Blocking request handlers in Google App Engine

Question

I'm developing a GAE application using python and i want it to serve requests concurrently. For this effect I have set threadsafe to true. I have defined two methods in my request handler:

POST - which enqueues a pull task to execute asynchronously, which will later itself enqueue a pull task as a response:

def post(self):
    message = self.request.body
    taskqueue.add(url='/process', params={'message': message}) #this task will enqueue a pull task as a response

GET - polls the pull task queue for the response until there's one available (I know I should find a way to improve that loop, but I'm just trying it out):

def get(self):
    (...)
    tasks = queue.lease_tasks_by_tag(1, 100, tag)
    while tasks.__len__() == 0:
        time.sleep(1)
        tasks = queue.lease_tasks_by_tag(1, 100, tag) 

The client application calls POST and then GET. I put a breakpoint on the task that is enqueued by the POST method, but it's never called while there's a request handler stuck in the loop. I thought the application was supposed to have several threads serve the requests, so how come it is not executing the push request handler concurrently? I'd appreciate any help with this.

Answer 1

I suspect that you are seeing the behaviour as described in your question from the development server, which cannot serve more than one request at a time. So when you put your breakpoint in the POST method, the dev server is effectively paused. To see this behaviour as intended, upload to App Engine.

Further, instead of polling using a GET HTTP request, try using App Engines Channel API. Using this, you can push a response back to the client directly from your task queue handler to notify that the process is complete (or whatever).