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Tienda de Playeras includes a built-in sell workflow that lets you record sales directly from the product catalog. When a sale is confirmed, the system atomically decrements the product’s stock in MySQL using a guarded UPDATE statement, ensuring the count can never drop below zero — even under concurrent requests.

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Processing a Sale

1

Locate the product card

Navigate to the catalog at http://localhost:8000. Find the product you want to sell in the card grid. Each card displays the current stock count (“X piezas”) so you can confirm availability at a glance.
2

Enter the sale quantity

In the card footer, type the desired quantity into the numeric input field. The input enforces min=1 and max=current stock, so you cannot accidentally enter a value that exceeds available inventory from the UI.
3

Confirm the sale

Click ”🛒 Confirmar Venta” to submit the sale. If stock is zero, this button is replaced by an “Agotado” label and is disabled — no action can be taken until the product is restocked.
4

Request reaches Laravel

The form POSTs to /vender/{id} with the cantidad field. Laravel’s PlayerasController forwards the request to the Java backend:
Http::asForm()->post($this->apiUrl, [
    'accion'   => 'vender',
    'id'       => (int)$id,
    'cantidad' => (int)$request->cantidad
]);
5

Backend processes the transaction

ProductoServlet receives accion=vender, id, and cantidad, then delegates to ProductoDAO, which executes an atomic SQL update against the MySQL database:
UPDATE productos SET stock = stock - ? WHERE id = ? AND stock >= ?
The AND stock >= ? condition is the server-side guard — the row is only updated when sufficient stock exists. If the condition is not met, zero rows are affected and the stock remains unchanged.
6

Confirm success

On a successful sale you are redirected back to the catalog and a green flash message appears: “¡Venta procesada con éxito!” The updated stock count is immediately reflected in the product card.

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Inventory Guard

The core of oversell prevention is a single atomic SQL statement executed by ProductoDAO: 
UPDATE productos SET stock = stock - ? WHERE id = ? AND stock >= ?
Because the stock check and decrement happen inside one UPDATE, the database engine handles the atomicity — there is no window between a SELECT and an UPDATE where a concurrent request could read the same stock value and both succeed. If two sale requests for the last unit arrive simultaneously, only one will match the AND stock >= ? condition; the other will affect zero rows and leave the stock at zero without going negative.
The sell button is also disabled in the UI whenever a product’s stock reaches zero, using the following Blade expression:
{{ $playera['stock'] <= 0 ? 'disabled' : '' }}
This client-side guard provides immediate visual feedback and prevents accidental form submissions, complementing the server-side SQL check.

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Out-of-Stock Behavior

When a product’s stock reaches 0, the catalog card changes automatically:
  • The sell button label changes to “Agotado” and becomes disabled.
  • The quantity input is also disabled, preventing any input.
  • No sale can be submitted from the UI until stock is replenished.
To restock a product, use the Edit Product workflow: click “✏️ Editar” on the product card, update the stock field to the new available quantity, and save. The sell controls will re-enable as soon as the updated page loads.
The quantity input defaults to 1, so a single click on ”🛒 Confirmar Venta” is all it takes to record a one-unit sale. You can increase the value up to the product’s current stock before confirming if you need to log a multi-unit transaction.

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