System Design

The 45-Minute Design Interview Framework

1. 1

   Clarify requirements (5–8 min)

   Separate functional (features) from non-functional (latency, availability, consistency, durability). Confirm MVP vs nice-to-haves, mobile/web, auth, and geographic scope.

2. 2

   Back-of-envelope estimation (3–5 min)

   Estimate DAU/MAU, QPS (avg + peak), storage, and bandwidth. State assumptions loudly. Round to powers of 10 — interviewers want reasoning, not precision.

3. 3

   API & data model sketch (5 min)

   List core endpoints (REST or RPC). Define primary entities and relationships. Pick read vs write patterns before choosing SQL/NoSQL.

4. 4

   High-level design (10 min)

   Draw clients → CDN → LB → services → cache/DB/queue. Walk through the hottest user flow end-to-end before adding boxes.

5. 5

   Deep dives (15–20 min)

   Interviewer picks 2–3 areas: sharding, caching, fan-out, consistency, bottlenecks. Explain tradeoffs and why you chose X over Y.

6. 6

   Wrap-up (3–5 min)

   Failure modes, monitoring/SLOs, security basics, and what breaks at 10× traffic. Mention phased rollout if time allows.