Applying message-ttl server policy to existing queue

[Michał Łęgocki]

Hello,

According to rabbitmq documentation (https://www.rabbitmq.com/ttl.html) "When a TTL policy is being applied to a queue which already has messages in it, matching messages will be discarded immediately".

In my scenario i have queue with some messages in it and i want to move them to temporary queue. What im doing is:

• create new exchange and queue for messages:
• rabbitmqadmin declare exchange name=temp.exchange type=fanout
• rabbitmqadmin declare queue name=temp.queue
• rabbitmqadmin declare binding source=temp.exchange destination=temp.queue
  
• set policy on queue with message-ttl:0 and dead letter exchange pointing to temp.exchange
• rabbitmqadmin declare policy name=temp.disabled pattern=original.queue definition='{"message-ttl":0,"dead-letter-exchange":"temp.exchange"}'
  

Problem is that even though in my management screen I see policy being applied to queue, existing messages aren't dead lettered and they block new incoming messages from dead lettering.

When I consume them they unblock flow and everything is working fine from now on, but i think this behaviour doesn't confirm what I quoted from documentation above.

Am I doing something wrong or have I misunderstood something?

[Michael Klishin]

The docs also mention "with some caveats" and link to https://www.rabbitmq.com/ttl.html#per-message-ttl-caveats.

If your goal is to move messages from queue A to B temporarily, I'd use a temporary dynamic Shovel.

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MK

Staff Software Engineer, Pivotal/RabbitMQ

[Michał Łęgocki]

Thank you for your answer.

I've read caveats but i thought it doesn't apply to per-queue TTL. Quote:

"While consumers never see expired messages, only when expired messages reach the head of a queue will they actually be discarded (or dead-lettered). When setting a per-queue TTL this is not a problem, since expired messages are always at the head of the queue."

I will probably use dynamic shovels but still I think that either documentation is not clear or there may be a problem with the way per-queue ttl works.