E-commerce systems often face the challenge of ensuring that inventory management is robust and accurate. When implementing the SAGA pattern to manage complex processes such as order creation and payment cancellation, it is essential to address issues related to inventory. Asynchronous actions from end users can lead to scenarios where the SAGA pattern inadvertently results in negative inventory stock. In this article, we will explore the concept of Inventory Reservation and demonstrate how it can be integrated into the SAGA pattern to maintain data consistency and prevent inventory-related issues.

Understanding the Challenge

Consider an e-commerce system where users can place orders and cancel payments. One part of the system increases the inventory when an order is created, and another part cancels the payment if necessary. However, if the payment cancellation fails while a user simultaneously buys the last item in stock, it can lead to a negative inventory stock. This asynchronicity can pose a significant challenge in maintaining data consistency.

Inventory Reservation Solution

To address this challenge, we can implement an Inventory Reservation system. Inventory reservation is a technique that temporarily allocates or reserves items in stock for specific orders or customers. By reserving items, we ensure that they are not available to other buyers while a transaction or order is being processed. Here's how it can be implemented:

Step 1: Create an Inventory Reservation Table/Database:

First, create a database or table to manage inventory reservations. This table should store information about reserved items, the associated order or customer, and the reservation status.

CREATE TABLE inventory_reservation (
   id INT AUTO_INCREMENT PRIMARY KEY,
   product_id INT,
   order_id INT,
   reserved_quantity INT,
   status VARCHAR(10)
);

Step 2: Reserve Inventory During the Ordering Process:

When a user places an order, reserve the inventory for the ordered items.
Ensure there is sufficient inventory available for each item in the order.
If available, create a reservation record in the database.

@Service
public class InventoryService {
   @Autowired
   private InventoryReservationRepository reservationRepository;

   @Transactional
   public void reserveInventory(Order order) {
       for (OrderItem item : order.getOrderItems()) {
           int productId = item.getProductId();
           int quantity = item.getQuantity();

           // Check if enough inventory is available
           int availableQuantity = getAvailableInventory(productId);

           if (availableQuantity >= quantity) {
               // Reserve inventory
               InventoryReservation reservation = new InventoryReservation();
               reservation.setProductId(productId);
               reservation.setOrderId(order.getId());
               reservation.setReservedQuantity(quantity);
               reservation.setStatus("RESERVED");
               reservationRepository.save(reservation);
           } else {
               // Handle insufficient inventory
               throw new InsufficientInventoryException("Not enough inventory for product " + productId);
           }
       }
   }

   // Implement logic to retrieve available inventory from your inventory database
   private int getAvailableInventory(int productId) {
       // ...
   }
}

Step 3: Release or Confirm Reservations:

After the order is successfully processed, confirm the reservations, indicating that the items are no longer available for other orders.
If the order fails or is canceled, release the reservations.

@Transactional
public void confirmReservation(Order order) {
   for (OrderItem item : order.getOrderItems()) {
       int productId = item.getProductId();
       int quantity = item.getQuantity();

       // Find the reservation for this product and order
       InventoryReservation reservation = reservationRepository.findByProductIdAndOrderId(productId, order.getId());

       if (reservation != null && reservation.getReservedQuantity() == quantity) {
           reservation.setStatus("CONFIRMED");
           reservationRepository.save(reservation);
       } else {
           // Handle inconsistency or unexpected conditions
       }
   }
}

@Transactional
public void releaseReservation(Order order) {
   for (OrderItem item : order.getOrderItems()) {
       int productId = item.getProductId();
       int quantity = item.getQuantity();

       // Find the reservation for this product and order
       InventoryReservation reservation = reservationRepository.findByProductIdAndOrderId(productId, order.getId());

       if (reservation != null && reservation.getReservedQuantity() == quantity) {
           // Release the reservation
           reservationRepository.delete(reservation);
       } else {
           // Handle inconsistency or unexpected conditions
       }
   }
}

Concurrent requests

When dealing with concurrent requests for creating orders in an e-commerce system, it's important to ensure that you handle the requests properly to avoid issues with inventory reservation. Messaging systems, like message brokers, can indeed help manage concurrent requests and maintain the integrity of the inventory reservation process. Here's how you can achieve this with a message broker:

Request Parallelism: When two or more concurrent requests are made to create orders, you want to ensure that inventory reservation is handled effectively, especially in cases where multiple orders involve the same product(s). Using a message broker, you can create a queue or topic where order creation requests are placed.
Messaging System Setup: Configure your messaging system (e.g., Apache Kafka, RabbitMQ, or ActiveMQ) to handle order creation requests. Each request is placed as a message on the queue or topic. The messaging system ensures that these messages are processed sequentially, preventing conflicts in inventory reservation for the same product.
Order Creation Process: Each order creation request should include information about the products and quantities being ordered. The order creation service should process these requests by checking product availability and then placing a reservation request in the messaging system.
Inventory Reservation Processing: A separate component, such as a microservice, can listen to the order creation queue or topic. When it receives an order creation message, it can process the inventory reservation as described in the previous responses.
Inventory Reservation Logic: Within the inventory reservation component, follow the logic to reserve inventory as outlined in the previous response. This component can handle reservations in parallel, ensuring that product availability is checked and items are reserved accordingly.
Handling Insufficient Inventory: If there's insufficient inventory for a product in a concurrent request, the inventory reservation component should handle this gracefully by possibly placing the request in a retry queue or informing the order creation service to handle the situation, like notifying the user about the unavailability of a product.
Confirming Reservations: Once the inventory reservation component has successfully reserved the required items, it can confirm the reservations, marking them as "CONFIRMED." This should also be done sequentially to maintain data consistency.

Using a message broker in this way helps serialize order creation and inventory reservation processes. It ensures that concurrent requests are processed one after the other, reducing the risk of overselling or inventory conflicts. Additionally, message brokers provide mechanisms for retrying failed requests and managing the flow of messages to handle exceptional situations, making your system more robust and resilient.

Keep in mind that the exact implementation details may vary depending on the messaging system you use and the technologies you employ in your e-commerce system.

Conclusion

Integrating Inventory Reservation into the SAGA pattern can significantly improve inventory management in e-commerce systems. It helps prevent issues such as negative inventory stock resulting from asynchronicity in user actions. By reserving inventory during the ordering process and appropriately handling confirmations and releases, data consistency is maintained. This approach enhances the reliability and robustness of your e-commerce system, ensuring a seamless and reliable shopping experience for your customers.

Implementing Inventory Reservation is a vital part of managing inventory in the SAGA pattern, reducing the risk of overselling or conflicts. It's essential to adapt this approach to your specific use case and database setup, and consider additional features like reservation expiration and orphaned reservation handling for a complete solution.

Managing Inventory Reservation in SAGA Pattern for E-commerce Systems