Capacity Estimation

Estimation is not about being right. It is about being right to within an order of magnitude, quickly, so you can rule out whole categories of design before spending a week on one.

"Ten million users" tells you nothing on its own. Ten million users generating 100 GB a day and ten million generating 100 TB a day are completely different systems — one fits on a laptop and one needs a distributed store. Five minutes of arithmetic separates them.

The habit to build: convert every requirement into requests per second, bytes per day, and bytes at rest. Those three numbers determine most of your architecture.