On 01/20/2010 03:18 PM, Andrew L wrote:
I am writing a custom exchange for qpid. It attempts to deliver messages in a specific way, and if it fails, the messages are stored in a "holding" queue. After some state changes, the exchange needs to try to redeliver the held messages. How is this best implemented within the custom exchange?
I wouldn't advise this strategy. There are no guarantees around any API for custom exchanges so that route will likely lead to a lot of maintenance on your part.
You can define an alternate exchange for an exchange. If messages aren't routed to one or more queues by the primary exchange then they will be routed to the alternate exchange and from there perhaps into any matching queue.
You can then also specify an alternate-exchange for a queue. When a queue is deleted, any messages on it at that time will be routed through the alternate exchange.
So one (perhaps a little clumsy) method of achieving what you want would be to use a dead-letter exchange (lets call it dle) of type fanout. You would set the alternate exchange of your 'real' exchange (lets call that my-exchange) to be dle and you would bind a queue (dlq) to dle to hold all the unroutable messages.
You would then have to create a temporary queue (dummy-q) whose alternate-exchange was set to be my-exchange, move the messages from dlq to dummy-q (using the QMF move command), then delete dummy-q, causing all those messages to be re-routed through the original my-exchange. (This last part could be done by a simple script that you ran via cron or similar).
I am looking for something similar to the Queue::move method. The exception being I would like to move the messages from a queue to an exchange for routing (and not directly into another queue). I also do not want to implement this via an external client which consumes messages in the holding queue and sends them back to the exchange (although, that will be the solution if it is impossible from within the exchange).
The above solution does involved an external client, but doesn't require the messages to all be routed through it. The client merely triggers internal re-routing periodically.
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