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Architect system-design and trade-off interviews

45-minute pacing​

  • 0–5: clarify journeys, scale, quality attributes, constraints/non-goals.
  • 5–10: invariants, trust/data authority and rough estimates.
  • 10–20: high-level client/backend/data flow.
  • 20–32: one write/read, offline/retry/races/process death/security.
  • 32–39: scaling, observability, testing, rollout/recovery.
  • 39–45: trade-offs, alternatives, evolution and questions.

Prompt bank​

Design offline commerce, chat, banking transfer, ride tracking, media download, news feed, BLE health, super-app, enterprise multi-brand app, photo backup, collaborative notes or feature-flag platform. Use Chapter 47 as reference only after attempting.

Follow-up questioning​

Interviewers change constraints to test reasoning:

  • “Now users are offline seven days.” → durability, staleness, queue/conflict/storage.
  • “Traffic grows 100×.” → identify measured bottleneck/partition/cache/fan-out, not wholesale rewrite.
  • “One team becomes ten.” → ownership/contracts/platform governance/build/release.
  • “This contains health/payment data.” → threat/privacy/audit/authorization/recovery.
  • “Rollback must be instant.” → challenge installed clients/data/external effects; flags plus compatibility/forward repair.
  • “Minimum devices are low-end.” → budgets, paging/media/memory/startup/device test.

Trade-off language​

Use: “Given requirement X and constraint Y, I prefer A because it improves Q. It costs Z and fails under W. I would mitigate/measure with M and revisit when trigger T occurs.”

Do not answer every challenge by adding a service, repository, queue or abstraction. Simplicity is an option with benefits and limits.

Evaluation dimensions​

Requirement discovery, correctness/invariants, mobile depth, data/security, reliability/operations, scale/evolution, trade-off clarity, communication and prioritization. A beautiful diagram with no failure model is weak.

Exercise​

Run three prompts with 15-, 30- and 45-minute limits. Record assumptions, changes after follow-ups and the one risky claim you need to verify.