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Ontology Management

The core of Knora's ontology management logic is OntologyResponderV2. It is responsible for:

  • Loading ontologies from the triplestore when Knora starts.
  • Maintaining an ontology cache to improve performance.
  • Returning requested ontology entities from the cache. Requests for ontology information never access the triplestore.
  • Creating and updating ontologies in response to API requests.
  • Ensuring that all user-created ontologies are consistent and conform to knora-base.

When Knora starts, the ontology responder receives a LoadOntologiesRequestV2 message. It then:

  1. Loads all ontologies found in the triplestore into suitable Scala data structures, which include indexes of relations between entities (e.g. rdfs:subClassOf relations), to facilitate validity checks.
  2. Checks user-created ontologies for consistency and conformance to knora-base, according to the rules described in Summary of Restrictions on User-Created Ontologies.
  3. Caches all the loaded ontologies using CacheUtil.

The ontology responder assumes that nothing except itself modifies ontologies in the triplestore while Knora is running. Therefore, the ontology cache is updated only when the ontology responder processes a request to update an ontology.

By design, the ontology responder can update only one ontology entity per request, to simplify the necessary validity checks. This requires the client to construct an ontology by submitting a sequence of requests in a certain order, as explained in Ontology Updates.

The ontology responder mainly works with ontologies in the internal schema. However, it knows that some entities in built-in ontologies have hard-coded definitions in external schemas, and it checks the relevant transformation rules and returns those entities directly when they are requested (see Generation of Ontologies in External Schemas).