CareerPath SysDesign 20

Learn — the 45-minute structure, used every time: 0–5 scope and cut, 5–10 estimate (QPS, storage/year, bandwidth), 10–15 API and data model, 15–30 high-level design plus a request walkthrough, 30–40 deep dive on the bottleneck you named, 40–45 failure modes.

Do — one question per session, out loud, on a blank page with nothing open. Reading design write-ups feels productive and transfers almost nothing.

Check — the four things interviewers actually score:

1. You drove requirements and scale numbers before drawing
2. You produced a back-of-envelope estimate
3. You named the bottleneck yourself and gave two options with a trade-off
4. You covered one failure mode — what happens when this component dies

Ship — 20 design docs: diagram, estimates, trade-off table, failure modes.

The tell. Candidates who list technologies get downlevelled; candidates who state trade-offs get levelled up. "We would use Kafka" is worth nothing. "We need at-least-once delivery and can tolerate duplicates because writes are idempotent, so a log-based queue beats a task queue" is the whole interview.