private function getConnectionType(array $config)
    {
        $isGrpcExtensionLoaded = $this->isGrpcLoaded();
        $defaultTransport = $isGrpcExtensionLoaded ? 'grpc' : 'rest';
        $transport = isset($config['transport'])
            ? strtolower($config['transport'])
            : $defaultTransport;

        if ($transport === 'grpc') {
            if (!$isGrpcExtensionLoaded) {
                throw new GoogleException(
                    'gRPC support has been requested but required dependencies ' .
                    'have not been found. ' . $this->getGrpcInstallationMessage()
                );
            }
        }

        return $transport;
    }