Design Amazon/E-commerce

def reserve_inventory(product_id, quantity, db):
    while True:
        # Current stock এবং version পড়ো
        product = db.get(f"SELECT id, stock, version FROM products WHERE id={product_id}")

        if product.stock < quantity:
            raise InsufficientStockError("Out of stock")

        # Version check দিয়ে atomic update
        rows_updated = db.execute("""
            UPDATE products
            SET stock = stock - %s, version = version + 1
            WHERE id = %s AND version = %s AND stock >= %s
        """, (quantity, product_id, product.version, quantity))

        if rows_updated == 1:
            # Success! কেউ আগে update করেনি
            return "Reserved"
        else:
            # Conflict! কেউ আগে stock নিয়ে গেছে, retry করুন
            continue  # বা raise error

class PlaceOrderSaga:
    def execute(self, cart, payment_info):
        steps_completed = []
        try:
            # Step 1: Reserve inventory
            self.inventory.reserve(cart.items)
            steps_completed.append('inventory')

            # Step 2: Process payment
            payment_id = self.payment.charge(payment_info, cart.total)
            steps_completed.append('payment')

            # Step 3: Create order
            order = self.order.create(cart, payment_id)
            steps_completed.append('order')

            # Step 4: Async notifications (Kafka)
            self.events.publish("order.placed", order)
            return order

        except Exception as e:
            # Compensating transactions (rollback)
            if 'payment' in steps_completed:
                self.payment.refund(payment_id)
            if 'inventory' in steps_completed:
                self.inventory.release(cart.items)
            raise e