Thanks for re-locating my question, Gurpreet - it’s greatly appreciated.
Green/Blue deployment isn’t quite what I’m looking for, however, as I would like to have both server ‘pairs’ (i.e. both database servers and both web servers) active at the same time. I can handle sticky sessions (if necessary) through my load balancer (i.e. HAProxy), and I’d imagine Master-Master replication should be okay, given UUIDs are used for Primary keys, rather than auto-incremented integers - if need be, however, I could always add an auto-increment offset to one of the servers.
Anyway, here’s a rough overview of what I’m aiming to achieve. Please forgive the rushed quality of the diagram. 
Thanks heaps!
Andrew
