If you have 30 seconds. "Don't duplicate data" is a rule that fails in production: caches are legitimate, and you ship them anyway. The rule that actually holds is finer-grained — every stored value that resembles a duplication is either Live, Snapshot, or Cache , and each category has a distinct implementation. Skip the classification and you ship the same kind of bug I just found multiplied by 560 rows: a column quietly going stale because nobody decided whether it was supposed to track its source or not. Useful if you have backend tables you stopped trusting. A column nobody updated Saturday April 18th, 2026, mid-morning. I'm reading the dashboard with Hélène, who has kept the school's books for nineteen years. Her notebook is open, mine is the screen. She points at one row — a ceramics student, third year, a perfectly ordinary case — and frowns. Rembrandt says she still owes us €1,159 . Hélène's payment schedule says €2,262 . We cross-check on paper. Eleven monthly installments at €205.65.…