As we have seen, data migrations have to be executed in several steps. Rollout as well as rollback scripts are executed right before the deployment. This implies that the application supports N-1 compatibility as well as that only one deployment is being executed at a time.
The migration process requires to perform several software releases, each of them consistent in application code and schema migration scripts. Engineers need to plan their commits accordingly. It's advisable to perform the full schema migration in a timely manner, to keep the database schema clean and to ensure that ongoing migrations aren't simply forgotten about.
It's in the nature of the corresponding model refactoring, whether existing data needs ...
