I kept rebuilding the same pattern across products (idempotency keys, Redis TTLs, unique constraints) just to avoid duplicate side effects from retries and race conditions. Eventually I turned it into a focused API.

ExactOnce lets you create an action and consume it exactly once. The transition from active → consumed is enforced atomically via a DynamoDB conditional write. Under concurrency, one request succeeds; the rest deterministically get a typed 409 already_used. No distributed locks, no cross-service transactions.

Use cases:

+ magic login links + coupon redemption + webhook deduplication + one-time download links + ticket scanning + license activation

Try it without signing up:

curl --request POST https://api.exactonce.com/v1/demo

That returns a temporary action (5‑minute TTL) and a reveal URL. Open it once — it consumes. Open again — already used.

Docs: https://exactonce.com/api-docs How it works: https://exactonce.com/how-it-works

Private beta is free. Happy to answer questions about the DynamoDB implementation or the atomic consume pattern