def poll():
  """ Callback function that polls for new tasks based on a schedule. """
  deployment_id = helper.get_deployment_id()
  # If the deployment is not registered, skip.
  if not deployment_id:
    return

  # If we can't reach the backup and recovery services, skip.
  nodes = helper.get_node_info()
  http_client = tornado.httpclient.HTTPClient()
  for node in nodes:
    br_host = node[helper.NodeInfoTags.HOST]
    request = tornado.httpclient.HTTPRequest(br_host)
    try:
      response = http_client.fetch(request)
      if json.loads(response.body)['status'] != 'up':
        logging.warn('Backup and Recovery service at {} is not up.'
          .format(br_host))
        return
    except (socket.error, __HOLE__):
      logging.exception('Backup and Recovery service at {} is not up.'
        .format(br_host))
      return

  logging.info("Polling for new task.")

  # Send request to AppScale Portal.
  url = "{0}{1}".format(hermes_constants.PORTAL_URL,
      hermes_constants.PORTAL_POLL_PATH)
  data = urllib.urlencode({JSONTags.DEPLOYMENT_ID: deployment_id})
  request = helper.create_request(url=url, method='POST', body=data)
  response = helper.urlfetch(request)

  if not response[JSONTags.SUCCESS]:
    logging.error("Inaccessible resource: {}".format(url))
    return

  try:
    data = json.loads(response[JSONTags.BODY])
  except (TypeError, ValueError) as error:
    logging.error("Cannot parse response from url '{0}'. Error: {1}".
      format(url, str(error)))
    return

  if data == {}:  # If there's no task to perform.
    return

  # Verify all necessary fields are present in the request.
  if not set(data.keys()).issuperset(set(hermes_constants.REQUIRED_KEYS)):
    logging.error("Missing args in response: {0}".format(response))
    return

  logging.debug("Task to run: {0}".format(data))
  logging.info("Redirecting task request to TaskHandler.")
  url = "{0}{1}".format(hermes_constants.HERMES_URL, TaskHandler.PATH)
  request = helper.create_request(url, method='POST', body=json.dumps(data))

  # The poller can move forward without waiting for a response here.
  helper.urlfetch_async(request)
dataset/ETHPy150Open AppScale/appscale/Hermes/hermes.py/poll