[orisho]

Sure, this problem can be solved as can any other -- but there's a cost to it in development time. For every feature, the dev working on it had to do it in 3 steps to ensure backwards compatibility and handle partial migration failures gracefully. Imagine doing this for a small feature - the ops effort dwarfs the actual feature! Many small features would probably be dropped.

[gomox]

Multi-single-tenant actually makes it easier to do transactional schema upgrades because these operations only impact one customer at a time. No fiddling with middle-of-the-migration state.

[411111111111111]

Depends on your uptime guarantees.

It becomes a non-issue if you're able to go down for a few moments. Not necessarily viable for some software though, it just depends on what you're running.

(Especially because you can apply the migration on slave and subsequently switch master, making this downtime as quick as the time your system needs to do a database switch.)

This doesn't apply to big services with millions of users, however.