最近，我在开发一个移动运用(以下简称APP)的后台程序(Server)，该运用需要调用到另一平台产品(Platform)的API。对于这个系统来说，可选的一种实现方式方式是APP同时跟Server&Platform两者交互；另一种则在Server端封装掉Platform的API，APP只和Server交互。显然后一种方式的系统架构会清晰些，APP编程时也就相对简单。那么如何在Server端封装Platform的API呢，我首先考虑到的就是用代理的方式来实现。碰巧最近Tornado邮件群组里有人在讨论using Tornado as a proxy，贴主提到的运用场景跟我这碰到的场景非常的相似，我把原帖的代码做了些整理和简化，源代码如下：

# -*- coding: utf-8 -*-
#
# Copyright(c) 2011 Felinx Lee & http://feilong.me/
#
# Licensed under the Apache License, Version 2.0 (the "License"); you may
# not use this file except in compliance with the License. You may obtain
# a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
# License for the specific language governing permissions and limitations
# under the License.
 
import logging
 
import tornado.httpserver
import tornado.ioloop
import tornado.options
import tornado.web
import tornado.httpclient
from tornado.web import HTTPError, asynchronous
from tornado.httpclient import HTTPRequest
from tornado.options import define, options
try:
 from tornado.curl_httpclient import CurlAsyncHTTPClient as AsyncHTTPClient
except ImportError:
 from tornado.simple_httpclient import SimpleAsyncHTTPClient as AsyncHTTPClient
 
define("port", default=8888, help="run on the given port", type=int)
define("api_protocol", default="http")
define("api_host", default="feilong.me")
define("api_port", default="80")
define("debug", default=True, type=bool)
 
class ProxyHandler(tornado.web.RequestHandler):
 @asynchronous
 def get(self):
 # enable API GET request when debugging
 if options.debug:
 return self.post()
 else:
 raise HTTPError(405)
 
 @asynchronous
 def post(self):
 protocol = options.api_protocol
 host = options.api_host
 port = options.api_port
 
 # port suffix
 port = "" if port == "80" else ":%s" % port
 
 uri = self.request.uri
 url = "%s://%s%s%s" % (protocol, host, port, uri)
 
 # update host to destination host
 headers = dict(self.request.headers)
 headers["Host"] = host
 
 try:
 AsyncHTTPClient().fetch(
 HTTPRequest(url=url,
 method="POST",
 body=self.request.body,
 headers=headers,
 follow_redirects=False),
 self._on_proxy)
 except tornado.httpclient.HTTPError, x:
 if hasattr(x, "response") and x.response:
 self._on_proxy(x.response)
 else:
 logging.error("Tornado signalled HTTPError %s", x)
 
 def _on_proxy(self, response):
 if response.error and not isinstance(response.error,
 tornado.httpclient.HTTPError):
 raise HTTPError(500)
 else:
 self.set_status(response.code)
 for header in ("Date", "Cache-Control", "Server", "Content-Type", "Location"):
 v = response.headers.get(header)
 if v:
 self.set_header(header, v)
 if response.body:
 self.write(response.body)
 self.finish()
 
def main():
 tornado.options.parse_command_line()
 application = tornado.web.Application([
 (r"/.*", ProxyHandler),
 ])
 http_server = tornado.httpserver.HTTPServer(application)
 http_server.listen(options.port)
 tornado.ioloop.IOLoop.instance().start()
 
if __name__ == "__main__":
 main()

运行上面的代码后，访问 http://localhost:8888/ 将会完整显示飞龙博客的首页，即代理访问了http://feilong.me/的内容。

我考虑用程序的方式来做代理而不是直接用Nginx来做代理，其中一点是考虑到用程序可以很容易的控制Platform的哪些API是需要代理的，而哪些是要屏蔽掉的，还有哪些可能是要重写的(比如Server的login可能不能直接代理Platform的login，但却要调用到Platform的login API)。

以上这段代码只是做了简单的页面内容代理，并没有对页面进行进一步的解析处理，比如链接替换等，这些就交个有兴趣的朋友去开发了。基于以上这段代码，将其扩展一下，是完全可以实现一个完整的在线代理程序的。

这段代码我已放到了我的实验项目里，见https://bitbucket.org/felinx/labs，我将会放更多类似于这样的实验性质的小项目到这个repository里来，有兴趣的朋友可以关注一下。

转载请注明出处：http://feilong.me/2011/09/tornado-as-a-proxy