A data systems interviewer will often change one constraint after your first design. The daily export becomes an operational alert, a stable identifier disappears, or replay must run alongside live updates. Explain which part of the design changes and which evidence still supports the rest. Naming a technology without tying it to that constraint leaves the design incomplete.
These questions move from easy arithmetic to harder recovery and experiment decisions. You'll inspect a flawed pipeline, preserve source order during replay, and choose a denominator under an explicit population contract. Each answer has a checked reason. Treat that reason as the sentence you would say before adding more detail in an interview. When you cannot prove completeness, identify the missing boundary or retained evidence instead of promising that a retry will solve it.