Can You Re-run This Safely?

Idempotency: The #1 Senior Signal This is the highest-signal topic in the entire reliability interview. Mentioning idempotency in the first 30 seconds of a pipeline design answer immediately tells the interviewer you've operated real pipelines in production. Candidates who bring up idempotency unprompted consistently score higher on the 'production experience' rubric. Your answer framework: 'The first thing I'd ensure is that the pipeline is idempotent - running it once or ten times produces the same result. I'd use MERGE on the natural key for dimension tables and partition overwrite for event tables. That way, when a job fails at 3 AM and the on-call engineer reruns it, there are zero duplicates.' The interviewer wants to hear two specific patterns: MERGE/UPSERT and partition overwrite.