Query Engine

LDkit ships with a simple SPARQL query engine that lets you execute queries over a SPARQL endpoint. This engine is used by default, unless you specify a custom one.

If you want to query other RDF data sources like files or in-memory quad stores, you need to use a custom query engine like Comunica, or implement your own.

Default query engine

import { QueryEngine } from "ldkit";

const engine = new QueryEngine();

The QueryEngine follows RDF/JS Query specification and implements the StringSparqlQueryable interface.

The QueryEngine is configurable through context.

import { type Context, QueryEngine } from "ldkit";

const context: Context = {
  sources: ["https://example.com/sparql"], // required, must include one SPARQL endpoint
  fetch: customFetchFunction, // optional, must follow standard fetch interface
};
const engine = new QueryEngine();

const response = await engine.queryBoolean("ASK { ?s ?p ?o }", context);

Note: The default query engine supports all SPARQL endpoints that conform to the SPARQL 1.1 specification and can return data of MIME application/sparql-results+json for SELECT and ASK queries, and application/rdf+json for CONSTRUCT queries.

Custom query engine

You can use a custom query engine with LDkit - same as the included query engine, it needs to implement StringSparqlQueryable interface from the RDF/JS Query specification.

For advanced federated querying over multiple datasources you can also use Comunica, or a custom engine derived from that - see Query with Comunica.

Setting default query engine

A query engine instance needs to be passed to a data Lens as a parameter in order to query data, and there are two ways how to handle that. Either you can pass the engine directly as an argument when creating the Lens, or you can set an engine instance as a default one. If there is a default engine instance, then the Lens will use that engine, if you do not provide one directly.

import { createLens, setDefaultEngine } from "ldkit";

const engine = new MyCustomQueryEngine();
setDefaultEngine(engine);

const MyLens = createLens(MySchema); // will use the custom engine, which is now default