Reader question — Idempotency, backfills, and yesterday arriving twice

Reader question on Idempotency, backfills, and yesterday arriving twice, and the answer belongs somewhere more findable than an email.

The single property that separates a pipeline you can operate from one you have to babysit — and the operation that will test it every time.

The question was, roughly, "when does this stop applying?" — which is the right question and the one lessons routinely fail to answer. Every technique has a range of validity, and stating it is what separates a lesson from a recipe.

It sits in the Getting the data in module. The exercise is a thinking one rather than a query one — some of this material is about deciding what to compute, and that does not need a keyboard.

Ordering matters here more than in most courses. It follows Batch, streaming, and the honest difference and leads into Schema drift, and the contract that prevents it, and reading it out of sequence mostly works but costs you the setup.

Free means free, and it also means we can rewrite it whenever it is wrong. No edition, no errata PDF, nothing to repurchase. The whole course is 36 lessons and the fixes land the day we find them.