fd:paramType xsd:float ]
                           [ fd:paramName "longitude" ; fd:paramType xsd:float ]
                           [ fd:paramName "current" ; fd:paramType xsd:string ] ) .

# the corresponding wrapper with two RML triples maps
_:openMeteoWrapper
      fd:rmlTriplesMaps ( _:openMeteoTM1 _:openMeteoTM2 ) .

_:openMeteoTM1
      rml:logicalSource [ rml:source _:openMeteoRestEndpoint ;
                          rml:referenceFormulation rml:JSONPath ;
                          rml:iterator "$.current" ] ;
      rml:subjectMap [ rml:termType rml:BlankNode ;
                       rml:reference "time" ];
      rml:predicateObjectMap [ rml:predicateMap [ rml:constant ex:temperature ] ;
                               rml:objectMap [ rml:reference "temperature_2m" ; rml:datatype xsd:double ] ] .

_:openMeteoTM2
      rml:logicalSource [ rml:source _:openMeteoRestEndpoint ;
                          rml:referenceFormulation rml:JSONPath ;
                          rml:iterator "$.current" ] ;
      rml:subjectMap [ rml:termType rml:BlankNode ;
                       rml:reference "time" ];
      rml:predicateObjectMap [ rml:predicateMap [ rml:constant ex:windSpeed ] ;
                               rml:objectMap [ rml:reference "wind_speed_10m" ; rml:datatype xsd:double ] ] .

Neo4j

@prefix fd: <http://www.example.org/se/liu/ida/hefquin/fd#> .
@prefix lpg2rdf: <http://w3id.org/hefquin/lpg2rdf#> .
@prefix ex: <http://example.org/> .

ex:neo4jTest
    a fd:FederationMember ;
    fd:serviceURI <http://localhost:7474/neo4j> ;
    fd:interface [
        a fd:BoltInterface ;
        fd:endpointAddress <http://localhost:7474/db/neo4j/tx> ;
        fd:mappingConfiguration [
            a lpg2rdf:LPGToRDFConfiguration ;
            # ...
        ]
    ] .