ch02

Chapter 2 - Accessing Knowledge Graphs programmatically

• HTTP requests
• SPARQL Libraries : SPARQL Wrapper, RDFLib, Jena
• Manipulating output from SPARQL : rdflib.js, LDFlex, GraphQL-LD HyperGraphQL

HTTP Requests

1. SPARQL query to retrieve all rock groups, and their English labels, from Wikidata. - SPARQL

SELECT ?item ?itemLabel
WHERE {
  ?item wdt:P31 wd:Q5741069.
  SERVICE wikibase:label { bd:serviceParam wikibase:language "[AUTO_LANGUAGE],en". }
}

2. HTTP request retrieving rock groups from the Wikidata SPARQL endpoint service - direct link

GET https://query.wikidata.org/sparql?query=SELECT%20%3Fitem%20%3FitemLabel%20%0AWHERE%20%0A%7B%0A%20%20%3Fitem%20wdt%3AP31%20wd%3AQ5741069.%0A%20%20SERVICE%20wikibase%3Alabel%20%7B%20bd%3AserviceParam%20wikibase%3Alanguage%20%22%5BAUTO_LANGUAGE%5D%2Cen%22.%20%7D%0A%7D

3. Using curl to transfer HTTP data about rock groups from the Wikidata SPARQL endpoint. We set the HTTP header Accept to ask the endpoint to send back the response in JSON format. - bash file

curl -H'Accept: application/json' -X GET "https://query.wikidata.org/sparql?query=SELECT%20%3Fitem%20%3FitemLabel%20%0AWHERE%20%0A%7B%0A%20%20%3Fitem%20wdt%3AP31%20wd%3AQ5741069.%0A%20%20SERVICE%20wikibase%3Alabel%20%7B%20bd%3AserviceParam%20wikibase%3Alanguage%20%22%5BAUTO_LANGUAGE%5D%2Cen%22.%20%7D%0A%7D"

4. Send a SPARQL query to Wikidata via HTTP using the Python library requests. - Python Script

import requests

endpoint_url = "https://query.wikidata.org/sparql"
query_string = "SELECT ?item ?itemLabel WHERE { ?item wdt:P31 wd:Q5741069.  SERVICE wikibase:label { bd:serviceParam wikibase:language \"[AUTO_LANGUAGE],en\". }}"

params = {'query' : query_string}
headers = {'accept' : 'application/json'}
r = requests.get(endpoint_url, params=params, headers=headers)

print(r.text)

SPARQL Libraries

5. Sending a SPARQL query to an endpoint with SPARQL Wrapper - Python script

from SPARQLWrapper import SPARQLWrapper, JSON

sparql = SPARQLWrapper("http://dbpedia.org/sparql")
sparql.setQuery("""
    PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
    SELECT ?label
    WHERE {
        <http://dbpedia.org/resource/Barcelona> rdfs:label ?label
    } """)
sparql.setReturnFormat(JSON)
results = sparql.query().convert()

for result in results["results"]["bindings"]:
    print(result["label"]["value"])

6. Retrieving file-like RDF data (local or remote) and querying it with SPARQL and RDFLib - Python script

import rdflib

g = rdflib.Graph()
g.parse("http://example.org/rdf-data.nt")

qres = g.query(
    """
    PREFIX dbo: <http://dbpedia.org/ontology/>
    PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
    PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>

    SELECT ?bandname
    WHERE {
          ?a rdf:type dbo:Band ;
             rdfs:label ?bandname .
    } """)

for row in qres:
    print(row)

7. Retrieving file-like RDF data (local or remote) and querying it using RDFLib iterators - Python script

import rdflib
from rdflib import URIRef
from rdflib.namespace import RDF, RDFS

g = rdflib.Graph()
g.parse("http://example.org/rdf-data.nt")

for s,p,o in g.triples((None, RDF.type, URIRef("http://dbpedia.org/ontology/Band"):
    for u,v,s in g.triples((s, RDFS.label, None)):
        print(s)

8. Sending a SPARQL query to an endpoint with Jena - Java class

import org.apache.jena.query.*;

public class JenaTest {
    public static void main(String []args) {
        String endpoint = "http://dbpedia.org/sparql";
        String queryString =
           "PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>\n" +
           "SELECT ?label \n" +
           "WHERE {  \n" +
           "<http://dbpedia.org/resource/Barcelona> rdfs:label ?label"
           + "\n }";

        Query query = QueryFactory.create(queryString);
        QueryExecution qexec = QueryExecutionFactory.sparqlService(endpoint, query);
        try {
            ResultSet results = qexec.execSelect();
            ResultSetFormatter.out( results );
        }
        finally {
            qexec.close();
        }
    }
}

Manipulating output from SPARQL

8. Creating and manipulating RDF data in rdflib.js - JS script

// Define the DBO namespace
var DBO = Namespace("http://dbpedia.org/ontology/");
var RDFS = Namespace("http://www.w3.org/2000/01/rdf-schema#");

// The store is the object that contains all triples
var store = $rdf.graph();

var band = $rdf.sym('http://dbpedia.org/resource/AC/DC');

// Insert triples in the store
store.add(band, RDFS('label'), 'AC/DC');
store.add(band, DBO('genre'), $rdf.sym('http://dbpedia.org/resource/Hard_rock'));
store.add(band, DBO('genre'), $rdf.sym("http://dbpedia.org/resource/Rock_and_roll"));

// Retrieve all (each) or one (any) results, using wildcards
var genres = store.each(band, DBO('genre'), undefined)
for (var genre of genres)
    console.log(genre.uri);

10. Manipulating RDF data in LDFlex - JS script

const { PathFactory } = require('ldflex');
const { default: ComunicaEngine } = require('@ldflex/comunica');
const { namedNode } = require('@rdfjs/data-model');

// The context maps properties and URIs
const context = {
  "@context": {
    "@vocab": "http://dbpedia.org/ontology/",
    "label": "http://www.w3.org/2000/01/rdf-schema#label",
  }
};

// The query engine interact with the SPARQL endpoint
const endpoint = 'http://dbpedia.org/sparql';
const queryEngine = new ComunicaEngine(endpoint);

// The PathFactory is the access point to the data
const path = new PathFactory({ context, queryEngine });

// define the band node as subject for queries
const band = path.create({
  subject: namedNode('http://dbpedia.org/resource/AC/DC')
});
getGenres(band);

async function getGenres(band) {
  // Under the hood, SPARQL queries are executed
  console.log(`This band is ${await band.label}`);

  console.log(`${await band.label} usually plays:`);
  for await (const genre of band.genre.label)
    console.log(`- ${genre}`);
}

11. A query in GraphQL-LD, with its related context in JSON-LD.

{
  label @single
  album {
    label
  }
  genre(label_en: "Hard rock")  @single
}

{
  "@context": {
    "label": "http://www.w3.org/2000/01/rdf-schema#label",
    "label_en": { "@id": "http://www.w3.org/2000/01/rdf-schema#label", "@language": "en" },
    "album": { "@reverse": "http://dbpedia.org/ontology/album" },
    "genre": "http://dbpedia.org/ontology/genre"
  }
}

12. Configuration, schema and query in HyperGraphQL

{
    "name": "my-api",
    "schema": "schema.graphql",
    "server": {
       "port": 8080,
       "graphql": "/graphql",
       "graphiql": "/graphiql"
    },
    "services": [{
       "id": "dbpedia-sparql",
       "type": "SPARQLEndpointService",
       "url": "http://dbpedia.org/sparql/",
       "graph": "http://dbpedia.org",
       "user": "",  "password": ""
    }]
}

type __Context {
    Band:  _@href(iri: "http://dbpedia.org/ontology/Band")
    Genre: _@href(iri: "http://dbpedia.org/ontology/Genre")
    label: _@href(iri: "http://www.w3.org/2000/01/rdf-schema#label")
    genre: _@href(iri: "http://dbpedia.org/ontology/genre")
}
type Band @service(id:"dbpedia-sparql") {
    label: [String] @service(id:"dbpedia-sparql")
    genre: Genre @service(id:"dbpedia-sparql")
}
type Genre @service(id:"dbpedia-sparql") {
    label: [String] @service(id:"dbpedia-sparql")
}

{
  Band_GET(limit: 1) {
    _id
    _type
    label
    genre {
      _id
      label(lang: "en")
    }
  }
}