[rabbitmq-discuss] dotnet client - Increasing prefetch with BasicQoS during HandleBasicDeliver doesn't trigger retrieval of next message

Mon May 12 10:08:23 BST 2014

On 11/05/2014 04:55, Jon Stelly wrote:
> I have a sort of odd scenario.  I want to process 1 message at a time so
> I call basic qos with a prefetch of 1 and call BasicConsume(), but
> sometimes my processing will end in a long blocking task.  When that
> happens, I would like to increase the qos prefetch to 2 to trigger the
> next message to be received and processed.  The issue is that I don't
> want to ack the first message until after the long blocking process
> completes successfully.
>
> If I call BasicQoS before acking the first message, the 2nd message
> isn't received until after the long running task completes and the ack
> is sent.  Can anyone explain what's going on and possibly suggest an
> alternative?

You are running 3.3.x? The new semantics for consumer prefetch mean that 
setting basicQos after consuming will only define a prefetch for *new* 
consumers. You could work around this by setting global=true.

See http://www.rabbitmq.com/consumer-prefetch.html for more information.

I am starting to wonder if the global=false qos should set the prefetch 
for all consumers on the channel; you wouldn't be able to have one 
channel with multiple consumers with different prefetch, but this is an 
area that seems to be unintuitive...

Cheers, Simon