code: def check(self, check_req):
          
  
          self.start()
          res = self._check_aggregator.check(check_req)
          if res:
              _logger.debug(u'using cached check response for %s: %s',
                            check_request, res)
              return res
  
          # Application code should not fail because check request's don't
          # complete, They should fail open, so here simply log the error and
          # return None to indicate that no response was obtained
          try:
              transport = self._create_transport()
              resp = transport.services.Check(check_req)
              self._check_aggregator.add_response(check_req, resp)
              return resp
          except exceptions.Error:  # only sink apitools errors
              _logger.error(u'direct send of check request failed %s',
                            check_request, exc_info=True)
              return None
docs: Process a check_request.
  
          The req is first passed to the check_aggregator.  If there is a valid
          cached response, that is returned, otherwise a response is obtained from
          the transport.
  
          Args:
            check_req (``ServicecontrolServicesCheckRequest``): to be sent to
              the service control service
  
          Returns:
             ``CheckResponse``: either the cached response if one is applicable
              or a response from making a transport request, or None if
              if the request to the transport fails