[mewm]

well, you kinda just move the problem to another layer. You'll face same horizontal scaling issues once you need to scale on each user.

[c-smile]

Or just remove one [DB] layer completely. Such databases can be stored directly on machines executing http requests. And if you need to rebuild DB structure you don't need to stop the world - do them one by one.

Of course, all this depends on amount of data you need to store for each user and informational structure of the app.