You shipped a logic bug three months ago. How do you safely backfill and reprocess the affected data?
A backfill that double-counts or knocks over the live pipeline is worse than the bug it fixes. The signal is idempotent partition-scoped rewrites, keeping backfill compute isolated from live runs, and validating before the swap.
Updated Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.
A backfill that double-counts or knocks over the live pipeline is worse than the bug it fixes. The signal is idempotent partition-scoped rewrites, keeping backfill compute isolated from live runs, and validating before the swap.
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.
