DDD (Domain-Driven Design)

DDD models complex domains with bounded contexts (Payment, Ledger, Fraud), aggregates (consistency boundaries), entities, value objects, and domain events. Microservices often map 1:1 to bounded contexts with Kafka for integration.

// Bounded Context: Payments
// Aggregate: Payment (root)
public class Payment { // Aggregate Root
  private PaymentId id;
  private Money amount;
  private PaymentStatus status;

  public void authorize(AuthorizationService auth) {
    if (status != PaymentStatus.PENDING)
      throw new DomainException("Already processed");
    AuthResult result = auth.authorize(this);
    this.status = result.isApproved()
        ? PaymentStatus.AUTHORIZED : PaymentStatus.DECLINED;
    domainEvents.add(new PaymentAuthorized(id, result));
  }
}

// Anti-corruption layer between contexts
public class LegacyCbsAdapter implements AccountPort {
  // Translates domain model ↔ legacy CBS format
}