[c-smile]

Consider multi-user wordpress site for example. You can have one SQLite DB per user - there is simply no information to share between users. If so you can split all your users between multiple backend machines and do per user request routing - like one machine serving users from A to F. SQLlite has cheap DB loading sequence so you can open/close it for each page request. That would be perfect horizontal scaling as all your users will not fight for single DB access.

[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.