application是一个定义了__call__方法的WSGIHandler类实例，首先加载中间件，然后根据environ生成请求request，根据请求生成响应response，status和response_headers由start_response处理，然后返回响应body。
Django也自带了WSGI server，当然性能不够好，一般用于测试用途，运行runserver命令时，Django可以起一个本地WSGI server,django/core/servers/basehttp.py文件：
def run(addr, port, wsgi_handler, ipv6=False, threading=False, server_cls=WSGIServer):
server_address = (addr, port)
if threading:
httpd_cls = type('WSGIServer', (socketserver.ThreadingMixIn, server_cls), {})
else:
httpd_cls = server_cls
httpd = httpd_cls(server_address, WSGIRequestHandler, ipv6=ipv6)
if threading:
httpd.daemon_threads = True
httpd.set_app(wsgi_handler)
httpd.serve_forever()
实现的WSGIServer，继承自wsgiref:
class WSGIServer(simple_server.WSGIServer):
"""BaseHTTPServer that implements the Python WSGI protocol"""
request_queue_size = 10
def __init__(self, *args, ipv6=False, allow_reuse_address=True, **kwargs):
if ipv6:
self.address_family = socket.AF_INET6
self.allow_reuse_address = allow_reuse_address
super().__init__(*args, **kwargs)
def handle_error(self, request, client_address):
if is_broken_pipe_error():
logger.info("- Broken pipe from %s\n", client_address)
else:
super().handle_error(request, client_address)
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