"Design a [system] at scale."
System design interviews assess your ability to build scalable, reliable, distributed systems. They're common at mid-level to senior engineering interviews at tech companies. Unlike coding questions, there's no single correct answer — the interviewer wants to see your thought process, trade-off analysis, and communication.
Step 1 — Clarify requirements (5 min): Ask about scale (users, QPS, data volume), core features, and non-functional requirements (latency, availability, consistency).
Step 2 — Estimate scale (5 min): Calculate storage, bandwidth, and QPS to anchor your design decisions.
Step 3 — High-level design (10 min): Draw the major components (clients, load balancers, servers, databases, caches, CDN).
Step 4 — Deep dive (15 min): Pick the most critical/complex components and explain your design in detail. Discuss trade-offs explicitly.
Step 5 — Wrap up (5 min): Summarize, address bottlenecks, mention what you'd do differently at 10x scale.
Design Twitter's timeline feature: I'd clarify that we need to support ~100M users with a read-heavy workload (fanout on read vs. write trade-off). High-level: Client → CDN → API Gateway → Tweet Service + Timeline Service → Cassandra (tweets) + Redis (timeline cache). For the timeline, I'd use a hybrid approach: pre-compute timelines for users with <1000 followers (fanout on write), and do fanout on read for users following celebrities. This avoids the write amplification problem while keeping latency low for most users.
Always ask clarifying questions before drawing anything.
Think out loud — the interviewer wants to see your reasoning.
Explicitly state trade-offs. Don't just say "use a cache" — explain why and what the downsides are.
Know these core components well: Load balancers, SQL vs NoSQL, CDN, caching strategies, message queues, sharding.
Practice with HireMe AI's real-time copilot to get suggestions during actual interviews.
Design Twitter/X, Design a URL shortener, Design YouTube, Design a ride-sharing app, Design a key-value store, Design a notification system.
Very important. Use a whiteboard or shared screen to draw components and data flows. Visuals help both you and the interviewer stay aligned.
No. There is no perfect answer. What matters is demonstrating structured thinking, clear communication, and awareness of trade-offs.
Study distributed systems fundamentals (CAP theorem, consistency models), practice 20+ design questions, and use HireMe AI's mock interview for scored practice sessions.
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