[rabbitmq-discuss] RabbitMQ 2.7.0 is released

[Emile Joubert]

Hi Stefan,

On 22/11/11 07:12, Stefan Kaes wrote:
> As far as I understand it so far, the new behavior is designed to keep as
> much ordering in the queue as possible, whereas the old behavior was to
> simply move the rejected message to the end of the queue.
> 
> For us, this change is rather unfortunate.
> 
> The most common scenario for rejects in our messaging layer is that we want
> to process it at some later time, because some external service needed to
> process the queue is currently unavailable, or we know some other consumer
> is already processing a copy of a redundantly queued message.
> 
> See http://xing.github.com/beetle/
> 
> With the new behavior, a consumer will immediately see the rejected message
> again, even though there might be hundreds of messages in the queue which
> could be processed right away.
> 
> I think the new behavior is also very surprising given the name of the
> parameter to reject is "requeue".
> 
> Could you please explain what motivated the change?
> 
> Frankly, we'd prefer to get the old behavior back.

I'm sorry to hear that this change has a negative impact for you. The
requeue ordering behaviour is left unspecified leaving room for
implementations to differ in this regard. You should avoid relying on
unspecified behaviour for maximum interoperability.

The change was motivated by popular demand. Not being able to preserve
message order was a gap in rabbit's feature set which many customers
have asked us to correct.

We felt it important that requeue behaviour be consistent across all
methods of requeueing (closing channel, basic.recover, basic.reject and
basic.nack). It would be very surprising if some of these requeued at
the front of a queue while others requeued at the back.

We are aware that some customers use requeueing to emulate queue
browsing. This is discouraged. A database is more suitable for this use.
We made no attempt to facilitate this use case. The documentation for
nack says:

"""
The client MUST NOT use this method as a means of selecting messages to
process.
"""

Your options at the moment are:

Don't use requeueing. Match messages and consumers in such a way that
consumers do not need to requeue messages. Use separate queues for
different kinds of messages and design your system in such a way that
consumers can be guaranteed to be able to process the messages they receive.

Acknowledge and republish messages instead of requeueing. This will
place a message at the back of the queue. The "redelivered" flag will
not record the fact that a consumer has already seen the message, so you
may want to set your own header in this case.

There is work underway to implement dead letter exchanges. I can make no
guarantees about when it will be released, but this feature will provide
an alternative means to have a message republished to an exchange of
your choice. If urgency is low then you could wait for this feature and
see if suits your needs.

Don't upgrade to 2.7.0 or beyond. Rabbit is improving all the time, so
this is obviously not a real option if you need future enhancements and
bugfixes.






The FAQ was a victim of a recent website reorganisation effort. Here is
a copy of the relevant question. Hopefully the FAQ will be resurrected soon.

====
Q:
I have read that RabbitMQ preserves the order of messages in queues. Is
this correct?

A:
For the most part, yes. Say a publisher publishes messages M1, M2, M3
and M4 (in that order) on the same channel and with the same routing
information. If these messages are routed to a queue then they will end
up in the same order that they were published. Consuming on the queue
will yield M1, M2, M3 and then M4.

Message order is maintained if messages are 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. E.g. a publisher sends messages M1, M2, M3 and
M4 and these messages are routed to the same queue. A consumer fetches
two messages, rejecting (with requeue) the first and acknowledging the
second. The queue now contains M1, M3 and M4. The consumer again fetches
two messages without acknowledgement when a network error causes the
connection with the broker to be terminated. The queue contains M1, M3
and M4 (in that order) after the broker discovers that the consumer has
disconnected.

Section 4.7 of the AMQP 0-9-1 specification explains the conditions
under which ordering is guaranteed. RabbitMQ offers stronger guarantees
than required by the specification, as explained above.
====



-Emile