Basic Architecture of Sport Web

docs/adr/0011-money-as-integer-minor-units.md

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+# ADR-0011: Money as integer minor units
+
+- **Status:** Accepted
+- **Date:** 2026-08-11
+
+## Context
+
+`0.1 + 0.2 !== 0.3`. Floating-point money produces discrepancies that are invisible in
+testing and unfixable once they are in an order history.
+
+## Decision
+
+All monetary values are integers in the currency's minor unit, in the database
+(`priceAmount Int`), across the API (`{ amount, currency }`), and in TypeScript (`Money`).
+VND has a minor-unit scale of 0, so `250000` means ₫250.000. Conversion to a display string
+happens in exactly one function, `formatMoney`, using `Intl.NumberFormat`.
+
+## Consequences
+
+Arithmetic is exact. No conversion happens between layers because every layer holds
+the same integer. Multi-currency is already representable without a schema change.
+
+Developers must remember that `price.amount` is not a display value; the single formatter and
+the absence of any other division by 100 are what keep that from going wrong.
+
+## Alternatives considered
+
+`Decimal`/`numeric` columns — correct in the database but arrive in JavaScript as
+strings or a Decimal object that must be handled at every boundary. Floats — never.