Skip to content

00. Interview Case Studies — The Five-Year-Old Version

The interview is a stage. You're performing a design — live, under time pressure, with critics watching.

Imagine you're an architect pitching a building design to a city council. You have 45 minutes. The council asks: "Design us a library for 10,000 daily visitors."

You can't draw every brick. You start with the blueprint — the high-level structure. "Here's the main hall, here are the reading rooms, here's the parking structure." Then the council pokes: "What if 50,000 visitors show up?" You adjust the blueprint on the spot.

The council doesn't care if you memorize building codes. They care that you THINK through tradeoffs. "Should we add another floor or build a second location?" Both are valid — they want to hear your reasoning aloud. The inner monologue, made external.

You must also show you know what you DON'T know. "I'd need to check the soil report before committing to a basement." That is the honest gap — acknowledging unknowns is strength, not weakness. Interviewers reward self-awareness.

Every case study follows the same choreography: clarify requirements, estimate scale, propose high-level design, dive deep into one or two components, discuss tradeoffs, and address failure modes. The pattern is always the same. The content changes.

This module gives you 12 full case studies. Each one walks through the choreography step by step. Practice all 12 and the pattern becomes muscle memory. On interview day, you won't freeze — you'll perform.

What separates a "hire" from a "strong hire"? Three things:

First, structured communication. You talk in layers — start broad, go deep only when asked. You label your sections: "Let me start with requirements... Now scale estimation... Now the high-level design." The interviewer can follow your reasoning aloud without getting lost.

Second, tradeoff awareness. You don't just pick a solution — you explain what you're giving up. "I'll use eventual consistency here because strong consistency would add 50ms latency, and for a news feed, that's acceptable." The blueprint shows WHY, not just WHAT.

Third, knowing when to stop. You have 45 minutes. Spending 30 minutes on the database schema means you never discuss caching, scaling, or failure modes. The choreography has time-boxes. Respect them. Breadth first, depth on demand.

Common mistakes that sink candidates: - Jumping to solutions before clarifying requirements - Over-engineering (using Kafka for 100 events/day) - Ignoring failure modes (what if this service goes down?) - Not asking questions (the interviewer HAS the context) - Describing only the happy path

Each case study in this module covers one system. The format mirrors a real interview: requirements gathering, back-of- envelope estimation, high-level design, component deep-dives, and tradeoff discussion. Practice by setting a 45-minute timer and talking through each one aloud.

The final file is not a case study — it's the scoring rubric. Understanding how interviewers evaluate you helps you optimize your performance. Know the game you're playing.

The placeholders you will see called back

Placeholder Meaning
blueprint high-level architecture — the box-and-arrow diagram you draw first
reasoning aloud structured thinking — explaining WHY you chose each component/tradeoff
honest gap acknowledging unknowns — stating what you'd research or ask about
choreography the interview framework — requirements → scale → design → deep-dive → tradeoffs
stage the interview setting — time-boxed, interactive, with follow-up questions

Top resources

What's coming

  1. 01-design-url-shortener.md — TinyURL: hashing, base62, read-heavy, redirect latency
  2. 02-design-rate-limiter.md — token bucket, sliding window, distributed counters
  3. 03-design-notification-system.md — push, email, SMS; fan-out, preferences, delivery guarantees
  4. 04-design-news-feed.md — fan-out-on-write vs read, ranking, timeline assembly
  5. 05-design-chat-system.md — WebSockets, presence, message ordering, group chats
  6. 06-design-search-autocomplete.md — trie, prefix matching, ranking, and typeahead
  7. 07-design-youtube.md — upload pipeline, transcoding, CDN, adaptive streaming
  8. 08-design-distributed-cache.md — consistent hashing, eviction, cache stampede, replication
  9. 09-design-payment-system.md — idempotency, double-entry, reconciliation, and PCI compliance
  10. 10-design-ml-inference-platform.md — model serving, batching, autoscaling, A/B traffic
  11. 11-design-recommendation-engine.md — collaborative filtering, embeddings, real-time re-ranking
  12. 12-design-real-time-analytics.md — streaming aggregation, OLAP, approximate counting
  13. 13-answer-rubric.md — how interviewers score: what separates good from great

Bridge. Let's start with the classic: design a URL shortener. Simple enough to finish in 45 minutes, complex enough to show depth. → 01-design-url-shortener.md