Expand contract pattern - Continuous Delivery and Databases

Modifying the schema of a production database is *hard*. If something goes wrong, the impact on both customers and the team can be enormous. And it can be hard or even impossible to rollback a database schema change if things go wrong. And the same is true for any architectural change for a production application.

The Branch by Abstraction and Strangler Pattern makes significant application changes easier. Are there any similar patterns we can use to make production database changes less risky?

Indeed, there are. The Expand/Contract pattern is a blueprint for making the database migration. It makes the remodelling both reversible and safe. By *expanding* the application to accommodate both the old and the new schemas in parallel, we can give ourselves time to:

- Migrate any downstream dependencies on the old database schema
- Gain confidence that the migration is safe

We *contract* the application to the new version, once we’ve satisfied that the old schema is no longer needed.

The pattern helps to make significant, but necessary refactorings to your data model in a **continuous delivery way**. Most importantly, without threatening the robustness of your production applications.

1. Expand Contract
Pattern
Continuous Delivery for Databases
@leenasn

2. A few years back

3. https://pixabay.com/en/change-new-beginning-renewal-673006/

6. https://martinfowler.com/bliki/BranchByAbstraction.html
Branch by Abstraction

8. Database?

9. Slow

10. High risk
https://pixabay.com/en/balance-risk-courage-risky-3062272/

11. Less risky?

12. Database Refactoring

13. https://martinfowler.com/books/refactoring.html
Refactoring is a controlled technique for improving
the design of an existing code base. Its essence is
applying a series of small behavior-preserving
transformations, each of which "too small to be worth
doing".
However the cumulative effect of each of these
transformations is quite significant.

14. Split Column
customer
booking_date
status

trial_bookings

15. Migration

16. New Schema

17. Refactoring Process

18. Split table

19. Migration Time

20. New Schema

21. Refactoring Process

22. https://martinfowler.com/articles/evodb.html
Expand Contract

23. Expand

25. Contract

26. Basics

27. Versioning

28. Golden gate bridge safety first
Tests

29. https://pixabay.com/en/strategy-arrows-building-startup-3224515/

30. Principles of low risk releases

31. Incremental

32. Done

33. Decouple deploy from release

34. Small Batches

36. References
https://medium.com/continuousdelivery/split-table-a-database-refactoring-techni
que-5dec00449e51
https://medium.com/@leenasn/continuous-delivery-for-database-eabd7f280019
https://medium.com/@leenasn/expand-contract-pattern-and-continuous-delivery
-of-databases-4cfa00c23d2e
http://databaserefactoring.com/
http://www.informit.com/articles/article.aspx?p=1833567

37. Leena S N
@leenasn / [email protected]

https://medium.com/@leenasn