Logging

When error details are hidden from the visitor, the only way to learn about them is the log — a file where the application records what happened and when. Slim has no logger, but it accepts any logger that follows the PSR-3 standard. Monolog is the usual choice.

Installation

composer require monolog/monolog

Where the log lives

Create a var folder in the project root and log inside it. var holds files the application creates while running: logs and cache. They are not added to the repository — put var/ into .gitignore.

The container definition

// config/container.php
use Monolog\Handler\StreamHandler;
use Monolog\Logger;
use Psr\Log\LoggerInterface;
 
LoggerInterface::class => function () {
    $logger = new Logger('app');
    $logger->pushHandler(new StreamHandler(__DIR__ . '/../var/log/app.log'));
 
    return $logger;
},

The entry key is the LoggerInterface interface: classes ask for the interface in the constructor, not for a specific library. StreamHandler says the records go to the file var/log/app.log.

Writing from an action

// src/Action/CreatePostAction.php
use Psr\Log\LoggerInterface;
 
public function __construct(
    private PostRepository $posts,
    private LoggerInterface $logger,
    // ... other dependencies
) {}

$id = $this->posts->create(trim($data['title']), trim($data['body']));
 
$this->logger->info('Post created', ['id' => $id]);

Create a post through the form and open var/log/app.log — it has a line with the date, the INFO level, the text and the post number. The second argument is an array of extra data.

Levels

Errors in the log

To have the error middleware write exceptions to the same file, pass the logger as the fourth argument:

// config/middleware.php
use Psr\Log\LoggerInterface;
 
$logger = $app->getContainer()->get(LoggerInterface::class);
 
$errorMiddleware = $app->addErrorMiddleware($settings['debug'], true, true, $logger);

The writing is done by Slim's standard handler. Your own handler from the previous lesson replaces it, so it must write to the log itself. Add $logger to its function through use, plus one line:

// config/middleware.php
function (ServerRequestInterface $request, Throwable $exception) use ($app, $logger) {
    $status = $exception instanceof HttpException ? $exception->getCode() : 500;
 
    if ($status >= 500) {
        $logger->error($exception->getMessage(), ['exception' => $exception]);
    }
 
    // ... building the response
}

404 errors do not go to the log: they are not an application failure but a wrong address.

Checking

Temporarily add the line throw new RuntimeException('Log check'); to any action, open the page and look into var/log/app.log — there is an ERROR record with that text. Then remove the line.

Common mistake

Passwords, tokens and whole form contents must not be written to the log: many people read the log file, and it is kept for a long time. Record the user or post number, not the data. And watch the file size: RotatingFileHandler can replace StreamHandler — it starts a new file every day and deletes old ones.