Field note · September 25, 2024
Course note — Idempotency, backfills, and yesterday arriving twice
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.
Revised Idempotency, backfills, and yesterday arriving twice this week. The change was small and the reason was not.
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 first draft explained the mechanism and stopped. What was missing was the failure: the thing that happens when you get it wrong, described concretely enough that someone recognises it later. A lesson that only teaches the correct version leaves you unable to spot the incorrect one, which is the situation you will actually be in.
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.