def on_message(self, unused_channel, basic_deliver, properties, body):
        """Invoked by pika when a message is delivered from RabbitMQ. The
        channel is passed for your convenience. The basic_deliver object that
        is passed in carries the exchange, routing key, delivery tag and
        a redelivered flag for the message. The properties passed in is an
        instance of BasicProperties with the message properties and the body
        is the message that was sent.

        :param pika.channel.Channel unused_channel: The channel object
        :param pika.Spec.Basic.Deliver: basic_deliver method
        :param pika.Spec.BasicProperties: properties
        :param str|unicode body: The message body
        """
        _logger.debug('Received message # %s from %s: %s',
                      basic_deliver.delivery_tag, properties.app_id, body)
        if self.ws and body:
            self.messages.append(body)
        _logger.debug('out ws : %s', len(self.ws))
        _logger.debug('out messages : %s', len(self.messages))
        self.acknowledge_message(basic_deliver.delivery_tag)
Invoked by pika when a message is delivered from RabbitMQ. The
        channel is passed for your convenience. The basic_deliver object that
        is passed in carries the exchange, routing key, delivery tag and
        a redelivered flag for the message. The properties passed in is an
        instance of BasicProperties with the message properties and the body
        is the message that was sent.

        :param pika.channel.Channel unused_channel: The channel object
        :param pika.Spec.Basic.Deliver: basic_deliver method
        :param pika.Spec.BasicProperties: properties
        :param str|unicode body: The message body