@comunica/query-sparql-rdfjs

Comunica SPARQL RDF/JS

Comunica SPARQL is a SPARQL query engine for JavaScript for querying RDF/JS sources that implement the Source interface.

This module is part of the Comunica framework.

Install

$ yarn add @comunica/query-sparql-rdfjs

Query

const QueryEngine = require('@comunica/query-sparql-rdfjs').QueryEngine;
const N3Store = require('n3').Store;
const DataFactory = require('n3').DataFactory;

// This can be any RDFJS source
const store = new N3Store();
store.addQuad(DataFactory.quad(
  DataFactory.namedNode('a'), DataFactory.namedNode('b'), DataFactory.namedNode('http://dbpedia.org/resource/Belgium')));
store.addQuad(DataFactory.quad(
  DataFactory.namedNode('a'), DataFactory.namedNode('b'), DataFactory.namedNode('http://dbpedia.org/resource/Ghent')));

// Create our engine, and query it.
// If you intend to query multiple times, be sure to cache your engine for optimal performance.
const myEngine = new QueryEngine();
const bindingsStream = await myEngine.queryBindings(`
  SELECT ?s ?p ?o WHERE {
    ?s ?p <http://dbpedia.org/resource/Belgium>.
    ?s ?p ?o
  } LIMIT 100`, {
  sources: [store],
});

// Consume results as a stream (best performance)
bindingsStream.on('data', (binding) => {
  console.log(binding.toString()); // Quick way to print bindings for testing

  console.log(binding.has('s')); // Will be true

  // Obtaining values
  console.log(binding.get('s').value);
  console.log(binding.get('s').termType);
  console.log(binding.get('p').value);
  console.log(binding.get('o').value);
});
bindingsStream.on('end', () => {
  // The data-listener will not be called anymore once we get here.
});
bindingsStream.on('error', (error) => {
  console.error(error);
});

// Consume results as async iterable (easier)
for await (const binding of bindingsStream) {
  console.log(binding.toString());
}

// Consume results as an array (easier)
const bindings = await bindingsStream.toArray();
console.log(bindings[0].get('s').value);
console.log(bindings[0].get('s').termType);

Read more about querying in an application.

Update

const QueryEngine = require('@comunica/query-sparql-rdfjs').QueryEngine;
const N3Store = require('n3').Store;

// This can be any RDFJS store
const store = new N3Store();

// Create our engine, and query it.
const myEngine = new QueryEngine();
const query = `
PREFIX dc: <http://purl.org/dc/elements/1.1/>
INSERT DATA
{
  <http://example/book1> dc:title "A new book" ;
                         dc:creator "A.N.Other" .
}
`;

// Execute the update
await myEngine.queryVoid(query, {
  sources: [store],
});

// Prints '2' => the store is updated
console.log(store.size);

Read more about updating in an application.

Optimization

The RDFJS Source interface by default only exposed the match method. In order to allow Comunica to produce more efficient query plans, you can optionally expose a countQuads method that has the same signature as match, but returns a number or Promise<number> that represents (an estimate of) the number of quads that would match the given quad pattern. Certain Source implementations may be able to provide an efficient implementation of this method, which would lead to better query performance.

If Comunica does not detect a countQuads method, it will fallback to a sub-optimal counting mechanism where match will be called again to manually count the number of matches.