Setting up a SPARQL endpoint

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The SPARQL protocol allows clients to send SPARQL queries to Web servers over HTTP, and query results to be sent back to the client. Comunica SPARQL can be used to set up a SPARQL endpoint on top of any number of sources you want.

1. Installation

Since Comunica runs on Node.js, make sure you have Node.js installed on your machine.

Next, we can install Comunica SPARQL on our machine:

$ npm install -g @comunica/query-sparql

2. SPARQL endpoint over one source

After installing Comunica SPARQL, you will be given access to several commands including comunica-sparql-http, which allows you to start a SPARQL endpoint from the command line.

This command requires one or more URLs to be provided as sources to query over.

For example, the following command starts a SPARQL endpoint over DBpedia:

$ comunica-sparql-http https://fragments.dbpedia.org/2016-04/en

Given a URL, Comunica will automatically detect the type of source and handle it accordingly.

By default, the endpoint will be exposed on port 3000. Your endpoint will now be live on http://localhost:3000/sparql. Any client that understands the SPARQL protocol will now be able to send queries to this URL, such as fetch-sparql-endpoint, or even Comunica itself.

The URL http://localhost:3000/ will automatically redirect to http://localhost:3000/sparql.

You can easily test query execution over your endpoint using a tool such as curl. The SPARQL protocol allows sending queries via HTTP GET by passing a URL-encoded SPARQL query via the ?query= parameter:

$ curl -v "http://localhost:3000/sparql?query=CONSTRUCT%20WHERE%20%7B%3Fs%20%3Fp%20%3Fo.%7DLIMIT%20100"

3. SPARQL endpoint over multiple sources

One key feature of Comunica is its ability to query over multiple sources. For this, you can just supply any number of URLs as arguments.

$ comunica-sparql-http https://fragments.dbpedia.org/2016-04/en \
    https://www.rubensworks.net/ \
    https://ruben.verborgh.org/profile/

4. SPARQL endpoint over local files

First install Comunica SPARQL for files:

$ npm install -g @comunica/query-sparql-file

Then start the SPARQL server:

$ comunica-sparql-file-http path/to/my/file.ttl

5. Changing the port

Using the -p option, the port can be changed:

$ comunica-sparql-http https://fragments.dbpedia.org/2016-04/en \
  -p 3001

6. Increasing the number of worker threads

Using the -w option, the number of parallel worker threads can be set:

$ comunica-sparql-http https://fragments.dbpedia.org/2016-04/en \
  -w 4

Setting this to the number of available CPU cores tends to give the best performance.

7. Learn more

This guide only discussed the basic functionality of comunica-sparql-http. You can learn more options by invoking the help command:

$ comunica-sparql-http --help
comunica-sparql-http exposes a SPARQL endpoint

Recommended options:
  -p, --port     HTTP port to run on                                                                                                    [number] [default: 3000]
  -w, --workers  Number of worker threads                                                                                                  [number] [default: 1]
  -t, --timeout  Query execution timeout in seconds                                                                                       [number] [default: 60]
  -u, --update   Enable update queries (otherwise, only read queries are enabled)                                                     [boolean] [default: false]

Options:
  -c, --context          Use the given JSON context string or file (e.g., config.json)                                                                  [string]
      --to               Destination for update queries                                                                                                 [string]
  -b, --baseIRI          base IRI for the query (e.g., http://example.org/)                                                                             [string]
  -d, --dateTime         Sets a datetime for querying Memento-enabled archives                                                                          [string]
  -l, --logLevel         Sets the log level (e.g., debug, info, warn, ...)                                                            [string] [default: "warn"]
      --lenient          If failing requests and parsing errors should be logged instead of causing a hard crash                                       [boolean]
  -v, --version          Prints version information                                                                                                    [boolean]
      --showStackTrace   Prints the full stacktrace when errors are thrown                                                                             [boolean]
  -i, --invalidateCache  Enable cache invalidation before each query execution                                                        [boolean] [default: false]

Examples:
  comunica-sparql-http https://fragments.dbpedia.org/2016-04/en
  comunica-sparql-http https://fragments.dbpedia.org/2016-04/en https://query.wikidata.org/sparql
  comunica-sparql-http hypermedia@https://fragments.dbpedia.org/2016-04/en sparql@https://query.wikidata.org/sparql