Semantic repositories* are engines similar to the database management systems (DBMS) – they allow for storage, querying, and management of structured data. The major differences with the DBMS are can be summarized as follows:
- They use ontologies as semantic schemata. This allows them to automatically reason about the data.
- They work with flexible and generic physical data models (e.g. graphs). This allows them to easily interpret and adopt “on the fly” new ontologies or metadata schemata.
As a result, semantic repositories offer easier integration of diverse data and more analytical power. To illustrate the usefulness of the automated interpretation (or reasoning), consider a query about telecom companies in Europe; given a simple ontology, which defines the semantics of the location and industry sector nesting relationships, a semantic repository can return as a result a mobile operator operating in the UK.
Over the last decade, the Semantic Web emerged as an area where the semantic repositories become as important as the HTTP servers. This tendency led to very high interest and activity in the field and resulted in a number of robust metadata and ontology standards, delivered by the W3C-driven community processes, most notable among which are RDF(S) and OWL. The named standards have a role similar to the role SQL played for the development and for the spread of the relational DBMS. Although primarily designed for use within the Semantic Web, the standards were widely accepted in areas like Enterprise Application Integration and life sciences.
Sesame is one of the most popular semantic repositories that supports RDF(S)and all the major syntaxes and query languages related to it. GraphDB™ is another semantic repository, packaged as a storage and inference layer (SAIL) for Sesame. GraphDB™ uses the TRREE engine to combine RDFS, OWL DLP, and OWL Horst support with high-performance reasoning and reliable persistence strategy.
* – so far, there is no agreed upon and well-defined term for what were described above as a “semantic repository”. A non-exhaustive list of weak synonyms is as follows: reasoner, ontology server, semantic store, metastore, RDF database. As a rule, the different wording is a reflection of the differences underlying the implementation, performance, intended application, etc. The description given above tries to cover the core functionality offered by the vast majority of the tools summarized under the name “semantic repository”.