The principal tenet of the persistence model is that it abstracts over all the physical properties of data such as how long it is stored, where it is stored, how it is stored, what form it is kept in and who is using it. Experience with programming systems which support orthogonal persistence has shown that the simpler semantics and reduced complexity can often lead to a significant reduction in software production costs. Persistent systems are relatively new and it is not yet clear which of the many models of concurrency and distribution best suit the persistence paradigm. Previous work in this area has tended to build one chosen model into the system which may then only be applicable to a particular set of problems. This thesis challenges the orthodoxy by designing a persistent framework in which all models of concurrency and distribution can be integrated in an add-on fashion. The provision of such a framework is complicated by a tension between the conceptual ideas of persistence...

We address the problem of null values and other forms of irregular data in object-oriented databases. This problem is important because of such aspects of modern database technologies as conceptual modeling, database design, schema evolution, interoperability, integration of data from heterogeneous sources, data warehousing, as well as reliability, maintenance and documentation of databases and application programs. We analyze various approaches to null values postulated by the relational school, in particular the SQL approach and the default values. In response we propose a new universal approach to irregular data based on the idea of absent objects. We believe that our method solves the difficulties encountered in SQL. Moreover, it covers union types and can be smoothly combined with the idea of default values. We discuss the implications of the approach to query languages, and briefly introduce a new approach to polymorphic typing of irregular data. 1 Introduction The phenomenon of...