BackendCompilerPass.php

<?php

namespace Drupal\Core\DependencyInjection\Compiler;

use Symfony\Component\DependencyInjection\Alias;
use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
use Symfony\Component\DependencyInjection\ContainerBuilder;

/**
 * Defines a compiler pass to allow automatic override per backend.
 *
 * A module developer has to tag a backend service with "backend_overridable":
 * @code
 * custom_service:
 *   class: ...
 *   tags:
 *     - { name: backend_overridable }
 * @endcode
 *
 * As a site admin you set the 'default_backend' in your services.yml file:
 * @code
 * parameters:
 *   default_backend: sqlite
 * @endcode
 *
 * As a developer for alternative storage engines you register a service with
 * $yourbackend.$original_service:
 *
 * @code
 * sqlite.custom_service:
 *   class: ...
 * @endcode
 */
class BackendCompilerPass implements CompilerPassInterface {
    
    /**
     * {@inheritdoc}
     */
    public function process(ContainerBuilder $container) {
        $driver_backend = NULL;
        if ($container->hasParameter('default_backend')) {
            $default_backend = $container->getParameter('default_backend');
            // Opt out from the default backend.
            if (!$default_backend) {
                return;
            }
        }
        else {
            try {
                $driver_backend = $container->get('database')
                    ->driver();
                $default_backend = $container->get('database')
                    ->databaseType();
                $container->set('database', NULL);
            } catch (\Exception $e) {
                // If Drupal is not installed or a test doesn't define database there
                // is nothing to override.
                return;
            }
        }
        foreach ($container->findTaggedServiceIds('backend_overridable') as $id => $attributes) {
            // If the service is already an alias it is not the original backend, so
            // we don't want to fallback to other storages any longer.
            if ($container->hasAlias($id)) {
                continue;
            }
            if ($container->hasDefinition("{$driver_backend}.{$id}") || $container->hasAlias("{$driver_backend}.{$id}")) {
                $container->setAlias($id, new Alias("{$driver_backend}.{$id}"));
            }
            elseif ($container->hasDefinition("{$default_backend}.{$id}") || $container->hasAlias("{$default_backend}.{$id}")) {
                $container->setAlias($id, new Alias("{$default_backend}.{$id}"));
            }
        }
    }

}