I've been looking for a diagram explaining WSGI flow for some time, too. That's why I was very happy when I found this topic. I had high expectation of what I was going to see knowing how good Ian Bicking is at writing Python. Nevertheless, I gained literally nothing by looking at Ian's fancy tubes (diagram?). That's why I decided to draw one myself. I hope it will help someone understand how WSGI flow works. As long as you have suggestions how to make it better I'm open to modify it. It was created with LUCIDCHART webapp. The original diagram you can find here and the high quality PNG is here.
