Is LiteFS+SQLite suitable for something like a SaaS solution? For example - Fresh books, Trello or Craigslist - all three have different needs. So which kind of apps should NOT be built on top of LiteFS+SQlite combination?
Technical requirements for a SaaS app have a tendency to become something like a Jenga tower over time. You'll be able to sail through launch and initial customers but then slam hard into difficult architectural challenges as you suddenly onboard larger customers or unique use cases.
For SQLite my guess would be areas of high concurrent write throughput - like a seasonal holiday/rush, a viral influx of users, or the onboarding of a large client.
Its not that SQLite can't handle these situations with careful architectural decisions. Its that out-of-the-box solutions, like the kind people depend on to solve business-issues in short time frames, won't support it as readily as more mainstream options.
I agree with everything the OP said above. Typically if you need to scale writes in SQLite, you'll want to look at sharding. The "single writer" restriction is per database so you can split your SaaS customers across multiple databases.
If your SaaS is in the hundreds or thousands of customers then you could split each customer into their own database. That also provides nice tenant isolation. If you have more customers than that you may want to look at something like a consistent hash to distribute customers across multiple databases.
In scenarios where an individual customer/tenant can have isolated data this makes sense. Is there any reason why the client application itself can't be one of the nodes in the distributed system? Does LiteFS support a more peer-2-peer distribution model (similar to a git repo) where the client/customer's SQLite database is fully distributed to them and then it's just a matter of merging diffs?