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MassTransit - Increase the throughput of your consumers

While running some stress tests on our environment, I noticed that our queues started to fill up. When I took a look at our MassTransit consumers, they were processing 10 messages simultaneously but not more although the CPU on the server was not stressed at all.

What is going on?

The reason is due to the number of messages the RabbitMQ transport will “prefetch” from the queue. The default for this is 10, so we can only process 10 messages simultaneously. To increase this, we can do 2 things:

  1. Configure the prefetchcount for your endpoint using the PrefetchCount property on the IRabbitMqBusFactoryConfigurator class.
  2. Add a ?prefetch=X to the Rabbit URL.

Remark: As a general recommendation, PrefetchCount should be relatively high, so that RabbitMQ doesn't choke delivering messages due to network delays.

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