[pdonis]

> Does that help?

What you said up to that point didn't really. But then you said this:

> If you're using your database to handle concurrency, you need every request to start inside the transaction. You can't check the idempotency key outside of the transaction or you can't guarantee once-and-only-once behavior.

Which answers the question that what you said earlier in your post raised. If I'm understanding you right, "lookup the idempotency key" is also relying on the same database, so you need the whole operation to be inside a single transaction in that database.