Real World Example

During a "Big Billion Days" style sale, thousands of users hit "Buy" on the same phone in the same second. ShopEase uses three layers: an atomic stock update for the hot path, optimistic locking for order edits, and SKIP LOCKED workers for payment follow-up.

@Service
@RequiredArgsConstructor
public class CheckoutService {
    private final ProductRepository products;
    private final OrderRepository orders;

    @Transactional
    public Order placeOrder(User user, List<CartItem> cart) {
        // 1. Reserve stock atomically, in product-id order (no lost updates, no deadlocks)
        List<CartItem> sorted = cart.stream()
                .sorted(Comparator.comparing(ci -> ci.getProduct().getId()))
                .toList();
        for (CartItem item : sorted) {
            int updated = products.decrementStock(item.getProduct().getId(), item.getQuantity());
            if (updated == 0) throw new OutOfStockException(item.getProduct().getId());
        }
        // 2. Create the order (Order has @Version for later status changes)
        Order order = Order.from(user, sorted);
        return orders.save(order);
    }
}

@Component
@RequiredArgsConstructor
class PaymentCheckWorker {
    private final OrderRepository orders;
    private final PaymentGateway gateway;

    @Scheduled(fixedDelay = 2000)
    @Transactional
    public void run() {
        // each app instance gets different rows thanks to SKIP LOCKED
        for (Order o : orders.pickBatch(PageRequest.of(0, 20))) {
            o.setStatus(gateway.isPaid(o.getPaymentRef()) ? OrderStatus.PAID : OrderStatus.PAYMENT_FAILED);
        }
    }
}