Yes, I think there are many things I could write about "pains and gotchas" :)
Regarding RDS, is an interesting thing. We are migrating from a Rails app where all the servers were connecting to the same database, as also multiple Sidekiq workers. It required a lot of open connections to handle that.
We now have a quite simple setup where a small pool of connections can easily handle that, even with traffic always hitting the database :)
Yeah I guess that's a nice side effect, with Elixir you might get more control over your whole system (including "external" resources like databases) because it's so easy to reason about what is running concurrently.
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