1. Django 的内置web server是如何实现的
2. Django 的WSGI是如何实现的
3. Django middle ware是如何实现的
4. Django framework的workflow, 从request 进来到response 出去
5. Django 和数据库的连接，model的工作原理
Django 的 WSGI Server 用的是 wsgiref.simple_server
WSGI APP 是 core.handler.wsgi.wsgihander
WSGIRequest实际上就是一个dict 持有原始request里的数据
WSGIHandler 返回HTTPResponse (所以HTTPResponse也是iterable的, 像list 一样？)
WSGIHandler 会去读 urlconfig找到view,会去读setting,例如middleware的配置
middleware的调用也是在WSGIHandler中
WSGI APP 必须具有下面的接口
def __call__(self, environ, start_response):
start_response is also callable object created by WSGIServer.
WSGI APP will transfer the status and headers to WSGIServer through this callable object.
Also WSGI APP must return a list similar response to WSGIServer.
In the end, WSGIServer will collect the headers and response together and send back to client.
