Logging and error pages

A visitor of a live site is not shown an error, but the developer must learn about it. Symfony writes errors to a log through the Monolog library and gives the visitor a page that you can style to match your site.

Writing to the log

use Psr\Log\LoggerInterface;
 
#[Route('/order/{id}/pay', name: 'order_pay', methods: ['POST'])]
public function pay(Order $order, LoggerInterface $logger): Response
{
    $logger->info('Payment started', ['order_id' => $order->getId()]);
 
    // …
 
    $logger->error('The payment service is unavailable', ['order_id' => $order->getId()]);
 
    // …
}

The second argument is an array of details. During development entries go to var/log/dev.log and are visible in the profiler.

Levels

Level	When to use it
`debug`	Details for debugging
`info`	Ordinary events
`warning`	Something unusual, but work continues
`error`	An operation failed
`critical`	An important part of the system is down

Configuration

config/packages/monolog.yaml decides what is written and where:

when@prod:
    monolog:
        handlers:
            main:
                type: fingers_crossed
                action_level: error
                handler: file
            file:
                type: rotating_file
                path: '%kernel.logs_dir%/%kernel.environment%.log'
                max_files: 14

fingers_crossed collects every message of a request and writes them only if an error happened: the log stays small, yet the whole history sits next to the error. rotating_file starts a new file each day and keeps the last 14.

Custom error pages

The templates go into a special folder:

{# templates/bundles/TwigBundle/Exception/error404.html.twig #}
{% extends 'base.html.twig' %}
 
{% block body %}
    <h1>Page not found</h1>
    <a href="{{ path('home') }}">Go to the home page</a>
{% endblock %}

error404.html.twig is for code 404, error403.html.twig for 403, error.html.twig for all the rest.

Previewing during development

In development mode a detailed report is shown instead of these pages. To see your own page, open the service address:

http://localhost:8000/_error/404

Raising an error

throw $this->createNotFoundException('Post not found');
 
throw $this->createAccessDeniedException();
 
throw new \Symfony\Component\HttpKernel\Exception\HttpException(429, 'Too many requests');

Errors as JSON

An HTML page does not suit an API. A listener turns an exception into a JSON response:

use Symfony\Component\EventDispatcher\Attribute\AsEventListener;
use Symfony\Component\HttpFoundation\JsonResponse;
use Symfony\Component\HttpKernel\Event\ExceptionEvent;
use Symfony\Component\HttpKernel\Exception\HttpExceptionInterface;
 
#[AsEventListener]
class ApiExceptionListener
{
    public function __invoke(ExceptionEvent $event): void
    {
        if (!str_starts_with($event->getRequest()->getPathInfo(), '/api')) {
            return;
        }
 
        $e = $event->getThrowable();
        $status = $e instanceof HttpExceptionInterface ? $e->getStatusCode() : 500;
 
        $event->setResponse(new JsonResponse(['error' => $e->getMessage()], $status));
    }
}

Common mistake

The exception text must not be given to a visitor on a 500 error: it may contain table names, file paths and fragments of queries. In the example above it is better to return a general phrase for code 500 and leave the details to the log. For the same reason passwords, tokens and card numbers are not written to the log — only identifiers.

Thanks for reading!