Façade-X Data Access in SPARQL

This document specifies how a Façade-X view of a heterogeneous resource is accessed from within a SPARQL query. It defines the SERVICE clause and IRI scheme through which an engine is asked to build the Façade-X representation of a resource, the options (properties) that control how the resource is interpreted, and the magic properties that support querying the resulting containers. It is a companion to the Façade-X Concepts and Metamodel and Façade-X RDF Vocabulary documents: the former defines the model, the latter its RDF terms, and this document defines how that RDF is obtained and queried in SPARQL.

This document is currently in active development. The mechanism described here is the one provided by the reference implementation, SPARQL Anything; the naming of the IRI scheme in particular is expected to be revisited before standardisation (see the Issues section).

Issues

The reserved IRI scheme is currently x-sparql-anything:, the scheme used by the reference implementation. A vendor-neutral scheme name is expected to be chosen before this document reaches Recommendation status; until then, the implementation-specific scheme is used throughout.

This document types the root container fx:Root, in agreement with the RDF vocabulary document. Some implementations currently emit the lowercase fx:root; the casing is to be reconciled across the specifications and implementations.

The Façade-X SERVICE clause

A Façade-X view is requested through a SPARQL SERVICE block whose IRI uses the reserved scheme x-sparql-anything:. The general form is:

SERVICE <x-sparql-anything:[OPTIONS]> {
  [GROUP GRAPH PATTERN]
}

    

An engine that conforms to this specification MUST, on encountering a SERVICE block whose IRI uses the x-sparql-anything: scheme:

1. determine the resource and interpretation from the options provided (see Providing options), where exactly one source option — fx:location, fx:content, or fx:command — MUST be given;
2. construct the Façade-X representation of that resource as defined by the metamodel and realised by the RDF vocabulary; and
3. evaluate the enclosed group graph pattern against that representation, returning solutions as for any other SERVICE invocation.

The resulting representation is a single Façade-X data source: one container typed fx:Root, reachable from which are its slots, values, and nested containers. Where a resource yields more than one data source, each is produced in its own named graph, as described in Named Graphs.

Providing options

Options are supplied to the engine in either of two modalities, which MAY be combined.

SERVICE <x-sparql-anything:location=https://example.org/people.json,media-type=application/json> {
  ?root a fx:Root ; xyz:name ?name .
}

    

SERVICE <x-sparql-anything:https://example.org/people.json> {
  ?root a fx:Root ; xyz:name ?name .
}

    

SERVICE <x-sparql-anything:> {
  fx:properties fx:location   "https://example.org/people.json" ;
                fx:media-type "application/json" .
  ?root a fx:Root ; xyz:name ?name .
}

    

Source options

Exactly one source option MUST be provided. It identifies the resource whose Façade-X view is to be queried.

Interpretation options

The following options control how the source is mapped onto the Façade-X model. All are optional.

Formats define further, format-specific options (for example CSV delimiters and header handling, or JSON and XML path expressions). These are defined alongside each format and are out of scope for this document.

Execution options

The following options affect how the representation is built and evaluated, but not the mapping itself. All are optional.

Magic properties

Beyond the terms defined by the RDF vocabulary, an engine recognises a small number of magic properties within a Façade-X SERVICE block. These are not asserted triples but query-time constructs the engine evaluates specially.

PREFIX fx:  <http://sparql.xyz/facade-x/ns/>
PREFIX xyz: <http://sparql.xyz/facade-x/data/>

SELECT ?name ?pet WHERE {
  SERVICE <x-sparql-anything:> {
    fx:properties fx:location "people.json" .
    ?root a fx:Root ;
          xyz:name ?name ;
          xyz:pets ?pets .
    ?pets fx:anySlot ?pet .
  }
}

    

Engines additionally provide functions over container-membership properties — for example fx:cardinal(?p), which returns the numeric index of a membership property, and fx:before(?a, ?b) and fx:after(?a, ?b), which compare the order of two membership properties. A fuller library of helper functions is provided by implementations and is out of scope for this document.