def add_response(self, req, resp):
        """Adds the response from sending to `req` to this instance's cache.

        Args:
          req (`ServicecontrolServicesAllocateQuotaRequest`): the request
          resp (AllocateQuotaResponse): the response from sending the request
        """
        if self._cache is None:
            return
        signature = sign(req.allocateQuotaRequest)
        with self._cache as c:
            now = self._timer()
            item = c.get(signature)
            if item is None:
                c[signature] = CachedItem(
                    req, resp, self.service_name, now)
            else:
                # Update the cached item to reflect that it is updated
                item.last_check_time = now
                item.response = resp
                item.is_in_flight = False
                c[signature] = item