Python Tornado - Asynchronous Request is blocking

Question

The request handlers are as follows:

class TestHandler(tornado.web.RequestHandler):    #  localhost:8888/test
    @tornado.web.asynchronous
    def get(self):
        t = threading.Thread(target = self.newThread)
        t.start()

    def newThread(self):
        print "new thread called, sleeping"
        time.sleep(10)
        self.write("Awake after 10 seconds!")
        self.finish()

class IndexHandler(tornado.web.RequestHandler):           #   localhost:8888/ 
    def get(self):
        self.write("It is not blocked!")
        self.finish()

When I GET localhost:8888/test, the page loads 10 seconds and shows Awake after 10 seconds; while it is loading, if I open localhost:8888/index in a new browser tab, the new index page is not blocked and loaded instantly. These fit my expectation.

However, while the /test is loading, if I open another /test in a new browser tab, it is blocked. The second /test only starts processing after the first has finished.

What mistakes have I made here?

Answer 1

What you are seeing is actually a browser limitation, not an issue with your code. I added some extra logging to your TestHandler to make this clear:

class TestHandler(tornado.web.RequestHandler):    #  localhost:8888/test
    @tornado.web.asynchronous
    def get(self):
        print "Thread starting %s" % time.time()
        t = threading.Thread(target = self.newThread)
        t.start()

    def newThread(self):
        print "new thread called, sleeping %s" % time.time()
        time.sleep(10)
        self.write("Awake after 10 seconds!" % time.time())
        self.finish()

If I open two curl sessions to localhost/test simultaneously, I get this on the server side:

Thread starting 1402236952.17
new thread called, sleeping 1402236952.17
Thread starting 1402236953.21
new thread called, sleeping 1402236953.21

And this on the client side:

Awake after 10 seconds! 1402236962.18
Awake after 10 seconds! 1402236963.22

Which is exactly what you expect. However in Chromium, I get the same behavior as you. I think that Chromium (perhaps all browsers) will only allow one connection at a time to be opened to the same URL. I confirmed this by making IndexHandler run the same code as TestHandler, except with slightly different log messages. Here's the output when opening two browser windows, one to /test, and one to /index:

index Thread starting 1402237590.03
index new thread called, sleeping 1402237590.03
Thread starting 1402237592.19
new thread called, sleeping 1402237592.19

As you can see both ran concurrently without issue.