How to Add extra Data to Log Messages via a Processor

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Monolog allows you to process every record before logging it by adding some extra data. This is the role of a processor, which can be applied for the whole handler stack or only for a specific handler or channel.

A processor is a callable receiving the record as its first argument. Processors are configured using the monolog.processor DIC tag. See the reference about it.

Adding a Session/Request Token

Sometimes it is hard to tell which entries in the log belong to which session and/or request. The following example will add a unique token for each request using a processor:

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// src/Logger/SessionRequestProcessor.php
namespace App\Logger;

use Monolog\LogRecord;
use Monolog\Processor\ProcessorInterface;
use Symfony\Component\HttpFoundation\Exception\SessionNotFoundException;
use Symfony\Component\HttpFoundation\RequestStack;

class SessionRequestProcessor implements ProcessorInterface
{
    public function __construct(
        private RequestStack $requestStack
    ) {
    }

    // method is called for each log record; optimize it to not hurt performance
    public function __invoke(LogRecord $record): LogRecord
    {
        try {
            $session = $this->requestStack->getSession();
        } catch (SessionNotFoundException $e) {
            return $record;
        }
        if (!$session->isStarted()) {
            return $record;
        }

        $sessionId = substr($session->getId(), 0, 8) ?: '????????';

        $record->extra['token'] = $sessionId.'-'.substr(uniqid('', true), -8);

        return $record;
    }
}

Next, register your class as a service, as well as a formatter that uses the extra information:

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# config/services.yaml
services:
    monolog.formatter.session_request:
        class: Monolog\Formatter\LineFormatter
        arguments:
            - "[%%datetime%%] [%%extra.token%%] %%channel%%.%%level_name%%: %%message%% %%context%% %%extra%%\n"

    App\Logger\SessionRequestProcessor:
        tags:
            - { name: monolog.processor }

Finally, set the formatter to be used on whatever handler you want:

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# config/packages/prod/monolog.yaml
monolog:
    handlers:
        main:
            type: stream
            path: '%kernel.logs_dir%/%kernel.environment%.log'
            level: debug
            formatter: monolog.formatter.session_request

If you use several handlers, you can also register a processor at the handler level or at the channel level instead of registering it globally (see the following sections).

When registering a new processor, instead of adding the tag manually in your configuration files, you can use the #[AsMonologProcessor] attribute to apply it on the processor class:

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// src/Logger/SessionRequestProcessor.php
namespace App\Logger;

use Monolog\Attribute\AsMonologProcessor;

#[AsMonologProcessor]
class SessionRequestProcessor
{
    // ...
}

The #[AsMonologProcessor] attribute takes these optional arguments:

  • channel: the logging channel the processor should be pushed to;
  • handler: the handler the processor should be pushed to;
  • method: the method that processes the records (useful when applying the attribute to the entire class instead of a single method).

3.8

The #[AsMonologProcessor] attribute was introduced in MonologBundle 3.8.

Symfony's MonologBridge provides processors that can be registered inside your application.

DebugProcessor
Adds additional information useful for debugging like a timestamp or an error message to the record.
TokenProcessor
Adds information from the current user's token to the record namely username, roles and whether the user is authenticated.
SwitchUserTokenProcessor
Adds information about the user who is impersonating the logged in user, namely username, roles and whether the user is authenticated.
WebProcessor
Overrides data from the request using the data inside Symfony's request object.
RouteProcessor
Adds information about current route (controller, action, route parameters).
ConsoleCommandProcessor
Adds information about the current console command.

See also

Check out the built-in Monolog processors to learn more about how to create these processors.

Registering Processors per Handler

You can register a processor per handler using the handler option of the monolog.processor tag:

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# config/services.yaml
services:
    App\Logger\SessionRequestProcessor:
        tags:
            - { name: monolog.processor, handler: main }

Registering Processors per Channel

By default, processors are applied to all channels. Add the channel option to the monolog.processor tag to only apply a processor for the given channel:

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# config/services.yaml
services:
    App\Logger\SessionRequestProcessor:
        tags:
            - { name: monolog.processor, channel: 'app' }
This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.
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