Design an idempotent inventory decrement for a flash sale without overselling.
PICTURE THIS: HOW JAVA RUNS
1. You writeJava source
2. javacBytecode .class
3. JVMRuns on any OS
Simple meaning
Persist a unique reservation id per attempt and decrement with UPDATE ...
WHY — Idempotency instead of guessing?
Why interviewers care about Idempotency:
Idempotency questions separate people
who only read docs from people who shipped.
Keep it short, concrete,
and tied to Backend work.
Stay structured
Name the idea, why it exists, then one short example.
Close cleanly
End with when you use it and one common pitfall.
STEPS — What happens step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1Persist a unique reservation
id per attempt and decrement with UPDATE ...
- 2Why it exists
WHERE sku=?
- 3AND qty >= :n
RETURNING qty.
- 4If the unique reservation
exists, return the original result.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Say this line
“If the unique reservation exists, return the original result.”
Break into beats
Iftheuniquereservationexistsreturn
Speaking order
2987408337471632900
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
Persist a unique reservation id per attempt and decrement with UPDATE ... WHERE sku=?