Domain-Driven Design (DDD)

DDD is a software design approach focusing on modeling software to match a domain according to input from that domain’s experts. It is essential for tackling high complexity in the heart of software.

When to Use

• Complex business domains (e.g., Insurance, Banking, Logistics) where logic is intricate.
• When there is a communication gap between developers and business experts.
• decomposing a Monolith into Microservices (defining boundaries).

Quick Start

// Aggregate Root
public class Order {
    private OrderId id;
    private Money totalAmount;
    private OrderStatus status;
    private List<OrderItem> items; // Aggregates items

    // Behaviors (Rich Model), not just Getters/Setters
    public void addItem(Product product, int quantity) {
        if (this.status != OrderStatus.DRAFT) {
            throw new DomainException("Cannot modify confirmed order");
        }
        this.items.add(new OrderItem(product, quantity));
        recalculateTotal();
    }

    public void confirm() {
        if (items.isEmpty()) throw new DomainException("Order empty");
        this.status = OrderStatus.CONFIRMED;
        // Raise Domain Event
        DomainEvents.publish(new OrderConfirmed(this.id));
    }
}

Core Concepts

Ubiquitous Language

A common, rigorous language shared by developers and domain experts. If the expert calls it a “Policy”, the code must call it Policy, not UserPlan or Subscription.

Bounded Context

The specific boundary within which a particular domain model is defined and applicable. Ideally maps to a Microservice or a Module.

Aggregates

A cluster of associated objects treated as a unit for data changes. External objects can only hold references to the Aggregate Root.

Common Patterns

Value Objects

Immutable objects defined by their attributes, not identity (e.g., Money, Address, Email). Two Money(5) objects are equal.

Domain Events

Something that happened in the domain that domain experts care about (OrderShipped, AccountDebited). Used to decouple side effects.

Anti-Corruption Layer (ACL)

A layer that translates models from an external system (or legacy subsystem) into the model of the current Bounded Context to prevent pollution.

Best Practices

Do:

• Focus on Behaviors, not just Data (Anemic Domain Model vs Rich Domain Model).
• Use Event Storming sessions to discover the domain events and boundaries.
• Ensure Transactional Consistency within an Aggregate, and Eventual Consistency between Aggregates.

Don’t:

• Don’t apply DDD to simple CRUD domains (it’s overkill).
• Don’t let Aggregates reference each other by Object Pointer; use IDs.

Troubleshooting

References

• Domain-Driven Design (Eric Evans)
• Implementing Domain-Driven Design (Vaughn Vernon)