How are atomic locks used with requests?

[plain elm]

I tried to find the solution online, but most setups are basic.
Lets say you have a product in stock, and you want to make sure nobody buys the product if it isn't is stock.
Good, I will make a form request that checks the stock, then I will reduce the quantity in the controller.
But the problem is, with parallel requests, the product can reach negative value.

Now, most tutorials I saw online, will check the stock inside the controller, not inside the form request. But how to do it in my case? I wanna send the stock error back to the user. not throw an error. Do I have to duplicate my logic inside the controller as well?

[agile pawn]

You make a request validating the stock count before performing the buylogic. Once the stock displayed to the user is enough to fulfill the order, you retrieve the desired amount from the DB. So the next requests should consider this new value

You should do this using DB locks

Let's say the stock model performs this operation, so this would look like:

DB::transaction(function () {
    $stock= Stock::lockForUpdate()->where('product_id', x)->get/find();
    $stock->balance -= 100;
    $stock->save();
});

The lock needs to be done inside a transaction

This is just a simple example. In the ideal world, you need to add layers to avoid deadlocks and long waiting times

https://dev.to/bhaidar/understanding-database-locking-and-concurrency-in-laravel-a-deep-dive-2k4m

If you use some sort of cache layer, you need to lock them as well for operations

https://farshadth.medium.com/understanding-cache-lock-in-laravel-a-solution-to-race-conditions-b07dd68bc797

[plain elm]

But what if this cannot be achieved? It was only an example, the stock needs to be checked right when the product is bought

[agile pawn]

Make a query that locks the db to check stock availability

Then you keep the lock/open a new one right after to decrease the balance

If the first step fails due to insufficient stock, just cancel de request

You only reserve the stock for a sell if the payment process has started

They past beyond the cart and starting check-out

You'll need some sort of CQRS and Event Sourcing for the whole feature to work properly, but this i'm assuming a real software

[plain elm]

My problem was more into how to make the validation lock the stock

[plain elm]

But i think, after reading your response, the best solution to my case is to create an atomic lock (cant use lock for update) and then manually run the form request inside it

Thank you

[agile pawn]

No problem

[boreal pumice]

In that case would't it be easier to obtain and release the lock in middleware?

[plain elm]

Actually... yes, yes it is

I forgot about middlewares...

But i have to test if i can put $next($request) inside the cache block function

Without breaking anithing