A very well written introduction to reactive programming that implements Twitter recommendation using RX. Link by andrestaltz
What is reactive programming: how message-driven (has direction from sender to receiver) is different than event-driven (undirected) and how message-driven (Actor-based) avoids callback-hell.
I have not read the followings:
Fancy a PhD thesis on Concurrent Programming for Scalable Web Architecture by Benjamin Erb?
Futures and Promises from Scala Documentation