RDF Processing API
DSP provides an API for parsing and formatting RDF data and for working with RDF graphs. This allows DSP developers to use a single, idiomatic Scala API as a façade for a Java RDF library.
Overview
The API is in the package org.knora.webapi.messages.util.rdf. It includes:
-
RdfModel, which represents a set of RDF graphs (a default graph and/or one or more named graphs). A model can be constructed from scratch, modified, and searched. -
RdfNodeand its subclasses, which represent RDF nodes (IRIs, blank nodes, and literals). -
Statement, which represents a triple or quad. -
RdfNodeFactory, which creates nodes and statements. -
RdfModelFactory, which creates empty RDF models. -
RdfFormatUtil, which parses and formats RDF models. -
JsonLDUtil, which provides specialised functionality for working with RDF in JSON-LD format, and for converting between RDF models and JSON-LD documents.RdfFormatUtilusesJsonLDUtilwhen appropriate. -
ShaclValidator, which validates RDF models using SHACL shapes.
To work with RDF models, start with RdfFeatureFactory, which returns instances
of RdfNodeFactory, RdfModelFactory, RdfFormatUtil, and ShaclValidator.
JsonLDUtil does not need a feature factory.
To iterate efficiently over the statements in an RdfModel, use its iterator method.
An RdfModel cannot be modified while you are iterating over it.
If you are iterating to look for statements to modify, you can
collect a Set of statements to remove and a Set of statements
to add, and perform these update operations after you have finished
the iteration.
RDF stream processing
To read or write a large amount of RDF data without generating a large string
object, you can use the stream processing methods in RdfFormatUtil.
To parse an InputStream to an RdfModel, use inputStreamToRdfModel.
To format an RdfModel to an OutputStream, use rdfModelToOutputStream.
To parse RDF data from an InputStream and process it one statement at a time,
you can write a class that implements the RdfStreamProcessor trait, and
use it with the RdfFormatUtil.parseWithStreamProcessor method.
Your RdfStreamProcessor can also send one statement at a time to a
formatting stream processor, which knows how to write RDF to an OutputStream
in a particular format. Use RdfFormatUtil.makeFormattingStreamProcessor to
construct one of these.
SPARQL queries
In tests, it can be useful to run SPARQL queries to check the content of
an RdfModel. To do this, use the RdfModel.asRepository method, which
returns an RdfRepository that can run SELECT queries.
The configuration of the default graph depends on which underlying
RDF library is used. If you are querying data in named graphs, use FROM
or quad patterns rather than the default graph.
SHACL validation
On startup, graphs of SHACL shapes are loaded from Turtle files in a directory specified
by app.shacl.shapes-dir in application.conf, and in subdirectories of
that directory. To validate the default graph of an RdfModel using a graph of
SHACL shapes, call ShaclValidator.validate, specifying the relative path of the
Turtle file containing the graph of shapes.
Implementations
-
The Jena-based implementation, in package
org.knora.webapi.messages.util.rdf.jenaimpl. -
The RDF4J-based implementation, in package
org.knora.webapi.messages.util.rdf.rdf4jimpl.
TODO
- SHACL validation.