System Design Interview Questions: A Practical Primer
A repeatable system design framework, common questions, and scalability concepts with expert tips.
Introduction
This guide is for mid-level and senior engineers. System design interviews reward a repeatable framework and clear communication more than memorized architectures.
Read this if you want a structure you can apply to any design question.
Overview
- Clarify requirements (functional + non-functional) and scale first.
- Estimate capacity: RPS, storage, bandwidth.
- Sketch high-level components, then drill into bottlenecks.
- Discuss trade-offs: consistency, availability, latency, cost.
Preparation Tips
- Start with the API and data model before boxes.
- State your assumptions and numbers — silence kills design rounds.
- Pick one bottleneck (DB, cache, queue) and go deep.
- Always address failure: what breaks, and how you recover.
- Tie caching, sharding, and async to specific bottlenecks.
Top Interview Questions
1. Design a URL shortener.
Discuss encoding (base62), counter vs hash, collision handling, caching hot URLs, and analytics. Scale: read-heavy, cache aggressively.
2. Design a rate limiter.
Token bucket per key, sliding window, distributed via Redis; handle clock skew and hot keys. Choose client vs server-side limits.
3. Design a chat application.
WebSocket connections, message ordering, delivery guarantees, fan-out for group chats, and storage (hot vs cold). Discuss presence.
4. Design a news feed.
Fan-out on write vs read, ranking, caching, and denormalization. Trade latency vs storage for celebrity problem.
5. Design a key-value store.
Consistent hashing for sharding, replication, quorum reads/writes, and conflict resolution (vector clocks/LWW).
Common Mistakes
- Jumping to architecture before requirements and scale.
- Silent design — interviewers can't evaluate unspoken thinking.
- Ignoring failure modes and recovery.
- Listing every buzzword without tying to a bottleneck.
Expert Tips
- Use a 4-step script: requirements → estimation → design → bottlenecks.
- Drive the conversation; pause to check alignment every few minutes.
- Have one caching and one sharding story ready to deploy.
- Practice on a whiteboard or doc under a 40-minute timer.
Frequently Asked Questions
Is system design for juniors?
Usually from mid-level (L4/L5) up; juniors may get a simplified version.
Do I need to know specific technologies?
Concepts matter more; mention technologies as examples, not requirements.
How long is a design round?
Typically 40–45 minutes including questions and your design.
What's the most common mistake?
Designing in silence or over-scoping without confirming requirements.
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