Concurrent order/inventory system

 

Interview answer

This is one of the most important real-world Laravel/MySQL concurrency scenarios.

Suppose inventory is:

stock = 1

Two customers simultaneously try to purchase the final item.

A naive implementation:

$product = Product::find($id);

if ($product->stock > 0) {
    $product->stock--;
    $product->save();
}

can suffer from a race condition because both requests may read stock = 1.

Safer approach

Use a transaction and row locking:

DB::transaction(function () use ($productId) {

    $product = Product::whereKey($productId)
        ->lockForUpdate()
        ->firstOrFail();

    if ($product->stock < 1) {
        throw new RuntimeException('Out of stock');
    }

    $product->decrement('stock');

    Order::create([
        'product_id' => $product->id,
        'quantity' => 1,
    ]);
});

lockForUpdate() locks the selected row until the transaction completes.

Conceptually:

Request A                         Request B

BEGIN                             BEGIN
   ↓                                ↓
lock product                       waits
   ↓
stock = 1
   ↓
decrement → 0
   ↓
COMMIT
                                  gets lock
                                     ↓
                                  stock = 0
                                     ↓
                                  reject order

Production considerations

For a real e-commerce system, I would also consider:

• database transaction boundaries

• row-level locking

• appropriate indexes

• deadlock handling/retries

• idempotency for payment/order requests

• order state transitions

• inventory reservation versus immediate decrement

• payment failures and compensation

• unique constraints for duplicate order/payment requests

• queue-based processing where appropriate

For example, an inventory reservation might use:

available_stock
reserved_stock

rather than immediately treating every checkout attempt as a completed sale.