[mighty_warrior]

They provided data integrity with a background sync process. What would have happened had that sync process failed?

In the past I have either flagged records to say where the system should read the data from, or just built logic into the readers to say if there is no data in A, read from B.

Then you update the writers to migrate the data from A to B on every new update and remove the data from A. It is an expensive 1 time write to move the data, but then you don't have to worry about keeping data in sync across two storage locations.

What you end up with is all records that are actively getting worked on move first. At a later date you start migrating all those stale records from A to B with a background process. Once A is empty, remove the logic to read from A, remove the migration writes, remove the A datasource.