Domain-Driven Design (DDD) offers powerful strategic patterns that can transform how we approach complex software projects. In this article, I’ll explore how these patterns can be applied specifically in the context of Kotlin projects, where the language’s features provide unique advantages for domain modeling.

Understanding Bounded Contexts

The concept of Bounded Contexts is perhaps the most important strategic pattern in DDD. A Bounded Context defines the scope within which a particular model applies. It creates clear boundaries that allow teams to work with consistent terminology and concepts within that context.

In Kotlin, we can represent these boundaries explicitly through module structures and packages. For example:

// Billing bounded context
package com.example.billing

data class Invoice(
    val invoiceId: InvoiceId,
    val customer: CustomerId,
    val lineItems: List<LineItem>,
    val total: Money
)

// Shipping bounded context
package com.example.shipping

data class Shipment(
    val shipmentId: ShipmentId,
    val destination: Address,
    val packages: List<Package>,
    val status: ShipmentStatus
)

Notice how the same real-world concept might be modeled differently in each bounded context. In billing, we care about line items and totals, while in shipping, we focus on packages and delivery status.

Context Mapping

When different bounded contexts need to interact, we use context mapping strategies to manage these relationships. Kotlin’s type system and extension functions make implementing these patterns particularly elegant.

Using Anti-Corruption Layers

One common context mapping strategy is the Anti-Corruption Layer (ACL), which protects your domain model from external influences:

// External payment service model
data class ExternalPaymentResponse(
    val id: String,
    val amount: Double,
    val currency: String,
    val status: String
)

// Our domain model
data class Payment(
    val paymentId: PaymentId,
    val amount: Money,
    val status: PaymentStatus
)

// Anti-corruption layer
class PaymentTranslator {
    fun toDomain(external: ExternalPaymentResponse): Payment {
        return Payment(
            paymentId = PaymentId(external.id),
            amount = Money(
                value = external.amount,
                currency = Currency.valueOf(external.currency)
            ),
            status = mapStatus(external.status)
        )
    }
    
    private fun mapStatus(externalStatus: String): PaymentStatus {
        return when(externalStatus) {
            "succeeded" -> PaymentStatus.COMPLETED
            "pending" -> PaymentStatus.PENDING
            "failed" -> PaymentStatus.FAILED
            else -> PaymentStatus.UNKNOWN
        }
    }
}

Strategic Design with Domain Events

Domain events are a powerful way to communicate changes across bounded contexts while maintaining their independence. Kotlin’s sealed classes make domain events particularly expressive:

sealed class OrderEvent {
    abstract val orderId: OrderId
    abstract val timestamp: Instant
    
    data class OrderPlaced(
        override val orderId: OrderId,
        val items: List<OrderItem>,
        override val timestamp: Instant = Clock.System.now()
    ) : OrderEvent()
    
    data class OrderPaid(
        override val orderId: OrderId,
        val payment: Payment,
        override val timestamp: Instant = Clock.System.now()
    ) : OrderEvent()
    
    data class OrderShipped(
        override val orderId: OrderId,
        val shipment: Shipment,
        override val timestamp: Instant = Clock.System.now()
    ) : OrderEvent()
}

These domain events can then be published to an event bus, allowing other bounded contexts to react accordingly without tight coupling.

Conclusion

Strategic Domain-Driven Design provides powerful patterns for managing complexity in large software systems. By leveraging Kotlin’s expressive type system, immutable data classes, and functional features, we can implement these patterns with elegance and precision.

In future articles, we’ll explore tactical DDD patterns and how they complement these strategic approaches.