Linked Data Fragments API

Introduction

Linked Data Fragments presents "new ways of publishing Linked Data, in which the query workload is distributed between clients and servers", an alternative to SPARQL endpoints and data dumps.

dotNetRDF features a client for Triple Pattern Fragments, a flavor of Linked Data Fragments.

Basic Example

The following example prints the first one thousand triples from the DBpedia LDF endpoint:

using var dbpedia = new TpfLiveGraph(new("https://fragments.dbpedia.org/2014/en"));
foreach (var t in dbpedia.Triples.Take(1000))
	Console.WriteLine("{0}", t);

Note

The example above results in 11 network calls:

• One to obtain the TPF metadata from the endpoint.
• Ten more to retrieve ten pages of data, a hundred items each.

Important

Repeating the code above (i.e. two subsequent iteration of the same first 1000 items) results in twice the network calls because there is no caching. You can use a custom Loader to implement caching.

Warning

An unrestricted iteration over all triples of the DBpedia TPF graph above (e.g. foreach (var t in dbpedia.Triples)) would result in thousands of network calls.

Programmatic triple patterns

The following example retrieves the first 1000 subjects that are instances of the Character class from the Between Our Worlds TPF endpoint:

using var betweenOurWorlds = new TpfLiveGraph(new("https://data.betweenourworlds.org/latest"));

var a = betweenOurWorlds.CreateUriNode(UriFactory.Create(RdfSpecsHelper.RdfType));
var character = betweenOurWorlds.CreateUriNode(UriFactory.Create("https://betweenourworlds.org/ontology/Character"));

var characters = betweenOurWorlds.GetTriplesWithPredicateObject(a, character)
    .Select(c => c.Subject)
    .Take(1000);

foreach (var c in characters)
	Console.WriteLine("{0}", c);

Note

Observe the similarity between the invocation of GetTriplesWithPredicateObject method above and the resulting TPF query:

GET /2014/en?predicate=http%3A%2F%2Fwww.w3.org%2F1999%2F02%2F22-rdf-syntax-ns%23type&object=http%3A%2F%2Fdbpedia.org%2Fclass%2Fyago%2FWorldWideWebConsortiumStandards
Host: fragments.dbpedia.org

Note

See Selecting Triples for more details on programmatically working with graphs.

SPARQL querying

The following example uses SPARQL against the DBpedia TPF endpoint to find the English labels of W3C standards:

const string sparql = """
    PREFIX yago: <http://dbpedia.org/class/yago/>
    PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>

    SELECT ?label
    WHERE {
        ?standard
            a yago:WorldWideWebConsortiumStandards ;
            rdfs:label ?label .

        FILTER (LANG(?label) = 'en')
    }
    """;

using var betweenOurWorlds = new TpfLiveGraph(new("https://fragments.dbpedia.org/2014/en"));
foreach (var r in betweenOurWorlds.ExecuteQuery(sparql) as SparqlResultSet)
	Console.WriteLine("{0}", r);

Important

Due to implementation details of dotNetRDF's query engine, LIMIT statements in SPARQL queries are executed only once all relevant triples have been obtained from the TPF endpoint, so they cannot be used for restricting the number of network requests. In some circumstances you can achieve the desired effect by using limited inline selects.

Note

See Querying with SPARQL for more details.