[rabbitmq-discuss] RabbitMQ as a reliable message queue

Thu Jun 16 09:17:10 BST 2011

On 16/06/11 18:14, T-zex wrote:
> Unexpected shut down of a process (power failure) will result in
> "good" message being put to the end of the queue - a very unexpected
> behaviour. In case of a "bad" message application should decide that
> it is bad and what should be done with it - requeue (unlikely) or get
> rid of it.

In an ideal world, yes, but in practice it's much better to assume the 
worst.

> On Thu, Jun 16, 2011 at 2:14 AM, Toby Corkindale
> <toby.corkindale at strategicdata.com.au>  wrote:
>> 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
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>>

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