Living in Parallel Realities: Co-Existing Schema Versions with a Bidirectional Database Evolution Language.

We introduce end-to-end support of co-existing schema versions within one database. While it is state of the art to run multiple versions of a continuously developed application concurrently, it is hard to do the same for databases. In order to keep multiple co-existing schema versions alive -- which are all accessing the same data set -- developers usually employ handwritten delta code (e.g. views and triggers in SQL). This delta code is hard to write and hard to maintain: if a database administrator decides to adapt the physical table schema, all handwritten delta code needs to be adapted as well, which is expensive and error-prone in practice. In this paper, we present InVerDa: developers use the simple bidirectional database evolution language BiDEL, which carries enough information to generate all delta code automatically. Without additional effort, new schema versions become immediately accessible and data changes in any version are visible in all schema versions at the same time. InVerDa also allows for easily changing the physical table design without affecting the availability of co-existing schema versions. This greatly increases robustness (orders of magnitude less lines of code) and allows for significant performance optimization. A main contribution is the formal evaluation that each schema version acts like a common full-fledged database schema independently of the chosen physical table design.