System designHardsd-g186

Subject BackfillLevel Senior–Staff~50 minCommon in Networking & APIs interviewsIndustries Technology, Software development

Question

A bug in a transformation under-counted user watch-minutes for the last 90 days across a partitioned fact table (~3 TB/partition-day, 90 days). You must backfill the corrected values without taking the table offline for live dashboards, without double-counting, and without saturating the shared cluster that also runs daily pipelines and ad-hoc analyst queries. Design the backfill: how you reprocess 90 days correctly, how you swap in corrected data safely, and how you avoid starving production workloads.

What a strong answer looks like

Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts — data model, bottlenecks, consistency, failure modes — and name the trade-offs you are making.

Learn the concepts

Narrate your design

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Run or narrate your approach, then ask the coach.