A homework planner with around 5,000 authenticated users, migrated from one database to another behind a feature flag without losing a record.
TimeCrunch had years of students and their coursework sitting in Realtime Database, and a version three that needed a structure the old database could not express well.
The users were real and the data was theirs. Losing a semester of somebody's assignments during finals week was not an acceptable failure mode at any probability.
The migration runs on the client, per user, behind a flag, the first time they open the new version. Nobody is moved until their own data has been written successfully.
The old database stays in place as a read-only backup rather than being cleaned up on success. Keeping it costs almost nothing and it is the only way to prove afterward that the migration did what it was supposed to.
The previous version had a destructive archive path that wiped data on term rollover. That is gone. Terms are real objects now, and nothing is destroyed to make room for them.
Each account migrates independently the first time it opens version three, gated on a flag that only flips after the write succeeds. A failure affects one person and retries, rather than taking down a whole cohort at once.
Realtime Database was kept read-only rather than cleaned up after the cutover. Storage is cheap, and being able to show exactly what a user's data looked like beforehand is worth far more than the space it takes.
The old version archived and wiped data on term rollover. Version three deletes that path entirely, which meant going looking for code that quietly destroys user data before anyone reported it.
A watchOS app, home screen widgets, and an upcoming assignments extension, all reading the same store, so a student sees the same state wherever they look.
Around 5,000 authenticated users moved to the new database with no records lost and no support queue full of people missing their coursework.
Migrating a live user base is the work most people are quietly nervous about. It comes down to going slowly and keeping everything you might need to fall back on.