A load balancer is usually the first box you draw after the clients, and the depth you can give on it is a fast proxy for how senior you sound. This covers L4 vs L7, the algorithms and their failure modes, health checks, and the mistake of moving your single point of failure instead of removing it.

Every system wants to be fast, scalable, and reliable, which is exactly why saying so is worthless. This post is about the sharper version: p99 under 200 ms, 99.99% availability, eventual consistency on the timeline — and the architectural lever each of those numbers forces you to pull.

Most system design answers go wrong one step before the boxes and arrows: the candidate never works out how big the system is. Here are the four formulas, the latency numbers worth memorizing, and a worked Twitter example that turns four calculations into five architecture decisions.

Most system design interviews are lost in the first five minutes, not because the candidate lacks knowledge, but because they jump to boxes and arrows before understanding what they are building. Here is the six-step framework that fixes it, and why the order matters more than any individual step.