Metamodelling with datalog and classes: ConceptBase at the age of 21

ConceptBase is a deductive object-oriented database system intended for the management of metadata. A distinguishing feature of the Telos language underlying ConceptBase is the ability to manage rules and constraints across multiple levels of instantiation in so-called meta formulas, thus offering uniform consistency management across heterogeneous notations or ontologies. Originally developed in the context of model-driven database design in the late 1980's, ConceptBase has been used in several thousand installations all over the world for numerous applications in areas such as requirements engineering, engineering information management, model management, eLearning, cultural information systems, and data warehousing. The internal representation is based on a quadruple object structure, combined with advanced Datalog engines, such that many optimization techniques in ConceptBase have pioneered ideas later pursued in the implementation of XML databases and ontology-based reasoning and data management engines.