How to Add extra Data to Log Messages via a Processor

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How to Add extra Data to Log Messages via a Processor

Monolog allows you to process the record before logging it to add some extra data. A processor can be applied for the whole handler stack or only for a specific handler.

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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namespace App\Logger;

use Symfony\Component\HttpFoundation\Session\SessionInterface;

class SessionRequestProcessor
{
    private $session;
    private $sessionId;

    public function __construct(SessionInterface $session)
    {
        $this->session = $session;
    }

    public function __invoke(array $record)
    {
        if (!$this->session->isStarted()) {
            return $record;
        }

        if (!$this->sessionId) {
            $this->sessionId = substr($this->session->getId(), 0, 8) ?: '????????';
        }

        $record['extra']['token'] = $this->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).

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.
WebProcessor
Overrides data from the request using the data inside Symfony's request object.

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

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

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