Part VI — Step-by-Step Interview Walkthroughs

Part V solved problems in a compact, reference style. Part VI shows how the 45–60 minutes actually unfold: what you say, what you ask, what the interviewer answers, where you draw, and how the design changes as you learn more. Each walkthrough follows the §2 framework, step by step, with timings.

47. How to Use These Walkthroughs

Format of each walkthrough:

 The prompt (and the variants you'll hear at different companies)
 Step 0  Clarify the business goal          — dialogue: 🎤 Candidate / 🧑‍💼 Interviewer
 Step 1  Functional requirements + scope cuts
 Step 2  Non-functional requirements (numbers)
 Step 3  Estimates → the decisions they force
 Step 4  APIs + data model (partition keys!)
 Step 5  High-level architecture (diagram) + walk the write path and read path
 Step 6  Deep dives (the 2-4 places the system is actually hard)
 Step 7  Failure modes, 10× scale, cost, evolution, ownership
 Step 8  Wrap-up statement (what to say in the last minute)
 Then:   Interviewer follow-ups with strong answers · 🌍 real-world references ·
         ⚠️ corner cases · signals by level

How to practice with them:

Read only the prompt. Close the guide and do it yourself, out loud, in 45 minutes.
Compare your Steps 0–3 with the walkthrough. Most candidates lose the most points here, not in the deep dives.
Compare deep-dive choices. Did you find the same "hard parts"?
Practice the follow-ups as rapid-fire questions with a partner.

The dialogue shows one plausible path. Real interviewers steer differently. The goal is to learn the moves (clarify → quantify → decide → justify → stress-test), not to memorize the script.