Idempotency Patterns

1. What Idempotent Means Here

The word is used loosely enough to be confusing, so it is worth defining precisely before building anything on it.

An operation is idempotent when applying it twice leaves the system in the same state as applying it once.

Notice what the definition does not say. It does not say the code runs once. The code runs twice, both times to completion. The second run simply produces no additional change.

That distinction decides where the work goes. Preventing the second delivery is not possible, for the reasons in the previous lesson