There are other ways to handle this at the application level, to be clear, using dual read & write and backfill. More relevant when you have TB+++ of data.
Interesting. I've done dual-writes at the application level to migrate the datastore for a smaller feature (branch by abstraction), but never for an entire application. And the code path was quite simple, so it was easy to think about all of the edge cases at one time in your head.
Do you have any resources which talk through the read/write/backfill approach?
