Schema evolution support is an important facility for object-oriented database (00DB) systems. While existing OODB systems provide for limited forms of evolution, including modification to the database schema and reorganization of affected instances, we find their support insufficient. Specific deficiencies are 1) the lack of compatibility support for old applications, and 2) the lack of ability to install arbitrary changes upon the schema and database. This paper examines the limitations of existing schemes, and offers a more general framework for specifying and reasoning about the evolution of class definitions and the adaptation of existing, persistent instances to those new definitions.
We present an overview of the novel aspects of Avaion/Common Lisp: (1) support for remote evaluation through a new evaiuator data type; (2) a generalization of the traditional client/server model of computation, allowing clients to extend server interfaces and server writers to hide aspects of distribution, sucrras caching, from clients; (3) support for automatic commit and abort processing of transactions and automatic crash recovery of atomic data. These capabilities provide programmers with the flexibility to exploit the semantics of an application to enhance its reliability and efficiency.Avaion/Common Lisp has been implemented and runs on IBM RTs on the Mach operating system. Though the design of Avaion/Common Lisp exploits some of the features of Common Lisp, e.g., its packaging mechanism, all of the constructs are applicable to any Lisp-like language.<0 1989 Clamen, Leibengood, Nettles and Wing
Schema evolution support is an important facility for object-oriented database (OODB) systems. While existing OODB systems provide for limited forms of evolution, including modification to the database schema and reorganization of affected instances, we find their support insufficient. Specific deficiencies are 1) the lack of compatibility support for old applications, and 2) the lack of ability to install arbitrary changes upon the schema and database. This paper examines the limitations of existing schemes, and offers a more general framework for specifying and reasonipg about the evolution of class definitions and the adaptation of existing, persistent instances to those new definitions.
Algorithmic presentations are given for three calendars of historical interest, the Mayan, French Revolutionary, and Old Hindu.
Providing support for schema evolution allows existing databases to be adjusted for varying roles over time. This paper reflects on existing evolution support schemes and introduces a more general and functional mechanism to support schema evolution and instance adaptation for centralized and distributed object-oriented database systems. Our evolution support scheme is distinguished from previous mechanisms in that it is primarily concerned with preserving existing database objects and maintaining compatibility for old applications, while permitting a wider range of evolution operations. It achieves this by supporting schema versioning, allowing multiple representations of instances to persist simultaneously, and providing for programmer specification of how to adapt existing instances. The mechanism is general enough to provide much of the support necessarily for heterogeneous schema integration, as well as incorporating much of the features of object migration and replication.
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