Depends a lot on your infrastructure too.
I have a small server farm in my company (7 servers today) and we already have a pretty good PostgreSQL machine, so I told Discourse to use it. That’s pretty good, and if you consider that PostgreSQL 
is the go-to SGBD in most open-source stacks this will happen a lot. And we reuse a 12GB 12CPUs server so thats pretty fast 
.
Redis is also a default in-memory stuff, but since we doesn’t got a dedicated machine for it yet it’s togheter with Discourse on a pretty standard Ubuntu VM.
We can host a lot of small apps with it, sinatra cruds, go apps, and re-use the nginx to proxy pass to other apps.