For better integration between 5.x (symfony based) kernel and legacy (4.x) kernel, injection is used to inject settings, session and current siteaccess from 5.x into legacy using an event: kernel.event_subscriber

Injected settings

Several settings are injected to avoid having to duplicate settings across new and old kernel, this is done in eZ\Bundle\EzPublishLegacyBundle\LegacyMapper\Configuration.
For mappings below, yml settings prefixed with ezpublish are system settings, while the rest are settings you can define per siteaccess.

Database settings

The settings for "Server", "Port", "User", "Password", "Database" and "DatabaseImplementation" are set based on the current settings in your yml files, either from the explicit settings defined below, or the "dsn"

Mapping:

Storage settings

The settings for "VarDir" and "StorageDir" are set based on current settings in your yml files as shown below.

Mapping:

Image settings

Some of the settings for image sub system are set based on your yml files as shown below.

Mapping:

Using ImageMagick filters

The following block shows a valid ImageMagick filter usage example for ezpublish.yml:

ezpublish:
    imagemagick:
        filters:
            geometry/scale: "-geometry {1}x{2}"

Since ImageMagick filter usage changed from eZ Publish 4.x versions you can find the list of filters existing by default to use eZ Publish 5.x:

For more details on setting ImageMagick filters on image.ini please refer to the [imagemagick] / filters documentation.

Extending injected settings

It's possible to add your own kernel event subscriber and also inject your own settings by following how it is done in LegacyMapper\Configuration, and then at the end merge it with existing injected settings, like so:

        $event->getParameters()->set(
            "injected-settings",
            $settings + (array)$event->getParameters()->get( "injected-settings" )
        );

Site settings

You can inject the site.ini SiteAccessSettings in your ezpublish.yml file.

ezpublish:
    system:
        ezdemo_site_group:
            content:
                tree_root:
                    # Root locationId. Default is top locationId
                    location_id: 123
                    # Every URL aliases starting with those prefixes will be considered
                    # being outside of the subtree starting at root_location.
                    # Default value is an empty array.
                    # Prefixes are not case sensitive.
                    excluded_uri_prefixes: [ /media, /images ]

Injected Behavior

In addition to injected settings, some injection of behavior is also performed.

eZFormToken (CSRF) integration

If your config.yml setting have framework.csrf_protection.enabled set to true, then both kernel.secret and framework.csrf_protection.field_name will be sent to ezxFormToken class so csrf protection in legacy uses the same token and form field name.

By making sure all your Symfony forms uses the provided csrf protection, forms with intention=legacy can be set up to send data to legacy kernel:

$formOptions = array( 'intention' => 'legacy' );
$form = $this->createFormBuilder( null, $formOptions )
    ->...
    ->getForm();

ezpEvent (Cache clear) integration

A listener is set up for both content/cache and content/cache/all to make sure Symfony (Internal proxy or Varnish with custom vcl) HTTP cache is cleared when cache is cleared in eZ Publish admin interface.