A formal model for temporal schema versioning in object-oriented databases

The problem of supporting temporal schema versioning has been extensively studied in the context of the relational model. In the object-oriented environment, previous works were devoted to the study of the different aspects of schema evolution or (non-temporal) versioning in branching models, due to the traditional origination of the object-oriented model from CAD/CAM and CIM. Nowadays, the common use of the object-oriented model for a wide class of applications, extends temporal versioning requirements and expectations also to this model. In this paper we present a formal model for the support of temporal schema versions in objectoriented databases. Its definition is partially based on a generic (ODMG compatible) object model and partially introduces new concepts. The proposed model supports all the schema changes which are usually considered in the OODB literature, for which an operational semantics and a formal analysis of their correct behaviour is provided. Semantic issues arising from the introduction of temporal schema versioning in a conventional or temporal database (concerning the interaction between the intensional and extensional levels of versioning and the management of data in the presence of multiple schema versions) are also considered.