Mastering System Design

The System Design interview is not a test of your knowledge. It is a test of your Seniority. The interviewer wants to see if you can lead a room and make high-stakes technical decisions.

1. Don't start drawing

The biggest mistake is drawing a box before you've asked a single question. **Architect Rule:** Spend the first 10 minutes defining **Requirements** and **Constraints**. - "How many users?" - "Is it Read-heavy or Write-heavy?" - "What's the required latency?" This proves you don't build things until you understand WHY they need to be built.

2. The "Back-of-the-Envelope" Calculation

A Lead doesn't guess. They calculate. Practice estimating QPS (Queries Per Second) and Storage requirements on the fly. - "100 Million users * 10 requests/day = ~12,000 QPS." Doing this proves you can predict infrastructure costs, which is a senior-level trait.

4. Career Mastery

Q: "How do I handle a trade-off I'm not sure about?"

Architect Answer: "Be honest and verbalize the logic. 'I could use Redis for speed, but if we need 100% data durability, we might want to stick to a SQL DB with aggressive indexing. In a real-world scenario, I'd benchmark both, but for this design, let's prioritize durability.' This shows you understand that engineering is a series of compromises."