Activity Idempotency

Goal: for every activity you write or review, be able to answer one question with evidence from the code — if this body runs twice with the same input, is the state of the outside world the same as if it had run once?

If the answer is no, the activity is not finished, however well it works on the happy path.

The guarantee you are designing against

The engine guarantees each scheduled activity runs at least once. It does not guarantee exactly once, and no configuration makes it do so.

An activity runs a second time because: