protected function handleSources(array $sources)
    {
        $responses = array();

        foreach ($sources as $source) {
            list($adapter, $request) = $source;
            $responses[] = $this->handleSource($adapter, $request);
        }

        $iterator = new \AppendIterator();

        /** @var Response $response */
        foreach ($responses as $response) {
            $iterator->append($response->getIterator());
        }

        return $iterator;
    }