I Thought I Knew System Design Until I Met a Google L7 Interviewer

A single whiteboard question revealed the gap between knowing patterns and actually designing systems that scale.

The marker was in my hand. The problem seemed straightforward enough: design a URL shortener. I’d done this before, or at least I thought I had. Hash the URL, store it in a database, return the short code. Classic stuff. I started drawing boxes for the API layer, the cache, the database. The interviewer just watched, nodded a bit, then asked: “What happens when you hit 10 million writes per second?”

I completely froze.

Not because I didn’t know about sharding or replication. I’d spent weeks studying those patterns. I froze because in that moment I realized something kind of uncomfortable: I’d been memorizing architectures without actually understanding why they existed in the first place. Like learning to recite a poem in a language you don’t speak.

You know that feeling when you suddenly see yourself from the outside? That was it.

Turns out I’m not alone in this. There was this survey last year of about 800 engineering candidates, and something like 67% could draw a load balancer perfectly, but only 19% could actually explain when horizontal…