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@comunica/actor-init-sparql

1.7.3 • Public • Published

Comunica SPARQL

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Linked Data on the Web is being published in different ways, such as data dumps, subject pages, results of SPARQL queries, and Triple Pattern Fragments. This client is able to solve queries over such heterogeneous interfaces.

Concretely, Comunica SPARQL is a module that is preconfigured with a configuration file to initialize the Comunica engine with actors to evaluate SPARQL queries over heterogeneous interfaces.

It's main distinguishing features are the following:

  • High modularity enabling easy extensions and customization.
  • Federated querying over heterogeneous interfaces.
  • Can run using Node.JS, in the browser, and via the command-line.

Comunica can either be invoked dynamically using a configuration file, or statically using a pre-compiled configuration file. The latter will be faster to start because the dependency-injection phase can be avoided.

This actor can not query over local files for security reasons, but Comunica SPARQL file can.

Installation

Comunica requires Node.JS 8.0 or higher and is tested on OSX and Linux.

The easiest way to install the client is by installing it from NPM as follows:

$ [sudo] npm install -g @comunica/actor-init-sparql

Alternatively, you can install from the latest GitHub sources. For this, please refer to the README of the Comunica monorepo.

Execute SPARQL queries

This actor can be used to execute SPARQL queries from the command line, HTTP (SPARQL protocol), within a Node.JS application, or from a browser.

Usage from the command line

Show 100 triples from http://fragments.dbpedia.org/2015-10/en:

$ comunica-sparql http://fragments.dbpedia.org/2015-10/en "CONSTRUCT WHERE { ?s ?p ?o } LIMIT 100"

Show all triples from http://dbpedia.org/resource/Belgium:

$ comunica-sparql file@http://dbpedia.org/resource/Belgium "CONSTRUCT WHERE { ?s ?p ?o }"

Combine multiple sources:

$ comunica-sparql http://fragments.dbpedia.org/2015-10/en file@http://dbpedia.org/resource/Belgium "CONSTRUCT WHERE { ?s ?p ?o } LIMIT 100"

Show the help with all options:

$ comunica-sparql --help

The dynamic variant of this executable is comunica-dynamic-sparql. An alternative config file can be passed via the COMUNICA_CONFIG environment variable.

When you are working with this module in the Comunica monorepo development environment, this command can be invoked directly as follows (when inside the packages/actor-init-sparql folder):

node bin/query.js http://fragments.dbpedia.org/2016-04/en "CONSTRUCT WHERE { ?s ?p ?o } LIMIT 100"

Use bin/query-dynamic.js when running dynamically inside the Comunica monorepo development environment.

Usage as a SPARQL endpoint

Start a webservice exposing http://fragments.dbpedia.org/2015-10/en via the SPARQL protocol, i.e., a SPARQL endpoint.

$ comunica-sparql-http "\"sources\": [{ \"type\"\"hypermedia\"\"value\" : \"http://fragments.dbpedia.org/2015/en\" }]}"

(Use \"type\": \"file\" if you want to query over remote RDF files)

Show the help with all options:

$ comunica-sparql-http --help

The SPARQL endpoint can only be started dynamically. An alternative config file can be passed via the COMUNICA_CONFIG environment variable.

Use bin/http.js when running in the Comunica monorepo development environment.

Usage within application

The easiest way to create an engine (with default config) is as follows:

const newEngine = require('@comunica/actor-init-sparql').newEngine;
 
const myEngine = newEngine();

Alternatively, an engine can also be created dynamically with a custom config:

const newEngineDynamic = require('@comunica/actor-init-sparql').newEngineDynamic;
 
const myEngine = await newEngineDynamic({ configResourceUrl: 'path/to/config.json' });

Once you have created your query engine, you can use it to call the async query(queryString, context) method, which returns an output of type that depends on the given query string.

For example, a SELECT query can be executed as follows:

const result = await myEngine.query('SELECT * WHERE { ?s ?p <http://dbpedia.org/resource/Belgium>. ?s ?p ?o } LIMIT 100',
  { sources: [ { type: 'hypermedia', value: 'http://fragments.dbpedia.org/2015/en' } ] })
result.bindingsStream.on('data', (data) => console.log(data.toObject()));

For CONSTRUCT and DESCRIBE queries, results can be collected as follows.

const result = await myEngine.query('CONSTRUCT { ?s ?p <http://dbpedia.org/resource/Belgium> } LIMIT 100',
  { sources: [ { type: 'hypermedia', value: 'http://fragments.dbpedia.org/2015/en' } ] })
result.quadStream.on('data', (data) => console.log(data.toObject()));

Finally, ASK queries return async booleans.

const result = await myEngine.query('ASK { ?s ?p <http://dbpedia.org/resource/Belgium> }',
  { sources: [ { type: 'hypermedia', value: 'http://fragments.dbpedia.org/2015/en' } ] })
const isPresent = await result.booleanResult;

Context options:

Key Description
sources An array of data sources, e.g. [ { type: 'hypermedia', value: 'http://fragments.dbpedia.org/2015/en' } ]. Sources can be one of the following types: hypermedia, sparql, file
initialBindings Variables that have to be pre-bound to values in the query, using the Bindings datastructure, e.g. Bindings({ '?s': literal('sl') }).
queryFormat Name of the provided query's format. Defaults to sparql, can also be graphql
baseIRI Base IRI for relative IRIs in SPARQL queries, e.g. http://example.org/.
log Logger to use, e.g. new LoggerPretty({ level: 'warn' }).
datetime Datetime to handle time travel with Memento, e.g. new Date().

GraphQL-LD

Instead of SPARQL queries, you can also define GraphQL queries (with a JSON-LD context).

If you want to convert your results to a GraphQL tree, then you will need the @comunica/actor-sparql-serialize-tree dependency, otherwise you can consume the bindings stream manually.

const newEngine = require('@comunica/actor-init-sparql').newEngine;
const bindingsStreamToGraphQl = require('@comunica/actor-sparql-serialize-tree').bindingsStreamToGraphQl;
 
const myEngine = newEngine();
const context = {
  sources: [ { type: 'hypermedia', value: 'http://fragments.dbpedia.org/2016-04/en' } ],
  queryFormat: 'graphql',
  "@context": {
    "label": { "@id": "http://www.w3.org/2000/01/rdf-schema#label", "@singular": true },
    "label_en": { "@id": "http://www.w3.org/2000/01/rdf-schema#label", "@language": "en" },
    "writer": { "@id": "http://dbpedia.org/ontology/writer", "@singular": true },
    "artist": { "@id": "http://dbpedia.org/ontology/musicalArtist", "@singular": true },
    "artist_label": { "@singular": true }
  }
};
myEngine.query('{ label writer(label_en: \"Michael Jackson\") artist { label } }',context)
  .then(function (result) { return bindingsStreamToGraphQl(result.bindingsStream, context); })
  .then(console.log);

Logging

Optionally, a custom logger can be used inside Comunica. By default, @comunica/logger-void is used, which will simply void all log calls. (This default can be changed in the configuration file)

Alternatively, @comunica/logger-pretty, @comunica/logger-bunyan, or a custom logger implementing the Logger interface can be used.

These loggers can be configured through the context as follows:

import {LoggerPretty} from "@comunica/logger-pretty";
 
const context = {
  log: new LoggerPretty({ level: 'warn' });
};
myEngine.query('...', context);

Usage within browser

(Just want to quickly demo queries in the browser? Have a look at our Web client)

This engine can run in the browser using Webpack. To create a web-packed version of the engine, run yarn run browser (when inside the packages/actor-init-sparql folder) to create comunica-browser.js. Alternatively, just use a pre-built version from our CDN.

Include this file in your webpage as follows:

<script src="path/to/comunica-browser.js"></script>

After that, Comunica.newEngine can be called via JavaScript.

const myEngine = Comunica.newEngine();
myEngine.query('SELECT * { ?s ?p <http://dbpedia.org/resource/Belgium>. ?s ?p ?o } LIMIT 100',
  { sources: [ { type: 'hypermedia', value: 'http://fragments.dbpedia.org/2015/en' } ] })
  .then(function (result) {
    result.bindingsStream.on('data', function (data) {
      console.log(data.toObject());
    });
  });

The browser script is pre-compiled using a config file and can therefore only be invoked dynamically. See the prepare and browser scripts in package.json to compile using a custom config file.

If you want to use GraphQL-LD here as well, you can do this similar as in the Node.JS API using Comunica.bindingsStreamToGraphQl

install

npm i @comunica/actor-init-sparql

version

1.7.3

license

MIT

homepage

github.com

repository

Gitgithub

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