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TBH, I am a big fan of actor- and reactor-like programming patterns (also known as Finite State Machines. While you’re not strictly required to use reactors and might be able to get away without them – they DO provide several very substantial benefits.

(Re)Actors, page 2:

On Zero-Side-Effect Interactive Programming, Actors, and FSMs

Abstract: “WHY are functional programming languages not popular for interactive programming purposes?” and “WHAT we can do about it?”
Quote: “IMNSHO, deterministic Actors are the very best thing in existence for interactive programming, with lots of very practical benefits (from production post-mortem, to protection of in-memory state against server faults).”

Unity 5 vs UE4 vs Photon vs DIY for MMO

Quote: “You can still use HLAPI despite its shortcomings”
Another Quote: “If you’re using one of the engines above (and not your own one), and your game requires Client-Driven Development Flow, you may want to start with a single-player Unity 5, or with a single-player UE4.”

Asynchronous Processing for Finite State Machines/Actors: from plain event processing to Futures (with OO and Lambda Call Pyramids in between)

Quote: “With ‘callback pyramid’ it is not easy to express the concept of ‘wait for more than one thing to complete’ , which leads to unnecessary sequencing, adding to latencies (which may or may not be a problem for your purposes, but still a thing to keep in mind).”
Another Quote: “Don’t even think of converting all of your code to a so-called Continuation-Passing-Style”

Server-Side Architecture. Front-End Servers and Client-Side Random Load Balancing

Quote: “[about Round-Robin DNS] one of these returned IPs can get cached by a Big Fat DNS server, and then get distributed to many thousands of clients”
Another Quote: “As a rule of thumb, Front-End Servers are a Good Thing™”

Server-Side MMO Architecture. Naïve, Web-Based, and Classical Deployment Architectures

Quote: “If you disrupt the game-event-currently-in-progress for more than 0.5-2 minutes, for almost-any synchronous multi-player game you won’t be able to get the same players back, and will need to rollback the game event anyway.”
Another Quote: “However, keep in mind, that all fall-tolerant solutions are complicated, costly, and for the games realm I generally consider them as an over-engineering (even by my standards).”

Client-Side. Client Architecture Diagram, Threads, and Game Loop

Quote: “To have a good concurrency model, it is not strictly necessary to program in Erlang”
Another Quote: “Most of developers agree that FSM-based programming is beneficial in the medium- to long-run.”

Client-Side. On Debugging Distributed Systems, Deterministic Logic, and Finite State Machines

Quote: “After your logic has failed in production, you can “replay” this inputs-log on your functionally identical in-house system, and the bug will be reproduced at the very same point where it has originally happened.”
Another Quote: “You can implement your Finite State Machine as a deterministic variation of a usual event-driven program”