Nithin Bekal@cosmin sqlite on rails had had some massive improvements in the past year, and should easily handle smaller apps. In fact Basecamp's campfire app now uses sqlite.
Highly recommend this write-up by @fractaledmind and his railsconf talk to see how well it performs out of the box with rails now.
https://fractaledmind.github.io/2024/04/15/sqlite-on-rails-the-how-and-why-of-optimal-performance/
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