code: def qos(self, prefetch_size=0, prefetch_count=0, apply_global=False):
          
          return self.backend.qos(prefetch_size, prefetch_count, apply_global)
docs: Request specific Quality of Service.
  
          This method requests a specific quality of service.  The QoS
          can be specified for the current channel or for all channels
          on the connection.  The particular properties and semantics of
          a qos method always depend on the content class semantics.
          Though the qos method could in principle apply to both peers,
          it is currently meaningful only for the server.
  
          :param prefetch_size: Prefetch window in octets.
              The client can request that messages be sent in
              advance so that when the client finishes processing a
              message, the following message is already held
              locally, rather than needing to be sent down the
              channel.  Prefetching gives a performance improvement.
              This field specifies the prefetch window size in
              octets.  The server will send a message in advance if
              it is equal to or smaller in size than the available
              prefetch size (and also falls into other prefetch
              limits). May be set to zero, meaning "no specific
              limit", although other prefetch limits may still
              apply. The ``prefetch_size`` is ignored if the
              :attr:`no_ack` option is set.
  
          :param prefetch_count: Specifies a prefetch window in terms of whole
              messages. This field may be used in combination with
              ``prefetch_size``; A message will only be sent
              in advance if both prefetch windows (and those at the
              channel and connection level) allow it. The prefetch-
              count is ignored if the :attr:`no_ack` option is set.
  
          :keyword apply_global: By default the QoS settings apply to the
              current channel only. If this is set, they are applied
              to the entire connection.