This is an entry on schema evolution. General papers on O2 are included.
We have implemented in O2 schema updates in our first release but without NO IMPACT on the database (we have a design to implement deferred update, but it is a paper design). However, users manage to convert their instances by hand, using their O2 programs written themselves, and with the aid of the following tools:
1- There is a set of predefined classes whose instances contain objects representing a schema (i.e., a Meta-schema). These classes may be used in a conversion program, they may even be extended by the programmer.
2- There is a save-restore program that allows to take an O2 database, save it on a file or a tape in a logical way (i.e., independent of the physical format of objects on disk), and restore it again on a (perhaps new release) of the system, in an empty database. Currently, when saving a database its schema is also saved. The next extension to this save/restore program will be to save the database without saving its schema, and then restore the database on a new version of that schema. The restore program will be able to perform automatically some conversions like "add attribute" or "delete attribute".
Schema updates with impact on the database will be implemented in future releases.
[Fernando Velez <email@example.com>]
For more information on O2, consult the following REFERENCES:
Francois Bancilhon, Claude Delobel, Paris Kanellakis. "Building an Object-Oriented Database System: The Story of O2". Morgan Kaufmann Series in Data Management Systems, San Mateo, Calif., 1992. F. Bancilhon, G. Barbette, V. Benzaken, C. Delobel, S. Gamerman, C. Lecluse, P. Pfeffer, P. Richard, and F. Velez. "The Design and Implementation of O2, and Object-Oriented Database System". Advances in Object-Oriented Database Systems, Springer Verlag. (Lecture Notes in Computer Science series, Number 334.)
C. Lecluse, P. Richard, and F. Velez. "O2, an Object-Oriented Data Model". Proceedings of SIGMOD88. Also appears in Zdonik and Maier, "Readings in Object-Oriented Database Systems", Morgan Kaufmann, 1990.
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