# Predefined query endpoint (beta)

Since 4.1.0.

The predefined query endpoint enables the specification of custom low-latency Web APIs in a declarative manner. It relies on two main components:

  1. A JSON-based configuration file, describing the API interface, its parameters and optionally a JSON-LD frame (opens new window) for shaping the returned data.
  2. A SPARQL query in charge of retrieving and perhaps transforming the data. The values of the API parameters are plugged as constants into the SPARQL query.

WARNING

At the moment only CONSTRUCT queries are supported.

This endpoint brings one important benefit in terms of performance with respect to the SPARQL endpoint: it is able to reuse reformulations from similar queries. This is particularly valuable for "lookup" requests involving many optional triples, and for which latency is critical.

This endpoint can be deployed using the command ontop endpoint or the Docker image ontop/ontop-endpoint (opens new window).

# HTTP API

GET /predefined/{queryId}?{param1}={value1}&{param2}={value2}&{param3}={value3}

# Example

# Configuration

{
    "queries": {
        "lodging": {
            "queryType": "graph",
            "name": "Lodging businesses",
            "description": "JSON-LD snippets for lodging businesses, based on schema.org.",
            "return404IfEmpty": true,
            "resultStreaming": false,
            "parameters": {
                "id": {
                    "description": "Internal identifier of the lodging business",
                    "type": "xsd:string",
                    "safeForRandomGeneration": true,
                    "required": true
                },
                "language": {
                    "description": "Language (e.g. it, de, en, ru). Default: en",
                    "type": "xsd:string",
                    "safeForRandomGeneration": false,
                    "required": false
                }
            },
            "frame": {
                "@context": "https://schema.org",
                "@type": [
                  "LodgingBusiness"
                ]
            }
        }
    }
}

# Predefined queries

[lodging]
query="""
BASE <http://noi.example.org/data/accommodation/>
PREFIX schema: <http://schema.org/>
PREFIX : <http://noi.example.org/ontology/odh#>
 CONSTRUCT {
   ?h a schema:LodgingBusiness ; schema:name ?nameStr ; schema:priceRange ?priceRange .
 }
 WHERE {
  # Convert incoming parameters
  BIND (IRI(?id) AS ?h)
  BIND (COALESCE(?language, "en") AS ?l)
  ?h a schema:LodgingBusiness .
  OPTIONAL { 
    ?h schema:name ?name .
    BIND(str(?name) AS ?nameStr)
    FILTER(lang(?name) = ?l)
  }
  OPTIONAL {
    ?h schema:priceRange ?priceRange .
  }
}
"""

# Cache for the JSON-LD contexts

Optionally, JSON-LD context documents can be stored in a cache and adapted if needed. When they are not cached, these documents are directly fetched from the Web.

{
  "https://schema.org": {
    "@context": {
      "@vocab": "http://schema.org/"
    }
  }
}

# Request

curl http://localhost:8080/predefined/lodging?id=32E7BE648E7B11D181AB006097B896BA&language=de

Results:

{
  "@context": "https://schema.org",
  "@id": "http://noi.example.org/data/accommodation/32E7BE648E7B11D181AB006097B896BA",
  "@type": "LodgingBusiness",
  "name": "Alpenblick - Gasthaus"
}

# Entries

# Top-level key

Key Type Description
queries Map of predefined queries Groups predefined queries by id.

# Query keys

Key Type Description
queryType String At the moment, only graph is supported. Corresponds to CONSTRUCT queries.
name String Name of the predefined query.
description String Description of the predefined query.
return404IfEmpty Boolean If true, returns a 404 status code when the result set is empty.
resultStreaming Boolean If true, uses the chunked transfer encoding.
parameters Map of parameters Groups parameters by id.
frame JSON object JSON-LD frame (opens new window)) for shaping the result set when the JSON-LD format is selected.

# Parameter keys

Key Type Description
description String Description of the parameter.
type String Type of the parameter. Can be iri for an IRI, any XSD datatype like xsd:string with the prefix xsd: or the IRI string of an RDF datatype.
safeForRandomGeneration Boolean If true, replacing the parameter value by a random value for generating a "reference" reformulated query can be considered as safe. Replacing afterwards the random value but the real parameter value in the reference reformulated query should produce the expected result. Usually, identifiers are safe while languages are not because the columns to consider differ from a language to another.
required Boolean If true, a value must be given for this parameter in the HTTP request.