Is there a way to append meta data without creating a race condition?

Question

Our site has a lengthy AJAX function that appends content to the user's cart, which is stored as an array in the usermeta table.

The function looks something like this:

$cart = get_user_meta($userID, 'cart', true);
$cart[] = $newItemID;
update_user_meta($userID, 'cart', $cart);

We're running into a problem where, when the user tries to add multiple items to their cart in rapid succession, the final call to update_user_meta will sometimes overwrite the cart with old data.

From what I understand, this is a race condition which is partly caused by the fact that WordPress stores array data as a serialized string rather than as a set of table rows.

I know that I could just create a cart table and use queries to manage it, but I was hoping that there was a way to do it with the existing get_user_meta/update_user_meta functions.

Answer 1

This happens due to the nature of ajax calls - they are asynchronous. When you have a lengthy function (or heavy plugins and slow site loading), several ajax calls are running at the same time, and nobody can predict, which one will get access to db data faster.

Usage of own tables is not a solution in this situation.

The easiest way to solve the problem is to block further adding of a product by Javascript until current ajax is finished.

So, sequence of actions in js should be:

• by the click on the add to cart button set flag blocking other clicks

• run ajax

• in .complete function delete the flag so allowing further add to cart clicks