SPARQL Endpoint list to single graph

Extractor, allows the user to extract RDF triples from a SPARQL endpoint using a series of CONSTRUCT queries. It is especially suitable for querying multiple SPARQL endpoints.

Number of threads to use
Number of threads to be used for querying in total.
Query time limit in seconds (-1 for no limit)
Some SPARQL endpoints may hang on a query for a long time. Sometimes it is therefore desirable to limit the time waiting for an answer so that the whole pipeline execution is not stuck.
Encode invalid IRIs
Some SPARQL endpoints such as DBpedia contain invalid IRIs which are sent in results of SPARQL queries. Some libraries like RDF4J then can crash on those IRIs. If this is the case, choose this option to encode such invalid IRIs.
Fix missing language tags on rdf:langString literals
Some SPARQL endpoints such as DBpedia contain rdf:langString literals without language tags, which is invalid RDF 1.1. Some libraries like RDF4J then can crash on those literals. If this is the case, choose this option to fix this problem by replacing rdf:langString with xsd:string datatype on such literals.
Limit number of tasks running in parallel in a group (0 for no limit)
To avoid overloading a single SPARQL endpoint, set a maximum number of tasks running in parallel for a group. Then, place all tasks targeting a single endpoint to a single group. Tasks in different groups will still run in parallel in the number of threads specified.
Query commit size (0 to commit all triples at once)
This is used to control the number of triples committed to a repository at once. Limiting the number may be necessary for large query results.

Characteristics

ID
e-sparqlendpointlist
Type
extractor
Inputs
RDF single graph - Configuration
RDF single graph - Tasks
Outputs
RDF single graph - Report
RDF single graph - Output
Look in pipeline
SPARQL Endpoint list to single graph

The SPARQL Endpoint list to single graph component queries a list of remote SPARQL endpoints using SPARQL CONSTRUCT queries. The typical scenarios include discovery tasks such as determining which classes are used in which endpoints, etc. On the input, the component expects a list of tasks specifying endpoints and queries. The Output contains the collected results in a single RDF graph. The Report output contains potential error messages encountered when querying the SPARQL endpoints.

Tasks specification

Below you can see sample task specification for the component. This task queries for a list of classes and number of their instances used in the endpoint. 

@prefix sel: <http://plugins.linkedpipes.com/ontology/e-sparqlEndpointList#> .

<https://nkod.opendata.cz/sparql> a sel:Task;
  sel:endpoint "https://nkod.opendata.cz/sparql";
  sel:group "https://nkod.opendata.cz/sparql";
  sel:query """PREFIX adhoc: <http://linked.opendata.cz/ontology/adhoc/>
CONSTRUCT {
  [] adhoc:class ?Class ;
     adhoc:endpointUri \"https://nkod.opendata.cz/sparql\";
    adhoc:numberOfInstances ?numberOfInstances .
} WHERE {
  {
    SELECT ?Class (COUNT(?resource) AS ?numberOfInstances)
    WHERE {
      ?resource a ?Class.
    }
    GROUP BY ?Class
  }
}""" .