Prefetch Count when using RabbitMQ clustering - concurrency

Question

I'm using RabbitMQ(3.8.14 ver) and Masstransit(8.0.1 ver).
I need to guarantee each message given Ack, then another packet starts consuming. (actually, in my queue, I generate a unique incremental key, then I must ensure at the moment only a packet gets consumed.)
this is noticeable to say, my rabbit MQ was configured as a cluster with three nodes.
for that, I configured the prefetch count with a "1" value.
but in the load test before giving Ack for the first message, the second message was consumed. despite descriptions given about "prefetch count".
sample code:
public class Caller
{
///ctor inections
private static int number= 0;
public async Task publish()
{
await _publishEndpoint.Publish(new GeneratedChipsEvent { messageNumber = ++number}); ;
}
}
public class GeneratedChipsEvent
{
public int messageNumber { get; init; }
}
public class GeneratedChipsEventConsumer : IConsumer<GeneratedChipsEvent>
{
///ctor inections
public async Task Consume(ConsumeContext<GeneratedChipsEvent> context)
{
_logger.LogInformation($"starting consuming the messsage:{context.Message.messageNumber}");
await Task.Delay(System.TimeSpan.FromSeconds(3));
_logger.LogInformation($"ending consuming the messsage:{context.Message.messageNumber}");
} }
logs in my local machine(non-cluster) that wait for ack first message, then starts consuming the second message:
[10:06:08 INF] starting consuming the messsage:1
[10:06:11 INF] ending consuming the messsage:1
[10:06:11 INF] starting consuming the messsage:2
[10:06:14 INF] ending consuming the messsage:2
[10:06:14 INF] starting consuming the messsage:3
[10:06:17 INF] ending consuming the messsage:3
logs in the server(as a cluster), that don't wait for ack first message:
[10:16:33 INF] starting consuming the messsage:1
[10:16:33 INF] starting consuming the messsage:2
[10:16:33 INF] starting consuming the messsage:3
[10:16:36 INF] ending consuming the messsage:1
[10:16:36 INF] ending consuming the messsage:2
[10:16:36 INF] ending consuming the messsage:3

Answer 1

If you are running three instances of your service, you'll see three messages consumed at a time, one on each instance. The prefetch count is per instance, it is not a global lock.
To only have one consumer active at a time, that can be specified on the receive endpoint configurator:
e.SingleActiveConsumer = true;