Monolog

A log line records something that happened so a person can read it afterwards. Symfony does not implement logging itself. It ships MonologBundle, which wires Monolog, a standalone logging library, into the service container and exposes a logger as a service.

You never construct that logger. Type-hint a constructor argument as Psr\Log\LoggerInterface and autowiring passes the configured one in. That interface comes from PSR-3, a standard several libraries implement, so your class never names Monolog.

The eight levels

PSR-3 defines eight severities, one method each. debug: detail only useful while reproducing something. info: a normal event worth keeping. notice: normal but unusual, such as a fallback being used. warning: recoverable trouble somebody reads tomorrow. error: a request or job failed and a person should look. critical: a dependency is gone and part of the product is down. alert: page someone now. emergency: the system is unusable.

Minimum level

A handler is a destination for records, configured in config/packages/monolog.yaml. Each handler has a level, the lowest severity it accepts: debug in development, much higher in production.

What trips people up

Levels are a contract with whoever is on call. Log routine events as error and the real errors stop being visible; no configuration repairs that.

// src/Service/RefundPolicy.php
declare(strict_types=1);

namespace App\Service;

use Psr\Log\LoggerInterface;

final class RefundPolicy
{
    public function __construct(private readonly LoggerInterface $logger)
    {
    }

    public function isRefundable(string $orderId, int $daysSincePurchase): bool
    {
        if ($daysSincePurchase <= 30) {
            $this->logger->info('Refund allowed', ['order' => $orderId]);

            return true;
        }

        // A declined refund is a decision somebody will ask about later.
        $this->logger->warning('Refund declined: window expired', [
            'order' => $orderId,
            'days' => $daysSincePurchase,
        ]);

        return false;
    }
}

// $policy->isRefundable('A-1187', 44);
// Output: false   (plus one WARNING record on the 'app' channel)

use Monolog\Level;
use Psr\Log\LogLevel;

// Shorthand: one method per level.
$logger->warning('Temperature high', ['celsius' => 31.4]);

// Longhand: the same record, level passed as an argument.
$logger->log(LogLevel::WARNING, 'Temperature high', ['celsius' => 31.4]);

// Monolog 3 also accepts its own enum here.
$logger->log(Level::Warning, 'Temperature high', ['celsius' => 31.4]);

# config/packages/monolog.yaml
when@dev:
    monolog:
        handlers:
            main:
                type: stream
                path: '%kernel.logs_dir%/%kernel.environment%.log'
                level: debug

when@prod:
    monolog:
        handlers:
            main:
                type: stream
                path: php://stderr
                level: error