About the shopping station, at which point does the customer change the total number of products? How to solve it

About the shopping station, at which point does the customer change the total number of products?
The first time I made a shopping station. A logical problem has been encountered. Please help the experts to analyze how to solve.

The shopping station. If the product has only one item left. is the product considered to be sold out after the user has joined the shopping cart? or after the purchase of the order is sold out, or after the successful payment of the transaction is considered sold out.

If it is added to the cart, it is considered sold out. So if the user is playing. Do not pay after placing in cart. Wouldn't that affect other users ' shopping.

If the order is paid after payment is considered sold out. So if there's only one thing left in the product. In the case of a user ordering but not yet paid. b the user ordered again. Wouldn't that be enough?

How the hell did this all work out?

------Solution--------------------
The inventory number should be reduced by one when "order is generated" (orders cannot be generated if it has reached 0). Too early (when put into the shopping cart) sellers suffer, too late (in the time of payment) sellers passive.

At the same time should consider adding a reminder function, in the shopping cart more than the number of inventory, remind the seller to pay attention, timely replenishment or active contact with the buyer.


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