A few years ago my side project (and my second ever ruby/rails project) unexpectedly became quite popular.

Last couple of years it's been doing about 50,000 (user facing) api requests per second during high load, with almost a million concurrent users. It's 99% rails.

We (me and a second guy) have ran this system on ~10 (cheap) servers, five of which run the actual rails app. The rails app has probably been the easiest component to scale for us. We obviously do some caching - but it's not like our database could live without that caching anyways.

And even though we have been an ad-financed bootstrapped company the hosting/hw costs has been very manageable. There are probably many valid reasons to chose something else than rails, but I wanted to share my experience w.r.t. making it work for our non-SAS needs.