Wsgi in openstack

Hi All, I am new to python coding and trying to understand how wsgi is implemented in nova or glance api servers. What would be the best way to understand the design here ? Are there any useful documents available over the internet to understand these ? I understand the WSGI standard. I can't figure out where the start_response functions are coded in nova-api wsgi implementation. Any useful tips are highly appreciated. Thanks so much for your time. Thanks Y.G