Read an interesting architecture article, adopted by PayPal.
I'm wondering how did Paypal take a billion hits a day system that might traditionally run on a 100s of VMs and shrink it down to run on 8 VMs. At the same time, it stays responsive even at 90% CPU at transaction densities.
Paypal has never seen before, with jobs that take 1/10th the time, while reducing costs and allowing for much better organizational growth without growing the compute infrastructure accordingly.
Answer is tha PayPal moved to an Actor model based on Akka. PayPal told their story here: Squbs: A New, Reactive Way for PayPal to Build Applications. Itz available at https://www.paypal-engineering.com/2016/05/11/squbs-a-new-reactive-way-for-paypal-to-build-applications/
They open source squbs and you can find squbs on GitHub. Ref: https://github.com/paypal/squbs