Using the RDF model to represent all system knowledge allows an easy interoperability between the stored data and the conceptual models and instances. Therefore, if the latter are enriched, extended or partially replaced, it is not necessary to change the implementation considerably. However, the requirements for tracking of provenance, versioning and stored knowledge meta-data make the use of RDF triples insufficient. Therefore, we use RDF quintuples and a repository model that supports them.

We will expose an open API, based on the ORDI SG data model that can be implemented for integration with alternative back-ends. The ORDI SG model is presented here in further detail, as it is the basis for the RDFDB API.

The atomic entity of the ORDI SG tripleset model is a quintuple. It consists of the RDF data type primitives - URI, blank node and literal, as follows:

{P, O, C, {TS1, ..., TSn}, where:

• S is the subject of a statement; of the type URI or blank node;
• P is the predicate of a statement; of the type URI;
• O is the object of a statement; of the type URI, blank node or literal;
• C is the context of a statement (i.e. Named Graph); of the type URI or blank node;
• {TS1, ..., TSn} is an unordered set of identifiers of the triplesets to which the statement is associated; of the type URI or blank node.

The ORDI data model is realized as a directed labeled multi-graph. For backward compatibility with RDF, SPARQL and other RDF-based specifications, a new kind of information is introduced to the RDF graph. The tripleset model is a simple extension of the RDF graph enabling an easy way for adding meta-data to the statements.

It is a new element in the RDF statement, previously expressed as a triple or a quadruple, to describe the relation between the statement and an identifiable group of statements. This new term is introduced to distinguish the new model from several similar, already existing, RDF extensions and the terms associated with them:

• Context, as defined in several RDF APIs like Sesame 2.0, YARS and others;
• Datasets, as defined in SPARQL;
• Named-graphs, as introduced at http://www.w3.org/2004/03/trix/, implemented in Jena and used in SPARQL.

The following is true for the tripleset model:

• Each contextualized triple {S, P, O, C} could belong to multiple triplesets;
• The triples are not "owned" by the contexts; a triple can be disassociated from a tripleset without being deleted;
• When a triple is associated with several triplesets it should be counted as a single statement, e.g. a single arc in the graph;
• A tripleset can contain triples from different contexts;
• Each tripleset can be converted to a multi-set of triples, as one triple can correspond to multiple contextualized triples, belonging to the tripleset.

Figure 4 below is a diagram of the relationship between the major elements in the ORDI data model.

Figure 4 - Entity-Relationship Diagram of the ORDI Data Model