"What Happens If Your Pipeline Runs Twice?"

What They're Really Testing The Unlock Idempotency is not a feature you add. It is a property that emerges from how you write data. The mental model: every write operation should be a function where f(x) = f(f(x)). If you INSERT, running twice creates duplicates. If you DELETE-then-INSERT the same partition, running twice produces the same result. The write pattern determines idempotency. The 60-Second Framework Saying this unprompted in the first 60 seconds of a pipeline design question puts you ahead of 90% of candidates. Most candidates never mention idempotency at all. The interviewer has to ask 'what happens if it runs twice?' and then watches the candidate stumble. Why Companies Care At Airbnb, a non-idempotent booking pipeline double-counted revenue for 48 hours after an Airflow ret