Do the Hard Things First: Refactoring Legacy Monoliths for Scale

<?php

declare(strict_types=1);

namespace App\Actions\Checkout;

use App\Models\Order;
use App\Models\User;
use App\DTOs\CheckoutData;
use App\Events\OrderPlacedEvent;
use App\Services\Payment\MpesaPaymentGateway;
use Illuminate\Support\Facades\DB;
use Illuminate\Support\Facades\Log;
use Throwable;

final class ProcessCheckoutAction
{
    public function __construct(
        private readonly MpesaPaymentGateway $paymentGateway
    ) {}

    public function execute(User $customer, CheckoutData $data): Order
    {
        return DB::transaction(function () use ($customer, $data): Order {
            $order = Order::create([
                'user_id' => $customer->id,
                'total_amount' => $data->totalAmount,
                'status' => 'pending_payment',
                'shipping_address' => $data->shippingAddress,
            ]);

            foreach ($data->items as $item) {
                $order->items()->create([
                    'product_id' => $item->productId,
                    'quantity' => $item->quantity,
                    'unit_price' => $item->unitPrice,
                ]);
                $item->product->decrementStock($item->quantity);
            }

            $stkResponse = $this->paymentGateway->initiateStkPush(
                phone: $data->phoneNumber,
                amount: $data->totalAmount,
                accountReference: "ORD-{$order->id}",
                transactionDesc: "Payment for Order #{$order->id}"
            );

            $order->update([
                'merchant_request_id' => $stkResponse->merchantRequestId,
                'checkout_request_id' => $stkResponse->checkoutRequestId,
            ]);

            event(new OrderPlacedEvent($order));
            Log::info("Checkout completed for Order #{$order->id}");
            return $order;
        });
    }
}