Yea, keeping your application consistent with two different schema versions. And I'm not saying from a blue/green standpoint, from whatever pgroll does instead so when the rollback happens you don't lose data.
I don't see the need to keep your application consistent with both schema versions. During a migration pgroll exposes two Postgres schema - one for the old version of the database schema and another for the new one. The old version of the application can be ignorant of the new schema and the new version of the application can be ignorant of the old.
pgroll (or rather the database triggers that it creates along with the up and down SQL defined in the migration) does the work to ensure that data written by the old applications is visible to the new and vice-versa.
A rollback in pgroll then only requires dropping the schema that contains the new version of the views on the underlying tables and any new versions of columns that were created to support them.
Eh, isn't this making it easier on the application side? Today, when I make a change in my app that needs a migration, I need multiple steps to make sure I don't break old instances of the app still running. With this it looks like one can avoid all that?
That is what this project is trying to achieve. By allowing your client apps to access both the old and the new schema at the same time, you can have the old instances of your application working while the new ones (using the new schema) get deployed.
They can work in parallel for a while until you complete the rollout and call the migration as done.
