[rabbitmq-discuss] Initial Questions on RabbitMQ and AMQP

[Matthias Radestock]

Mark,

On 25/10/11 21:51, Mark Petrovic wrote:
> 1) No ONCE-ONLY semantics. Messages may be sent twice by RabbitMQ to
> the consumer(s)

That only happens if something went wrong and, in effect, the message is 
"retried".

Once-only / exactly-once are tricky to define, and impossible to 
implement if taken literally. There has been some previous discussion on 
the subject. A quick search of the archives brings up


../rabbitmq-server/scripts/rabbitmqctl delete_user test_user_no_perm
Deleting user "test_user_no_perm" ...
Error: {no_such_user,<<"test_user_no_perm">>}
make[3]: [start_test_broker_node] Error 2 (ignored)


>
> 2) Unordered; not FIFO delivery
>
> I'm actually not certain (1) is a criticism of AMQP or Rabbit.  On its
> face, it seems morally neutral, yet at the same time it appears to
> make an important point.  Some interpretation from people in the know
> here would still be helpful and much appreciated.
>
> As for (2), I recognize it as an oversimplication, after reading this
> thread in this group:
>
> http://groups.google.com/group/rabbitmq-discuss/browse_thread/thread/19066ba6f2944df8/f9d7d4bea18f2b5a?lnk=gst&q=fifo#
>
> The explanation by Matthias Radestock is helpful, but even after
> reading his citation of the AMQP spec with respect to channels and
> "single paths", I am still left with the sense that somehow AMQP
> recognizes and codes to a world that is not quite so simple as "is- or
> isNot-FIFO".  But I cannot put my finger on why, or what exactly is
> being said.
>
> Would someone here be kind enough to talk about why section 4.7 of the
> 0.9.1 spec reads ever so slightly tortured?
>
> And a short discussion of (1) would be very helpful, too.
>
> Thank you very kindly.
>
> Mark P.
>