The management of changing types in an object-oriented database

We examine the problem of type evolution in an object-oriented database environment. Type definitions are persistent objects in the database and as such may be modified and shared. The effects of changing a type extend to objects of the type and to programs that use objects of the type. We propose a solution to the problem through an extension of the semantic data model. A change in the interface defined by a type may result in errors when programs use new or old objects of the type. Through the use of an abstraction of the type over time, timestamping and error handling mechanisms provide support for the type designer in creating compatible versions of the type. The mechanisms are incorporated into the behavior defined by the type and are inherited via the type-lattice.