How to Add extra Data to Log Messages via a Processor

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

    app.logger.session_request_processor:
        class: AppBundle\Logger\SessionRequestProcessor
        arguments:  ['@session']
        tags:
            - { name: monolog.processor }

monolog:
    handlers:
        main:
            type: stream
            path: '%kernel.logs_dir%/%kernel.environment%.log'
            level: debug
            formatter: monolog.formatter.session_request

Note

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).

Registering Processors per Handler

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

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# app/config/config.yml
services:
    app.logger.session_request_processor:
        class: AppBundle\Logger\SessionRequestProcessor
        arguments:  ['@session']
        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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# app/config/config.yml
services:
    app.logger.session_request_processor:
        class: AppBundle\Logger\SessionRequestProcessor
        arguments:  ['@session']
        tags:
            - { name: monolog.processor, channel: main }
This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.
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