Field note · October 25, 2025
Cutting a lesson down — 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.
A note from writing Idempotency, backfills, and yesterday arriving twice, which took three passes to get to something short.
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 hard part of writing this was cutting it. The first version covered every case; the useful version covers the case you hit on a Tuesday and names the rest in a sentence. Completeness is a property of reference material, not of teaching material, and confusing the two produces something nobody finishes.
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.