Minutes 0–5 — Clarify requirements
Functional requirements (what does it do), non-functional (QPS, latency, consistency), and out-of-scope. Write them on the board. Ambiguity ignored here kills you in minute 30.
The 45-minute system design loop, minute by minute.
TL;DR
A system design interview follows a predictable shape: requirements, API, storage, traffic, bottlenecks, deep dives. The candidates who pass aren't the ones who know every architecture — they're the ones who talk tradeoffs out loud and budget their time.
Minutes 5–10 — Estimate scale
Back-of-envelope: requests/sec, storage/year, read/write ratio. You don't need precision — you need an order-of-magnitude anchor for every later decision.
Minutes 10–20 — High-level design
Client → LB → service → DB. Draw it. Name your components. Explicitly say 'I'd use X because Y' for each edge. Silent drawing loses points.
Minutes 20–35 — Deep dive + tradeoffs
Interviewer picks a component. You deep-dive: schema, indexing, caching, sharding, replication. Narrate the tradeoff you're making and what breaks if you pick the other option.
Minutes 35–45 — Scale, reliability, open questions
How do you detect failure, what's the replication strategy, what's the rollout plan, what's unresolved. Naming what you haven't solved is a senior-level signal.
Frequently asked questions
- How long is a system design interview?
- Typically 45–60 minutes. FAANG companies tend toward 45; some hedge funds and startups extend to 90 with a heavy deep-dive.
- What does the interviewer score?
- Problem framing, scale estimation, high-level design correctness, deep-dive depth on one component, tradeoff reasoning, and communication. Different companies weight these differently.
- What if I don't finish?
- Finishing is overrated. Scoping a clear subset, nailing the deep-dive, and explicitly naming what's unfinished reads as senior. Rushing produces shallow boxes-and-arrows that score below average.
- Should I memorize famous architectures?
- Know 2–3 reference designs (URL shortener, news feed, chat) well enough to steal primitives from. Rote-reciting a Dropbox writeup reads as memorization and scores worse than original reasoning.
Practice this live.
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