WSGI 的全称是 Web Server Gateway Interface, 是为 Python 语言定义的 Web 服务器和 Web 应用程序之间的一种简单而通用的接口。类似的协议还有 CGI, FastCGI, uWSGI 等。该协议保证了 Python Web 应用和服务器之间的交互的通用性。有关WSGI的功能解释用一张图就可以解释。
WSGi 可以很好的将服务器和 Web 应用解耦开来，服务器负责解析 HTTP，应用负责处理业务逻辑。这里需要说明的是，Werkzeug 是 flask 本身自己的 Web Server；Tornado本身又可以作 Server 又可以作 Framework，可以不走 wsgi；Nginx可以提供更加强悍的网关功能，包括静态文件处理、负载均衡等，所以通常会暴露在 wsgi server 的外面。还要补充的一点是 wsgi 应用是同步IO的，即回调函数运行完后，才会进行下一个请求的操作，因此通常这种网络架构的并发问题由 server 和 proxy 部分完成，比如多线程的从 accept 池中获得请求并递交 application，当然如果对并发有更高的要求，则要再开多个进程，来利用更多的 CPU 资源。
简单说明一下 wsgi 的用法。作为一种通信的协议，往往靠标准来传递数据进行交互。wsgi通过两个参数来完成这个工作，一个是 environ，另一个则是 start_response。
environ：是一系列的环境变量，包含 wsgi 本身的信息和解析出来的HTTP请求信息、客户端信息。
start_response：是回调函数。它包含两个参数， 一个是status，另一个则是response_headers，分别代表着HTTP请求状态和响应请求的头信息。
通过 wsgi 将参数传给 application 之后，在处理完毕后，application运行回调函数传递相关信息之外，同时返回请求的回复信息，即response。接下来的工作则继续由 server 返给客户端。至此一个请求从接收到回答处理完毕。wsgi则很好的将有关网络的工作和有关业务的工作拆分开来。
附上一段wsgi的应用代码。不需要详细说明。
一个小的wsgi server：
# Tested with Python 2.7.9, Linux & Mac OS X import socket import StringIO import sys class WSGIServer(object): address_family = socket.AF_INET socket_type = socket.SOCK_STREAM request_queue_size = 1 def __init__(self, server_address): # Create a listening socket self.listen_socket = listen_socket = socket.socket( self.address_family, self.socket_type ) # Allow to reuse the same address listen_socket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1) # Bind listen_socket.bind(server_address) # Activate listen_socket.listen(self.request_queue_size) # Get server host name and port host, port = self.listen_socket.getsockname()[:2] self.server_name = socket.getfqdn(host) self.server_port = port # Return headers set by Web framework/Web application self.headers_set = [] def set_app(self, application): self.application = application def serve_forever(self): listen_socket = self.listen_socket while True: # New client connection self.client_connection, client_address = listen_socket.accept() # Handle one request and close the client connection. Then # loop over to wait for another client connection self.handle_one_request() def handle_one_request(self): self.request_data = request_data = self.client_connection.recv(1024) # Print formatted request data a la 'curl -v' print(''.join( '< {line}\n'.format(line=line) for line in request_data.splitlines() )) self.parse_request(request_data) # Construct environment dictionary using request data env = self.get_environ() # It's time to call our application callable and get # back a result that will become HTTP response body result = self.application(env, self.start_response) # Construct a response and send it back to the client self.finish_response(result) def parse_request(self, text): request_line = text.splitlines()[0] request_line = request_line.rstrip('\r\n') # Break down the request line into components (self.request_method, # GET self.path, # /hello self.request_version # HTTP/1.1 ) = request_line.split() def get_environ(self): env = {} # The following code snippet does not follow PEP8 conventions # but it's formatted the way it is for demonstration purposes # to emphasize the required variables and their values # # Required WSGI variables env['wsgi.version'] = (1, 0) env['wsgi.url_scheme'] = 'http' env['wsgi.input'] = StringIO.StringIO(self.request_data) env['wsgi.errors'] = sys.stderr env['wsgi.multithread'] = False env['wsgi.multiprocess'] = False env['wsgi.run_once'] = False # Required CGI variables env['REQUEST_METHOD'] = self.request_method # GET env['PATH_INFO'] = self.path # /hello env['SERVER_NAME'] = self.server_name # localhost env['SERVER_PORT'] = str(self.server_port) # 8888 return env def start_response(self, status, response_headers, exc_info=None): # Add necessary server headers server_headers = [ ('Date', 'Tue, 31 Mar 2015 12:54:48 GMT'), ('Server', 'WSGIServer 0.2'), ] self.headers_set = [status, response_headers + server_headers] # To adhere to WSGI specification the start_response must return # a 'write' callable. We simplicity's sake we'll ignore that detail # for now. # return self.finish_response def finish_response(self, result): try: status, response_headers = self.headers_set response = 'HTTP/1.1 {status}\r\n'.format(status=status) for header in response_headers: response += '{0}: {1}\r\n'.format(*header) response += '\r\n' for data in result: response += data # Print formatted response data a la 'curl -v' print(''.join( '> {line}\n'.format(line=line) for line in response.splitlines() )) self.client_connection.sendall(response) finally: self.client_connection.close() SERVER_ADDRESS = (HOST, PORT) = '', 8888 def make_server(server_address, application): server = WSGIServer(server_address) server.set_app(application) return server if __name__ == '__main__': if len(sys.argv) < 2: sys.exit('Provide a WSGI application object as module:callable') app_path = sys.argv[1] module, application = app_path.split(':') module = __import__(module) application = getattr(module, application) httpd = make_server(SERVER_ADDRESS, application) print('WSGIServer: Serving HTTP on port {port} ...\n'.format(port=PORT)) httpd.serve_forever()一个小的flask app：
from flask import Flask from flask import Response flask_app = Flask('flaskapp') @flask_app.route('/hello') def hello_world(): return Response( 'Hello world from Flask!\n', mimetype='text/plain' ) app = flask_app.wsgi_app 代码来源：https://ruslanspivak.com/lsbaws-part2/
