Middlewares

Doctrine DBAL supports middlewares. According to the DBAL documentation:

"A middleware sits in the middle between the wrapper components and the driver"

They allow to decorate the following DBAL classes:

• Doctrine\DBAL\Driver
• Doctrine\DBAL\Driver\Connection
• Doctrine\DBAL\Driver\Statement
• Doctrine\DBAL\Driver\Result

Symfony, for instance, uses a middleware to harvest the queries executed by the current page and make them available in the profiler.

You can also create your own middleware. This is an example of a (very) simple middleware that prevents database connections with the root user. The first step is to create the middleware:

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<?php

namespace App\Middleware;

use Doctrine\DBAL\Driver;
use Doctrine\DBAL\Driver\Middleware;

class PreventRootConnectionMiddleware implements Middleware
{
    public function wrap(Driver $driver): Driver
    {
        return new PreventRootConnectionDriver($driver);
    }
}
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As you can see in the wrap method, the principle of a middleware is to decorate Doctrine objects with your own objects bearing the logic you need. Now, the connect method of the driver must be decorated in PreventRootConnectionDriver to prevent connections with the root user:

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<?php

namespace App\Middleware;

use Doctrine\DBAL\Driver\Connection;
use Doctrine\DBAL\Driver\Middleware\AbstractDriverMiddleware;
use SensitiveParameter;

final class PreventRootConnectionDriver extends AbstractDriverMiddleware
{
    public function connect(array $params): Connection
    {
        if (isset($params['user']) && $params['user'] === 'root') {
            throw new \LogicException('Connecting to the database with the root user is not allowed.');
        }

        return parent::connect($params);
    }
}
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That's all! Connection with the root user is not possible anymore. Note that connect is not the only method you can decorate in a Connection. But thanks to the AbstractDriverMiddleware default implementation, you only need to decorate the methods for which you want to add some logic. Too see a more advanced example with a decoration of the Statement class, you can look at the middleware implementation starting in the class Symfony\Bridge\Doctrine\Middleware\Debug\Middleware of the Doctrine Bridge. Decorating the Result class follows the same principle.

The middleware we've just created applies by default to all the connections. If your application has several dbal connections, you can limit the middleware scope to a subset of connections thanks to the AsMiddleware PHP attribute. Let's limit our middleware to a connection named legacy:

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<?php

namespace App\Middleware;

use Doctrine\Bundle\DoctrineBundle\Attribute\AsMiddleware;
use Doctrine\DBAL\Driver;
use Doctrine\DBAL\Driver\Middleware;

#[AsMiddleware(connections: ['legacy'])]
class PreventRootConnectionMiddleware implements Middleware
{
    public function wrap(Driver $driver): Driver
    {
        return new PreventRootConnectionDriver($driver);
    }
}
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If you register multiple middlewares in your application, they will be executed in the order they were registered. If some middleware needs to be executed before another, you can set priority through the AsMiddleware PHP attribute. This priority can be any integer, positive or negative. The higher the priority, the earlier the middleware is executed. If no priority is defined, the priority is considered 0. Let's make sure our middleware is the first middleware executed, so that we don't set up debugging or logging if the connection will be prevented:

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<?php

namespace App\Middleware;

use Doctrine\Bundle\DoctrineBundle\Attribute\AsMiddleware;
use Doctrine\DBAL\Driver;
use Doctrine\DBAL\Driver\Middleware;

#[AsMiddleware(priority: 10)]
class PreventRootConnectionMiddleware implements Middleware
{
    public function wrap(Driver $driver): Driver
    {
        return new PreventRootConnectionDriver($driver);
    }
}
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priority and connections can be used together to restrict a middleware to a specific connection while changing its priority.

All the examples presented above assume autoconfigure is enabled. If autoconfigure is disabled, the doctrine.middleware tag must be added to the middleware. This tag supports a connections attribute to limit the scope of the middleware and a priority attribute to change the execution order of the registered middlewares.