Moss Market operates a fictional retail platform with a stock ledger, an event lake, and an experiment service. The tasks mix the Expert units without introducing a new technique. You'll choose a population, calculate an aggregate, and preserve correctness when an event arrives late or a historical job retries.
Read the required output before drawing. A metric for assigned users uses a different denominator from a metric for exposed users. A current-state replica needs a source boundary, while a historical analysis needs the historical events themselves. The same distinction appears in dashboard freshness: a recent browser request doesn't prove that the source is up to date. Use the supplied evidence to make each decision. Where the evidence is missing, choose a design that makes the limit visible instead of silently filling the gap.