Not built: a service mesh, a microservice per feature, a separate tier for every function. The whole estate is a list you can enumerate.
Outcome: 209,420,973 HTTP requests in one day (9 February 2016), served by 11 IIS web servers, four SQL servers, three Elasticsearch nodes, three Tag Engine servers, two Redis servers, four load balancers, two networks, two firewalls and two routers.
A system you can enumerate in a paragraph is a system you can patch, audit and reason about in an afternoon. Nothing in that architecture is clever. There is simply very little of it, and “very little of it” is most of why it was hard to attack.
The numbers are old by now, and that is part of the point: the argument was not that this was a temporary arrangement until the team could afford real infrastructure.
Source: Stack Overflow: The Architecture, 2016 Edition (17 February 2016)