[rabbitmq-discuss] How to configure

[Simon MacMullen]

This is a slightly unusual thing to want to do I think.

I don't think this is a shovel question specifically, it's more of a 
routing question.

Normally if messages get sent to an arbitrary number of consumers, then 
if the number of consumers drops to zero then the messages just don't 
get routed - clearly no-one's interested. And if each message must go to 
exactly one consumer then you use a shared queue.

It sounds like you want each message to get sent to n consumers, except 
when n=0, when the messages should be buffered until someone is 
interested again.

The simplest way to do this is probably to use the alternate-exchange 
feature. Declare your "public" exchange with an alternate-exchange of 
(say) "alternate". Then declare "alternate" as a fanout exchange with 
and bind a durable queue to it. Then consumers can consume from this 
queue (to pick up any messages that were published while no-one was 
listening) and their own queue (to make sure that if there are multiple 
consumers they each get a copy of appropriate messages).

Does this make sense?

Cheers, Simon

On 15/02/12 14:47, MELIQUE Denis (MORPHO) wrote:
> Hi every body.
>
> I’m quite new with RabbitMQ, so sorry if my question is stupid.
>
> I’am currently setting a Shovel configuration to forward messages from
> one broker to another,
>
> to have a two sites Active/Semi-Active configuration. Incoming messages
> must be processed by both sites.
>
> I have set a very small testing configuration with 2 hosts.
>
> ‘Active host’ is running RabbitMQ 2.7.1 on Windows, as ‘Semi-Active’ is
> running RabbitMQ 2.6.1 on RedHat6.
>
> The test configuration is as following:
>
> - messages are posted by clients to a direct Exchange named ‘public’
>
> - consumers bind to transient queues to the ‘public’ exchange, with a
> specific key
>
> Each consumer read a message and stores it on disk.
>
> - obviously I don’t want to lost any message
>
> My Shovel configuration has: (see attached rabbitmq.config files)
>
> 1-On the Active side:
>
> - a persistent queue bound to ‘public’ exchange with all possible keys.
> So every incoming message is read by the Shovel plug-In (that’s OK)
>
> - a Shovel declaration to forward messages to the distant exchange
> declared for the
>
> 2-On the Semi-Active side:
>
> - a topic exchange to which Shovel forwards messages (that’s also OK)
>
> - a persistent queue bound to this topic exchange
>
> - a Shovel declaration to forward messages into the local ‘public’ Exchange.
>
> Everything is fine and messages are going from Active side to
> Semi-Active one.
>
> Except when consumers are not running on the Semi-Active side.
>
> In that case, messages are sent to ‘public’ Exchange and are lost
> because nobody is bound to.
>
> despite the setting : ack_mode, on_confirm
>
> Nb: if the ‘Semi-Active side’ is not started at all, it’s OK messages
> are kept in the persistent queue
>
> On the Active side.
>
> I’am looking for a way of configuring Shovel plug-in to keep messages in
> persistent queue until
>
> a consumer can process them.
>
> I thing I am probably wrong with my configuration, but I can’t figure
> where ??
>
> Thank’s a lot.
>
> Denis
>
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-- 
Simon MacMullen
RabbitMQ, VMware