Promotions — version 1

Promotions bounded context — the deals and discounts the pizzeria runs. It owns the rule that anchors this context: a discount can lower an order’s total, but it can never drive that total below zero. The rule is enforced as an invariant on a Discount value object, exposed as a reusable spec, and computed by a pure domain operation.

Domain Model

classDiagram
    class DealKind {
        <<enumeration>>
        Percentage
        FixedAmount
        FreeDelivery
    }
    class Coupon {
        <<value object>>
        +String code
        +String /normalized
    }
    class Discount {
        <<value object>>
        +DealKind kind
        +Decimal orderTotal
        +Decimal amountOff
        +Decimal /discountedTotal
    }
    Discount *-- DealKind

Domain Types

Coupon — value object

A coupon code a customer can apply. A first-class value object so a coupon is never a bare string, validated by shape.

Field	Type	Description
code	String
normalized	String	derived

Business rules

• a coupon code cannot be blank
• a coupon must look like PIZZA10

Discount — value object

A discount applied to an order total. THE rule of this context: the discounted total can never be negative — a deal can take the total to exactly zero (a free pizza) but no further. Both the gross total and the discount are non-negative, and the invariant proves the result stays non-negative too.

Field	Type	Description
kind	DealKind
orderTotal	Decimal	The order total before the discount is applied. Never negative.
amountOff	Decimal	How much money the discount takes off. Never negative.
discountedTotal	Decimal	derived

Business rules

• the order total cannot be negative
• a discount cannot be negative
• a discount cannot drive the order total negative

DealKind — enum

The kind of deal a coupon represents. A plain enum used for reporting.

Values: Percentage, FixedAmount, FreeDelivery

Specifications

IsFreeOrder on Discount

Condition: discountedTotal == 0

Services

DiscountService

A domain service with pure operations (expression bodies). cap clamps a requested discount so it can never exceed the order total (the safe way to build a Discount); rate gives the percentage rate for a deal kind.

Operations

• cap(orderTotal: Decimal, requested: Decimal): Decimal

• rate(kind: DealKind): Decimal

Coverage

Kind	Covered	Total
enum	1	1
value	2	2

All declared constructs are documented. ✅