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Real-time databases are different from conventional databases in that they have timing constraints on data and on transactions upon the data. The timing constraints of data require that the current status of the real world must be close enough to the state represented by the contents of the database. The timing constraints of transactions are typically expressed in the form of deadline which indicates a certain time in the future by which a transaction must be completed. Conventional data models can not be directly applied to describe the conceptual data model of a real-time database, since there is no mechanism to deal with the representation of timing constraints. A real-time data model should simultaneously satisfy many goals. It should clearly and concisely provide support for specifying timing constraints on data and transactions, semantics of real-time data and real-time transactions, concurrency control mechanisms, and transactions scheduling policies to meet the timing constraints defined by the real-time applications. This paper presents a real-time object-oriented data model and its use in designing real-time extensions to the DB4O database management system. This data model supports expression of time-constrained data, time-constrained transactions, concurrency control mechanisms, and transactions scheduling policies. It also can be integrated easily in existing object-oriented method and permits automatic code generation of all the real-time code of an application.

The design of real-time databases needs to incorporate methods specifically developed to represent the timing constraints of the system under consideration. Real-time databases have timing constraints on data and on transactions upon the data. During the last few years, several research approaches have been directed towards using the relational model as a data model for real-time databases. However, neither approach adds a temporal dimension to the data model. As realtime applications complexity grows, the real-time database community migrated towards object-oriented technology, both because one can use its rich data semantics in timing transactions processing and because complex real-time applications may benefit from its capabilities for modeling, storing and manipulating complex objects. In this paper we explore the realtime object-oriented data models for real-time databases, performing an overview of the existing models and presenting a set of features suitable to evaluate the quality of each model.