Inventory / order system

class Stock {
  private int available;
  private int reserved;

  synchronized boolean tryReserve(int qty) {
    if (qty <= 0) throw new InvalidQty();
    if (available < qty) return false;
    available -= qty;
    reserved += qty;
    return true;
  }

  synchronized void confirm(int qty) {
    if (qty <= 0) throw new InvalidQty();
    if (reserved < qty) throw new OverConfirm();
    reserved -= qty; // sold; available already reduced at reserve
  }

  synchronized void release(int qty) {
    if (qty <= 0) throw new InvalidQty();
    if (reserved < qty) throw new OverRelease();
    reserved -= qty;
    available += qty;
  }
}

class Order {
  private OrderStatus status = OrderStatus.CREATED;
  void reserve() { /* CREATED → RESERVED */ }
  void pay() { /* RESERVED → PAID */ }
  void ship() { /* PAID → SHIPPED only */ }
  void expire() { /* RESERVED → EXPIRED */ }
  void cancel() { /* from CREATED or RESERVED */ }
}

class CheckoutService {
  private final Clock clock;
  Order place(Sku sku, int qty) {
    if (qty <= 0) throw new InvalidQty();
    Order o = new Order();
    if (!stock.tryReserve(qty)) throw new OutOfStock();
    try {
      o.reserve();
      o.holdUntil(clock.instant().plus(Duration.ofMinutes(10)));
      return o;
    } catch (RuntimeException ex) {
      stock.release(qty); // compensate if order creation fails after reserve
      throw ex;
    }
  }
}