How does long-polling work in Tornado?

Question

In Tornado\'s chat demo, it has a method like this:

@tornado.web.asynchronous
def post(self):
    cursor = self.get_argument("cursor", None)
    gl         


        

Answer 1

The right way to convert your test app into a form that won't block the IOLoop is like this:

from tornado.ioloop import IOLoop
import tornado.web
from tornado import gen
import time

@gen.coroutine
def async_sleep(timeout):
    """ Sleep without blocking the IOLoop. """
    yield gen.Task(IOLoop.instance().add_timeout, time.time() + timeout)

class MainHandler(tornado.web.RequestHandler):
    @gen.coroutine
    def get(self):
        print("Start request")
        yield async_sleep(4)
        print("Okay done now")
        self.write("Howdy howdy howdy")
        self.finish()

if __name__ == "__main__":
    application =  tornado.web.Application([
        (r'/', MainHandler),
    ])
    application.listen(8888)
    IOLoop.instance().start()

The difference is replacing the call to time.sleep with one which won't block the IOLoop. Tornado is designed to handle lots of concurrent I/O without needing multiple threads/subprocesses, but it will still block if you use synchronous APIs. In order for your long-polling solution to handle concurrency the way you'd like, you have to make sure that no long-running calls block.