Using prefetch to improve message consumption latency/throughput

[andrzejolszak]

Hello,

I'm looking at a scenario where I need to receive messages from a 3rd party publisher, and I'm looking for ways of minimizing/hiding latency.
I don't have any control over the publisher and its network stack, but can adjust the configuration on my side.

As the baseline, please consider this basic code:

var consumer = new EventingBasicConsumer(channel);
consumer.Received += OnMessageReceived;
channel.BasicConsume(queueName, autoAck: true, consumer: consumer);

My questions:

1. I believe no prefetching happens in the above example, as according to the documentation setting autoAck=true, makes the prefetching settings prefetchCount and pretechSize be ignored. Thus, the message transmission over the network will be completely sequential. Please correct me if I'm wrong?

2. If I enable prefetching (via explicit ACKs and channel.BasicQos), can this possibly make the messages arrive out of order from the point of view of the OnMessageReceived ?
   If yes, can DeliveryTag or some other property be used to check the ordering and reliably fix it afterwards?

[michaelklishin]

You misunderstand what prefetching means. Prefetch does not affect delivery order but re-queueing can.

Prefetching simply controls the number of "in flight" deliveries that RabbitMQ would allow when manual acknowledgements are used.

See the guide on Acknowledgements.

[andrzejolszak]

Thanks for help, this makes sense now.

[michaelklishin]

So the setting controls how many deliveries can be transferred on the channel, and dispatched to an OnMessageReceive handler. Those dispatched messages can and in practice will be processed in parallel assuming the runtime has enough cores.