private function getAdapterNode()
    {
        $node = $this->createRootNode('adapter');
        return $node
            ->isRequired()
            ->addDefaultsIfNotSet()
            ->children()
                ->enumNode('type')
                    ->values(['jsonapiorg', null])
                ->end()
                ->scalarNode('service')->cannotBeEmpty()->end()
            ->end()
            ->validate()
                ->always(function($v) {
                    $this->validateAdapter($v);
                    return $v;
                })
            ->end()
        ;
    }