[rabbitmq-discuss] 3.3.0: How to achieve channel.flow(0) with basic.qos?

Mon May 12 14:36:50 BST 2014

On 07/05/2014 17:58, mrogan wrote:
> Support for channel.flow(bit active) was removed in RabbitMq 3.3.0 (
> changelog <https://www.rabbitmq.com/release-notes/README-3.3.0.txt>  ). I
> was using it to suspend flow on a channel when initially consuming a queue
> with basic.consume. The reason is that I need to determine if I am the
> exclusive consumer of that queue and perform business logic before starting
> to receive already pending messages.
>
> I read that "basic.qos is superior in all cases" but I don't see how to
> completely suspend the flow by setting the prefetch-count. If I set the
> prefetch-count to zero then then it is effectively unlimited. This is not
> the same as channel.flow(0)!

This is true.

> I can work around this by buffering messages until I can perform my business
> logic after the (successful) basic.consume. But this would seem to be
> reimplementing the channel.flow that has disappeared!

You can set the basic.qos to 1 before consuming; that way you only have 
to buffer one message.

If that doesn't appeal you could also have your clients agree on the 
name of a queue to use as a mutex; exclusively declaring that queue 
could be taken as proving that the exclusive consume operation will 
succeed, and you could perform the business logic between declaring and 
consuming.

Cheers, Simon