How to use Monolog to write Logs¶

Monolog is a logging library for PHP 5.3 used by Symfony2. It is inspired by the Python LogBook library.

Usage¶

In Monolog each logger defines a logging channel. Each channel has a stack of handlers to write the logs (the handlers can be shared).

Tip

When injecting the logger in a service you can use a custom channel to see easily which part of the application logged the message.

The basic handler is the StreamHandler which writes logs in a stream (by default in the app/logs/prod.log in the prod environment and app/logs/dev.log in the dev environment).

Monolog comes also with a powerful built-in handler for the logging in prod environment: FingersCrossedHandler. It allows you to store the messages in a buffer and to log them only if a message reaches the action level (ERROR in the configuration provided in the standard edition) by forwarding the messages to another handler.

To log a message simply get the logger service from the container in your controller:

$logger = $this->get('logger');
$logger->info('We just got the logger');
$logger->err('An error occurred');

Tip

Using only the methods of the Symfony\Component\HttpKernel\Log\LoggerInterface interface allows to change the logger implementation without changing your code.

Using several handlers¶

The logger uses a stack of handlers which are called successively. This allows you to log the messages in several ways easily.

YAML

monolog:
    handlers:
        syslog:
            type: stream
            path: /var/log/symfony.log
            level: error
        main:
            type: fingers_crossed
            action_level: warning
            handler: file
        file:
            type: stream
            level: debug

XML

<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:monolog="http://symfony.com/schema/dic/monolog"
    xsi:schemaLocation="http://symfony.com/schema/dic/services http://symfony.com/schema/dic/services/services-1.0.xsd
                        http://symfony.com/schema/dic/monolog http://symfony.com/schema/dic/monolog/monolog-1.0.xsd">

    <monolog:config>
        <monolog:handler
            name="syslog"
            type="stream"
            path="/var/log/symfony.log"
            level="error"
        />
        <monolog:handler
            name="main"
            type="fingers_crossed"
            action-level="warning"
            handler="file"
        />
        <monolog:handler
            name="file"
            type="stream"
            level="debug"
        />
    </monolog:config>
</container>

The above configuration defines a stack of handlers which will be called in the order where they are defined.

Tip

The handler named “file” will not be included in the stack itself as it is used as a nested handler of the fingers_crossed handler.

Note

If you want to change the config of MonologBundle in another config file you need to redefine the whole stack. It cannot be merged because the order matters and a merge does not allow to control the order.

Changing the formatter¶

The handler uses a Formatter to format the record before logging it. All Monolog handlers use an instance of Monolog\Formatter\LineFormatter by default but you can replace it easily. Your formatter must implement Monolog\Formatter\FormatterInterface.

YAML

services:
    my_formatter:
        class: Monolog\Formatter\JsonFormatter
monolog:
    handlers:
        file:
            type: stream
            level: debug
            formatter: my_formatter

XML

<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:monolog="http://symfony.com/schema/dic/monolog"
    xsi:schemaLocation="http://symfony.com/schema/dic/services http://symfony.com/schema/dic/services/services-1.0.xsd
                        http://symfony.com/schema/dic/monolog http://symfony.com/schema/dic/monolog/monolog-1.0.xsd">

    <services>
        <service id="my_formatter" class="Monolog\Formatter\JsonFormatter" />
    </services>
    <monolog:config>
        <monolog:handler
            name="file"
            type="stream"
            level="debug"
            formatter="my_formatter"
        />
    </monolog:config>
</container>

Adding some extra data in the log messages¶

Monolog allows 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 it’s 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.

namespace Acme\MyBundle;

use Symfony\Component\HttpFoundation\Session;

class SessionRequestProcessor
{
    private $session;
    private $token;

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

    public function processRecord(array $record)
    {
        if (null === $this->token) {
            try {
                $this->token = substr($this->session->getId(), 0, 8);
            } catch (\RuntimeException $e) {
                $this->token = '????????';
            }
            $this->token .= '-' . substr(uniqid(), -8);
        }
        $record['extra']['token'] = $this->token;

        return $record;
    }
}

YAML

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

    monolog.processor.session_request:
        class: Acme\MyBundle\SessionRequestProcessor
        arguments:  [ @session ]
        tags:
            - { name: monolog.processor, method: processRecord }

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 the processor at the handler level instead of globally.