Running an async background task in Tornado

Question

Reading the Tornado documentation, it\'s very clear how to call an async function to return a response:

class GenAsyncHandler(RequestHandler):
    @gen.corou         


        

Answer 1

Here's my answer in 2019!

Start with some slow non-blocking code. See: http://www.tornadoweb.org/en/stable/faq.html#id2

async def _do_slow_task(self, pk):
    await asyncio.sleep(pk)
    logger.info(f'Finished slow task after {pk} seconds')

See here to understand the difference between async and blocking: http://www.tornadoweb.org/en/stable/guide/async.html#asynchronous-and-non-blocking-i-o

Then, make your request method a coroutine with the async/await syntax, which makes it non-blocking in order to handle multiple requests in parallel.

async def post(self):
    """Make a few requests with different pks and you should see that
        the numbers logged are in ascending order.
    """
    pk = self.get_query_argument('pk')
    try:
        record = await self.db_queryone(
            f"SELECT * FROM records WHERE id = {int(pk)};"
        )
    except Exception as e:
        self.set_status(400)
        self.write(str(e))
        return
    await self._do_slow_task(pk)
    self.write(f'Received {pk}')

Now, modify the method a bit to run in the background or

“fire and forget” a coroutine without waiting for its result

so that the client receives a response right away. See: http://www.tornadoweb.org/en/stable/guide/coroutines.html#how-to-call-a-coroutine

async def post(self):
    """Make a few requests with different pks and you should see responses
        right away, and eventually log messages with the numbers in
        ascending order.
    """
    pk = self.get_query_argument('pk')
    try:
        record = await self.db_queryone(
            f"SELECT * FROM records WHERE id = {int(pk)};"
        )
    except Exception as e:
        self.set_status(400)
        self.write(str(e))
        return
    IOLoop.current().spawn_callback(self._do_slow_task, pk)
    self.write(f'Received {pk}')