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name: ddd-aggregate-design description: Design rich domain aggregates with proper encapsulation, domain events, and the Export pattern. Use when implementing DDD aggregates with event sourcing patterns.

Rich Domain Aggregate Design

Design and implement rich domain aggregates with proper encapsulation, domain events, and the Export pattern.

When to Use This Skill

• Designing new domain aggregates from scratch
• Refactoring anemic models to rich domain models
• Adding domain events to existing aggregates
• Reviewing aggregate design for encapsulation

Core Rules

Read DDD-RULES.md for all aggregate design rules. Key principles:

1. Hide all fields - All struct fields must be private
2. Actionable methods only - Expose domain operations, not getters
3. Domain Events - Track state changes through events
4. Export pattern - Use Export() to expose state externally
5. Factory functions - Create aggregates only through validated factories

Quick Reference (Go Examples)

Factory Function Pattern

Copyfunc NewOrder(customerID CustomerID, items []OrderItem) (*Order, error) {
    if len(items) == 0 {
        return nil, ErrEmptyOrder
    }
    o := &Order{
        id:     NewOrderID(),
        status: OrderStatusPending,
        events: &eventRegister{},
    }
    o.addEvent(applayer.NewEvent(OrderCreated{OrderID: o.id}))
    return o, nil
}

Actionable Method Pattern

Copyfunc (o *Order) Ship(warehouse WarehouseID) error {
    if o.status == OrderStatusCancelled {
        return ErrOrderAlreadyCancelled
    }
    if o.status == OrderStatusShipped {
        return nil // Idempotent
    }
    oldStatus := o.status
    o.status = OrderStatusShipped
    o.addEvent(applayer.NewEvent(OrderShipped{OldStatus: oldStatus}))
    return nil
}

Export Pattern

Copyfunc (o *Order) Export() ExportedOrder {
    return ExportedOrder{
        ID:     o.id.String(),
        Status: string(o.status),
    }
}
func (o *Order) ID() OrderID { return o.id } // Only identity getter allowed