private function readOptions(array $options = [])
    {
        $options += [
            'readConsistency' => null,
            'transaction' => null
        ];

        $readOptions = array_filter([
            'readConsistency' => $options['readConsistency'],
            'transaction' => $options['transaction']
        ]);

        return array_filter([
            'readOptions' => $readOptions
        ]);
    }