如果同时有多个进程，我如何处理mysql插入？

Question

I'm trying to make a simple shopping cart. Normally if product quantity is 0 buy now button will disappear and showing out of stock text.

Let's say current product have 1 quantity left and at the same time have two buyer buy the product. Both of buyers already in checkout page which script is not calculate available quantity any more.

In this case, how do I validate this part? I want the process basis on first-served, who is first insert into MySQL will get the product if not, script will return back to product page.

Let me know..

Answer 1

The simples, and thus probably best solution - Use the same script to check if the item is out of stock on the insert page. If the item is out of stock, display the error message. If it it's not out of stock, let them buy it.

You just need to add that same select again.

Alternatively, depending on your schema, when you update the quantity after the sale, add a where clause to the update query: WHERE the qty >= the quantity purchased. Then, see how many rows were affected. If the number of rows affected is 0, then you know the sale didn't go through. Show the out of stock error message.

Alternatively, probably an undesirable soluation - lock out the page. When the first user goes to the page, you insert a row into a table that keeps track of who is on what pages, and prevents other uses from going to that page. Then, when the second user goes to the page, they get an error message. Then, when the first user completes the transaction, you make sure that the same session id as who locked the page is who is completing the transaction now.