[rabbitmq-discuss] RabbitMQ as a reliable message queue

Thu Jun 16 02:14:16 BST 2011

On 15/06/11 19:54, Emile Joubert wrote:
> On 14/06/11 17:08, Jason J. W. Williams wrote:
>>> Is it possible to use RabbitMQ as a reliable message queue? What I
>>> mean is to make it preserve the order of messages in queues even for
>>> non ack'ed durable messages.
>>> Putting non acked messages into the end of the queue just doesn't make sence.
>>
>> Actually, it would be difficult to preserve the order in the case of
>> non-ack. Let's say you've got M1, M2, M3, M4 in the queue (like the
>> FAQ describes). If you have two consumers, and C1 is consuming M1 and
>> C2 is consuming M2. C1 finishes, acks M1 and starts consuming M3.
>> However, C2 fails and disconnects without acking M2, where should M2
>> go in the order? M3 is already being consumed. The only sane approach
>> is to treat M2 as a new submission and append it to the end of the
>> queue.
>
> Yes, it is not obvious how to requeue in the presence of multiple
> consumers. But it is possible to do better in the case of a single
> consumer where the inconsistency can't arise. At present rabbit always
> requeues at the back of the queue, i.e. treat it as a new message. This
> is consistent with the spec which only guarantees ordering along the
> same path from a single producer to a single consumer.

I quite like the "send to the back of the queue" behaviour.
In the event of a bad message that is always failing/crashing on the 
consumer, it'll get pushed to the back of the queue to allow other good 
messages to be processed.

In a system where the messages are always delivered in-order, that bad 
message would effectively lock the system up.

Toby