[rabbitmq-discuss] question on message ordering guarantees

[Alexis Richardson]

If you use the consistent hashing exchange, then you can make sure
that two messages with the same key are sent to the same queue.  Then,
provided you do not allow parallel processing of those messages due to
how they are consumed, you can "process in order" as required.





On Thu, Oct 18, 2012 at 11:43 AM, Tim Watson <tim at rabbitmq.com> wrote:
> Hi
>
>
> On 10/18/2012 10:30 AM, Artur Ejsmont wrote:
>
> Hi there everyone,
>
> I am working on a rabbitmq based messaging system and i was wondering if you
> could help me out :)
>
> I was wondering how could i enforce events being processed in the order they
> are published?
>
>
> So from http://www.rabbitmq.com/semantics.html, we can see:
>
> "Section 4.7 of the AMQP 0-9-1 core specification explains the conditions
> under which ordering is guaranteed: messages published in one channel,
> passing through one exchange and one queue and one outgoing channel will be
> received in the same order that they were sent. RabbitMQ offers stronger
> guarantees since release 2.7.0.
>
> Messages can be returned to the queue using AMQP methods that feature a
> requeue parameter (basic.recover, basic.reject and basic.nack), or due to a
> channel closing while holding unacknowledged messages. Any of these
> scenarios caused messages to be requeued at the back of the queue for
> RabbitMQ releases earlier than 2.7.0. From RabbitMQ release 2.7.0, messages
> are always held in the queue in publication order, even in the presence of
> requeueing or channel closure.
>
> With release 2.7.0 and later it is still possible for individual consumers
> to observe messages out of order if the queue has multiple subscribers. This
> is due to the actions of other subscribers who may requeue messages. From
> the perspective of the queue the messages are always held in the publication
> order."
>
> So you've got to think about it in those terms. It's exactly the same with
> ActiveMQ message groups, because they are based on the Exclusive Consumer
> feature, which states: "The broker will pick a single MessageConsumer to get
> all the messages for a queue to ensure ordering. If that consumer fails, the
> broker will auto failover and choose another consumer."
> (http://activemq.apache.org/exclusive-consumer.html) except that groups are
> parallel, so: "... logically Message Groups are kinda like a parallel
> Exclusive Consumer. Rather than all messages going to a single consumer, the
> standard JMS header JMSXGroupID is used to define which message group the
> message belongs to. The Message Group feature then ensures that all messages
> for the same message group will be sent to the same JMS consumer - while
> that consumer stays alive. As soon as the consumer dies another will be
> chosen." (http://activemq.apache.org/message-groups.html).
>
> You might be able kludge something similar using the consistent hash
> exchange plugin
> (http://hg.rabbitmq.com/rabbitmq-consistent-hash-exchange/file/rabbitmq_v2_8_7/README.md)
> but you will *not* get the same kind of guarantees.
>
> We have been asked about this feature before and will probably implement
> something like it in a future release.
>
> Cheers,
> Tim
>
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