Slightly shooting from the hip here (as I'm still learning).

Low-ish volume- design your system such that data flows [at the relevant crucial points] through a single actor to ensure proper concurrency.

High volume- trickier but I think same idea in principle. First thought that comes to mind here is the new GenStage stuff in Elixir.

I'm not sure what you are suggesting. There is no getting around "at most/at least once." You can shift the posts, some, but at some point that is your choice. This is a good read on the problem: http://bravenewgeek.com/you-cannot-have-exactly-once-deliver...

"Exactly once delivery" is not the same as "One message at a time". Akka Actors process one message at a time. Akka does no provide exactly once delivery. It default defaults to "at most once".

So, then to go back to the original.

You may not have to do deal with locks at a local level. You absolutely have to deal with locks at a system level.

Correct. Message delivery strategy is separate from message processing once delivered. If you choose at-least-once delivery, you'll need to handle possible duplicates regardless of whether or not you can process a duplicate message in a lock-free manner.