A retried request must not double-apply.
Name it
Say what this problem really is, professionally — the concept + when it bites. practice the framing →
Explain it out loud
Say it in your own words — but hit these: idempotency key · unique constraint · same transaction · at-least-once · retries · return the original result
checking microphone…
Code it (7-step rhythm, narrate)
Clarify → approach → data model → narrate → edge → test → tradeoff. AI off. Lab: idempotency store →
Edge cases — say + handle each
Say the trade-off
“The trade-off is extra storage and a hot-path lookup, in exchange for correctness under retries.”
checking microphone…
Answer the follow-up
Where do you store the key, and how long do you keep it?
Catch the classic bug
Spot the wrong implementation in <60s. open the bug drill →