tornado异步（未完成）

tornado是有异步处理和长轮询的优势的。
在gen.coroutine出现之前tornado是用web.asynchronous来实现异步的。web.asynchronous是与web服务器建立长连接，直到self.finish()关闭连接。而gen.coroutine使用协程实现类似异步的处理效果，

tornado.gen is a generator-based interface to make it easier to work in an asynchronous environment. Code using the gen module is technically asynchronous, but it is written as a single generator instead of a collection of separate functions.

文档中的例子中说明有gen.coroutine装饰器存在异步变得简单，不再需要回调函数。文档例子：

For example, the following asynchronous handler:

class AsyncHandler(RequestHandler):
@asynchronous
def get(self):
        http_client = AsyncHTTPClient()
        http_client.fetch("http://example.com",
                          callback=self.on_fetch)

def on_fetch(self, response):
        do_something_with_response(response)
        self.render("template.html")

could be written with gen as:

class GenAsyncHandler(RequestHandler):
@gen.coroutine
def get(self):
        http_client = AsyncHTTPClient()
        response = yield \ http_client.fetch("http://example.com")
        do_something_with_response(response)
        self.render("template.html")

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那web.asynchronous应该可以用来建立长轮询。