A simple, lightweight module to send queries to SPARQL endpoints and retrieve their results in a streaming fashion.
All results are compatible with the RDFJS specification.
All SPARQL queries are supported, such as SELECT
, ASK
, CONSTRUCT
DESCRIBE
, INSERT
, DELETE
, ...
Internally, this library supports SPARQL results in SPARQL JSON, SPARQL XML, and Turtle.
This package can be installed via npm.
$ npm install fetch-sparql-endpoint
This package also works out-of-the-box in browsers via tools such as webpack and browserify.
import {SparqlEndpointFetcher} from "fetch-sparql-endpoint";
const myFetcher = new SparqlEndpointFetcher();
Optionally, you can pass an options object with the following optional entries:
const myFetcher = new SparqlEndpointFetcher({
fetch: fetch, // A custom fetch-API-supporting function
dataFactory: DataFactory, // A custom RDFJS data factory
prefixVariableQuestionMark: false // If variable names in bindings should be prefixed with '?', defaults to false
});
SPARQL SELECT queries returns a (promise to a) stream of bindings.
const bindingsStream = await fetcher.fetchBindings('https://dbpedia.org/sparql', 'SELECT * WHERE { ?s ?p ?o } LIMIT 100');
bindingsStream.on('data', (bindings) => console.log(bindings));
This will output bindings in the following form, where keys correspond to variables in the queries, and values and RDFJS terms:
{ s: namedNode('s1'), p: namedNode('p1'), o: namedNode('o1') }
{ s: namedNode('s2'), p: namedNode('p2'), o: namedNode('o2') }
{ s: namedNode('s3'), p: namedNode('p3'), o: namedNode('o3') }
...
Optionally, you can obtain a list of variables by listening to the 'variables'
event:
const bindingsStream = await fetcher.fetchBindings('https://dbpedia.org/sparql', 'SELECT * WHERE { ?s ?p ?o } LIMIT 100');
bindingsStream.on('data', (bindings) => console.log(bindings));
// Will print [ variable('s'), variable('p'), variable('o') ]
bindingsStream.on('variables', (variables) => console.log(variables));
SPARQL ASK queries answer with a (promise to a) boolean value.
const answer = await fetcher.fetchAsk('https://dbpedia.org/sparql', 'ASK WHERE { ?s ?p ?o }');
This will output true
or false
.
SPARQL CONSTRUCT and SPARQL DESCRIBE queries returns a (promise to a) stream of triples.
const tripleStream = await fetcher.fetchTriples('https://dbpedia.org/sparql', 'CONSTRUCT { ?s ?p ?o } LIMIT 100');
tripleStream.on('data', (triple) => console.log(triple));
This will output RDFJS triples as follows:
triple(namedNode('s1'), namedNode('p1'), namedNode('o1'))
triple(namedNode('s2'), namedNode('p2'), namedNode('o2'))
triple(namedNode('s3'), namedNode('p3'), namedNode('o3'))
...
SPARQL Update queries answer with a void promise.
await fetcher.fetchUpdate('https://dbpedia.org/sparql', 'INSERT DATA { <ex:s> <ex:p> <ex:o> }');
The await
fill throw an error if the update has failed.
If you want to know the query type
in order to determine which of the above fetch methods to call,
then you can use the getQueryType
method as follows:
fetcher.getQueryType('SELECT * WHERE { ?s ?p ?o } LIMIT 100'); // Outputs 'SELECT'
fetcher.getQueryType('ASK WHERE { ?s ?p ?o }'); // Outputs 'ASK'
fetcher.getQueryType('CONSTRUCT { ?s ?p ?o } LIMIT 100'); // Outputs 'CONSTRUCT'
This method will also throw an error if the query contains a syntax error.
A command-line tool is provided to quickly query any SPARQL endpoint:
Usage:
$ fetch-sparql-endpoint https://dbpedia.org/sparql [-q] 'SELECT * WHERE { ?s ?p ?o } LIMIT 100'
$ fetch-sparql-endpoint https://dbpedia.org/sparql -f query.sparql
$ cat query.sparql | fetch-sparql-endpoint https://dbpedia.org/sparql
This software is written by Ruben Taelman.
This code is released under the MIT license.