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RDF Processing API

Knora provides an API for parsing and formatting RDF data and for working with RDF graphs. This allows Knora developers to use a single, idiomatic Scala API as a façade for a Java RDF library. By using a feature toggle, you can choose either Jena or RDF4J as the underlying implementation.

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.

  • RdfNode and 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. RdfFormatUtil uses JsonLDUtil when appropriate.

To work with RDF models, start with RdfFeatureFactory, which returns instances of RdfNodeFactory, RdfModelFactory, and RdfFormatUtil, using feature toggle configuration.

JsonLDUtil does not need a feature factory.

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.

Feature toggle

For an overview of feature toggles, see Feature Toggles.

The RDF API uses the feature toggle jena-rdf-library:

  • on: use the Jena implementation.

  • off (the default): use the RDF4J implementation.

The default setting is used on startup, e.g. to read ontologies from the repository. After startup, the per-request setting is used.

What still uses RDF4J directly

Before this API was added, Knora mainly used the RDF4J API directly, and still does in some places:

  • Code that uses RDF4J's streaming API to process large amounts of data, especially to avoid constructing a large string in TriG format:

    • ProjectsResponderADM.projectDataGetRequestADM

    • HttpTriplestoreConnector.turtleToTrig

    • RepositoryUpdater

  • The repository update plugin tests, which use SPARQL.

  • TEIHeader: uses XSLT that depends on the exact format of the RDF/XML generated by RDF4J. The XSLT would need to be improved to handle rdf:Description.

  • GravsearchParser: uses RDF4J's SPARQL parser. This is probably not worth changing.

TODO

  • SHACL validation.

  • SPARQL querying.

  • A streaming parsing/formatting API for processing large graphs.