DTOs & Boundaries

DTOs vs domain models

A domain model carries business behavior; a DTO (Data Transfer Object) is a plain data shuttle between layers:

// domain — knows the rules
public record Account(String id, int balanceCents) {
    public Account withdraw(int amount) {
        if (amount > balanceCents) throw new InsufficientFundsException();
        return new Account(id, balanceCents - amount);
    }
}

// DTO — crosses the layer boundary, no behavior
public record AccountView(String id, String balance) {}

Rules of thumb:

• internal layers pass domain objects; boundaries expose DTOs
• the mapping function is explicit (toView(Account)) — never expose mutable domain internals by accident
• DTO fields are presentation-shaped ("1,234.00"), domain fields are computation-shaped (int cents)

Small apps can skip DTOs; the discipline matters when the API shape and the internal model want to evolve at different speeds.