Structural 3 — Facade

One call for the controller; the orchestration of four subsystems stays in one place.

public final class CheckoutFacade {
  private final InventoryClient inventory;
  private final PaymentService payments;
  private final OrderRepository orders;
  private final NotificationService notifications;

  public CheckoutFacade(InventoryClient i, PaymentService p,
                        OrderRepository o, NotificationService n) {
    this.inventory = i; this.payments = p; this.orders = o; this.notifications = n;
  }

  public OrderId checkout(Cart cart, PaymentMethod method) {
    Reservation r = inventory.reserve(cart.items());          // 1
    try {
      ChargeResult c = payments.charge(cart.total(), method);  // 2
      Order order = orders.save(Order.from(cart, c));          // 3
      notifications.orderConfirmed(order);                     // 4
      return order.id();
    } catch (RuntimeException e) {
      inventory.release(r);                                    // compensate
      throw e;
    }
  }
}
// Controller: checkoutFacade.checkout(cart, method); — knows nothing of the four subsystems.