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The schema versioning technique allows changes to the database schema with continued support of previous schemata and their corresponding data, which are retained without any change. The newly created schema version is (usually) used to accommodate new data insertions, modifications and deletions. This technique neither leads to loss of information nor to obsolescence of existing applications, as they can still work with old schema versions.
In a schema versioning environment, two related issues are involved: (1) compatibility between schema versions and applications and (2) data access facilities.
As far as the first issue is concerned, we distinguish between backward compatibility and forward compatibility. Backward compatibility means that data defined under the previous schema version can be processed by an application designed for the new schema version, whereas forward compatibility means that data introduced under a new schema version can be processed by an application designed for a previous schema version.
Key Terms in this Chapter
Schema Change Propagation: The effects of a schema change at instance level, involving suitable conversions necessary to adapt extant data to the new schema.
Temporal Schema Versioning: Schema versioning technique, in which different schema versions are indexed by timestamps.
Schema Version: A complete database schema as created by the initial design or by the application of schema changes to an existing schema.
Schema Change Semantics: The effects of a schema change at schema level.
Schema Evolution: A schema change modality that avoids the loss of extant data. No support is required for previous schemata.
Schema Versioning: A schema change modality that provides for the maintenance of previous schema versions, such that extant data are accessible through any of the supported schema versions.
Database Schema: The formal structure of the database, described by means of a data definition language.
Database Conversion/Migration: Process consisting in adapting data valid under a given schema version to another schema version.
Schema Changes: Operations performed on a (populated) database schema, in order to adapt it to new application requirements.