How to design the logic or database when two users choose one product?

Question

Assuming a e commerce web app has a high amount of requests, how do I prevent two users from choosing the only product left? Should I check the quantity when adding to shopping list or payment? Is it using a field to record quantity of selected product in DB is bad way? How does the large e commerce web app like amazon deal with conflict problem?

Answer 1

Several options that I know :

1. For the RDBMS that support ACID , you can use optimistic locking technique on the product table. Unless it is very often that many users hit the buying button on the same product at the nearly same times ,it should work pretty well.(For how many users does the 'many' means, you have to measure it. I think 1k should be no problem. Just my guess , don't take it for granted)

2. Do not check it and let users to buy it. Adjust the business flow to handle it. For example, when an user hits the buying button ,tell him his order is just accepted and will be processed but not guarantee he must able to buy it. Then in the later stage when you find that there is not enough inventory to ship the product to him , send an email to apologise and refund to him.

   Also in the real business , it is common that the product inventory can go to negative and still accepting orders but tell the user he will get the product at XXX days later. The business can then produce or order more product from the supplier after receiving the money.

   If you are buying iPhone on the Apple web site , it also works like this.

Answer 2

It really depends upon the number of concurrent users here. In the case of millions, the NoSQL approach is prefered to manage the basket with eventual consistency then the buying process would go with ACID to ensure the product can be sold.

For less users, you can rely on an ACID database.

If you are not sure, you may go with a database that has ACID capabilities but can as well allow you to work in an eventual consistency way or that can implement the concept of sharding for scalability purpose. To my knowledge Oracle can do these 3 things: COMMIT NO WAIT, COMMIT and Sharding deployment.

HTH