主程序的代码是：

 #!/usr/bin/env python
#
# Copyright 2009 Facebook
#
# 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.auth
import tornado.escape
import tornado.ioloop
import tornado.web
import os.path
import uuid

from tornado import gen
from tornado.options import define, options, parse_command_line

define("port", default=8888, help="run on the given port", type=int)


class MessageBuffer(object):
    def __init__(self):
        self.waiters = set()
        self.cache = []
        self.cache_size = 200

    def wait_for_messages(self, callback, cursor=None):
        if cursor:
            new_count = 0
            for msg in reversed(self.cache):
                if msg["id"] == cursor:
                    break
                new_count += 1
            if new_count:
                callback(self.cache[-new_count:])
                return
        self.waiters.add(callback)

    def cancel_wait(self, callback):
        self.waiters.remove(callback)

    def new_messages(self, messages):
        logging.info("Sending new message to %r listeners", len(self.waiters))
        for callback in self.waiters:
            try:
                callback(messages)
            except:
                logging.error("Error in waiter callback", exc_info=True)
        self.waiters = set()
        self.cache.extend(messages)
        if len(self.cache) > self.cache_size:
            self.cache = self.cache[-self.cache_size:]


# Making this a non-singleton is left as an exercise for the reader.
global_message_buffer = MessageBuffer()


class BaseHandler(tornado.web.RequestHandler):
    def get_current_user(self):
        user_json = self.get_secure_cookie("chatdemo_user")
        if not user_json: return None
        return tornado.escape.json_decode(user_json)


class MainHandler(BaseHandler):
    @tornado.web.authenticated
    def get(self):
        self.render("index.html", messages=global_message_buffer.cache)


class MessageNewHandler(BaseHandler):
    @tornado.web.authenticated
    def post(self):
        message = {
            "id": str(uuid.uuid4()),
            "from": self.current_user["first_name"],
            "body": self.get_argument("body"),
        }
        # to_basestring is necessary for Python 3's json encoder,
        # which doesn't accept byte strings.
        message["html"] = tornado.escape.to_basestring(
            self.render_string("message.html", message=message))
        if self.get_argument("next", None):
            self.redirect(self.get_argument("next"))
        else:
            self.write(message)
        global_message_buffer.new_messages([message])


class MessageUpdatesHandler(BaseHandler):
    @tornado.web.authenticated
    @tornado.web.asynchronous
    def post(self):
        cursor = self.get_argument("cursor", None)
        global_message_buffer.wait_for_messages(self.on_new_messages,
                                                cursor=cursor)

    def on_new_messages(self, messages):
        # Closed client connection
        if self.request.connection.stream.closed():
            return
        self.finish(dict(messages=messages))

    def on_connection_close(self):
        global_message_buffer.cancel_wait(self.on_new_messages)


class AuthLoginHandler(BaseHandler, tornado.auth.GoogleMixin):
    @gen.coroutine
    def get(self):
        if self.get_argument("openid.mode", None):
            user = yield self.get_authenticated_user()
            self.set_secure_cookie("chatdemo_user",
                                   tornado.escape.json_encode(user))
            self.redirect("/")
            return
        self.authenticate_redirect(ax_attrs=["name"])


class AuthLogoutHandler(BaseHandler):
    def get(self):
        self.clear_cookie("chatdemo_user")
        self.write("You are now logged out")


def main():
    parse_command_line()
    app = tornado.web.Application(
        [
            (r"/", MainHandler),
            (r"/auth/login", AuthLoginHandler),
            (r"/auth/logout", AuthLogoutHandler),
            (r"/a/message/new", MessageNewHandler),
            (r"/a/message/updates", MessageUpdatesHandler),
            ],
        cookie_secret="__TODO:_GENERATE_YOUR_OWN_RANDOM_VALUE_HERE__",
        login_url="/auth/login",
        template_path=os.path.join(os.path.dirname(__file__), "templates"),
        static_path=os.path.join(os.path.dirname(__file__), "static"),
        xsrf_cookies=True,
        )
    app.listen(options.port)
    tornado.ioloop.IOLoop.instance().start()


if __name__ == "__main__":
    main()

主要是class MessageUpdatesHandler(BaseHandler)那里，明明是用了异步的装饰器，但是我没看出来他是如何把他做成异步的，如果没有做成异步的，那这个东西会占用服务器的cpu的资源太多了，我想官方安装包下的demo应该不会出错的，请哪位大神帮我细致的解释一下，这段demo是如何唯美地实现了一个chatroom的功能？（其实它的大致意思我大概是能理解的，就是没发现他是如何在这个问题上用异步的，明明用了异步的装饰器，但是就是没有发现它使如何用异步的）

————————————
注：我理解异步，理解他这个chat的实现思路，只是认为他想做成异步的，但似乎实现异步的过程中是错的，没有用到异步，因为on new message那里虽然用到了异步装饰器，但是没有用到异步。求指出他在具体哪一步释放cpu线程的。对于二楼提出的他构造了一个异步的function的callback，但是callback回调函数不是异步的特有函数，是很平常的函数构造，我没看出他的构造。而我知道把异步的东西分开来构造的是func(args, callback=(yield gen.Callback(key)))
result = yield gen.Wait(key) 虽然说他没有gen，但是他根本没有用到他自己导入的那个tornado.web.asynchronous模块的任何功能。所以说感觉上他就没有实现异步。

其实这个问题的最佳实践方法是websocket，不过我就是觉得除了websocket的另外一个官方的直接不用websocket的代码应该不会有问题，明明用了异步装饰器，但是却没有发现他用到异步的功能。所以疑问。

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不过可能是我对tornado的理解不够深刻，也许二楼回答的就是对的。先采纳吧。谢谢大家的回答。