Live, Snapshot, Cache: the three-way decision before storing a derived value

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 so