[rabbitmq-discuss] Ordering of redelivered messages?

Fri Jan 28 13:41:44 GMT 2011

W dniu 28.01.2011 14:26, Marek Majkowski pisze:
> When a message is delivered to the consumer, with acks (no_ack=False),
> and the consumer goes away, the message will be redelivered as
> to the queue. It will appear as "last" element.
>
> So yes, in the event of a failure the ordering gets broken.
>
> You can reduce number of messages that will be delivered
> to the consumer by setting basic.qos(prefetch_count=1).
>
> That way the consumer will at any given moment have
> at most 1 unacknowledged message.

Thanks for your response.

I'm not sure I understand correctly, so please confirm or correct me in 
the following.

The messages were all considered delivered_but_not_yet_acked, even 
though the consumer wasn't even connected, so after the consumer 
returned, all the messages were REdelivered from RabbitMQ's perspective, 
which broke the ordering.

If prefetch_count=1 had been set, there would be at most one message in 
this state, so the reordering would happen only to it (so, no effect).

Right?

 > But you pay the latency costs - a new message
 > will be send only after the ack will be delivered to server.

Not a problem for me. In any case, I suppose I could set the 
prefetch_count to something comparable to the number of independent RRD 
archives as they're updated at roughly the same intervals so I'd get low 
latency and minimal risk of data loss.

Best regards,
  Grzegorz Nosek