I wrote this in response to the below comment, which is now edited and unfortunately dead, so posting here:

I understand, that wasn't a comment on your efforts back then, just that it is a solved problem today. But that does not mean other scaling problems are comparable or comparably solved. The universe of scaling problems is immense!

Worse, different problems occur at different scales. In the 3rd party API system, years after the first re-architecting, some use-cases developed issues at scale that exceeded the already high operational parameters we benchmarked at, and required us to re-architect the service again, including building out a whole new cluster so we could isolate that traffic entirely.

It is really hard to predict how things will break until they do.

(As an aside, I remember reading a lot of interesting things about Blizzard's technology, even if Blizzard didn't publish those themselves. There were many people who researched their products and published their findings. For instance, someone analyzed wireshark traces and published a very detailed report about how they tuned their server-side networking stack. One thing that stood out was Blizzard used TCP for WoW, whereas the conventional wisdom was UDP for real-time multiplayer!)