backfills

The pipeline-design question that exposes whether you've ever been paged. One property, idempotency, solves reruns, late data, and backfills at once, and there are exactly three patterns that deliver it.

The aggregate-restatement problem that breaks naive backfills: events arrive days late and out of order, so any partition you already published may be wrong. The staff answer is event-time partitioning plus bounded restatement, not a bigger watermark.