Design a backfill for an aggregate table that takes late and out-of-order events without double-counting or holes.
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.
Updated Aug 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.
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.
Lead with where the obvious approach breaks, because that is the judgment they are screening for — most candidates jump straight to the happy path and lose the room.
Then walk the failure back through the pipeline in order, naming the one metric the customer's exec sponsor actually cares about before you propose the fix.
