public function configuration_validation($data, $files, array $errors) {
        if (empty($errors['prefix'])) {
            $factory = cache_factory::instance();
            $config = $factory->create_config_instance();
            foreach ($config->get_all_stores() as $store) {
                if ($store['plugin'] === 'apcu') {
                    if (isset($store['configuration']['prefix'])) {
                        if ($data['prefix'] === $store['configuration']['prefix']) {
                            // The new store has the same prefix as an existing store, thats a problem.
                            $errors['prefix'] = get_string('prefixnotunique', 'cachestore_apcu');
                            break;
                        }
                    } else if (empty($data['prefix'])) {
                        // The existing store hasn't got a prefix and neither does the new store, that's a problem.
                        $errors['prefix'] = get_string('prefixnotunique', 'cachestore_apcu');
                        break;
                    }
                }
            }
        }
        return $errors;
    }