#scalability

Counting unique visitors sounds like a set: add every user id, read its size. At a few billion ids that set is tens of gigabytes, kept only to produce one number. Do you need to remember who visited to count them?

A celebrity posts and a million people tap the heart in ten minutes, and every one of them sees a count that seems to keep up. One row with a number in it can't take that. So what does?

A green tick appears before you've stopped typing, and behind it is a list of a billion taken names. It isn't one clever database query.

One product-page endpoint, the same code, ten times more traffic, then ten times more again. Each step breaks something different, and the last one breaks something extra capacity can't fix.

The interviewer asks how much traffic you're expecting. Round to powers of ten, turn per-day into per-second, and end every number on the design decision it forces: one node, a cache, or a shard.