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From Gordon Sim <g...@redhat.com>
Subject Re: Routing to second alternate exchange
Date Mon, 06 Aug 2012 09:13:04 GMT
On 08/06/2012 08:31 AM, Sitapati das / Joshua J Wulf wrote:
> How to reconcile these two seemingly contradictory things?
>
> 1. Messages must not be re-routed to a second alternate exchange
>
>
> http://qpid.2158936.n2.nabble.com/Alternate-Exchange-of-Alternate-Exchange-td7000176.html
> The AMQP 0-10 specification has a rule ('double-failure' on p216), that
> states:
>
>       A message which is being routed to a alternate exchange,
>       MUST NOT be re-routed to a secondary alternate exchange
>       if it fails to route in the primary alternate exchange.
>       After such a failure, the message MUST be dropped. This
>       prevents looping.
>
> 2. Orphaned/rejected messages no longer dropped if the exchange they are
> routed to first has an alternate exchange.
>   https://bugzilla.redhat.com/show_bug.cgi?id=679212

The rule from the specification is intended as a way of preventing 
circular routes. As soon as you allow three exchanges in a chain, you 
would have to deal with potential message looping.

In the case of a *queue's* alternate exchange, you can safely re-route 
to that exchanges alternate-exchange if defined without any risk of 
circularity; there are still only two exchanges in the chain.

So I would argue it is a case of adhering to the spirit of the law while 
perhaps bending the letter of the law just a fraction.


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