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From Gordon Sim <g...@redhat.com>
Subject Re: Use of subject for routing - moved thread to user list from earlier private discussion.
Date Fri, 29 Aug 2014 15:39:05 GMT
On 08/29/2014 03:42 PM, Rob Godfrey wrote:
> The to field indicates the destination in terms of what the sender wants to
> send it to.  It says nothing about where it actually arrives.

     "The to field identifies the node that is the intended
      destination of the message. On any given transfer this
      might not be the node at the receiving end of the link."


     "Examples of AMQP nodes are producers, consumers, and
      queues."

I can certainly see that it is useful to 'redirect' messages, i.e. to 
deliver them to nodes other than that 'identified' as the 'destination'. 
I can also certainly see that you might want a looser interpretation of 
the to field to be some sort of abstract or logical description of 
'message content and purpose' rather than the identity of a specific node.

I'm not opposed to change. However I don't think the use of subject for 
what might be termed 'routing' is obviously wrong or unintended by the 
current spec and therefore an illogical choice for anything 'designed 
around AMQP 1.0'.

As an example let's take a simple news service. Senders send in some 
news, recipients subscribe to receive news. Messages are classified with 
regard to the type of news, e.g. sports, weather, politics whatever.

In such a case I think it would be a perfectly understandable choice 
given the current spec to specify the type of news, the classification, 
in the subject. It is after all clearly indicating in a brief way the 
content and purpose of the news.

The c++ broker uses the subject when routing through pre 1.0 style 
exchanges because that's what the only document on the subject - namely 
the legacy filters, defined 'to allow a consistent mechanism for 
addressing legacy AMQP Exchanges over AMQP 1.0' - specifies.

There may be good reasons to change this and reinterpret the property 
descriptions. I don't think the wording of the current specification is 
one of them.

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