NServiceBus 7 concurrency doubts

Question

In NServicebus 7 you can set concurrency that means you can decide how many messages in queue your software can process in parallel. This can be done at NserviceBus Endpoint level.

I have few doubts about this concept:

1. the concurrency is per queue not per message Type? Right?
2. If I use satellites which means I’ll have N different queues (for example: one per message Type), the concurrency will still be per queue?

For example:

1. I have configured 1 endpoint (so 1 queue) and setted to 10 the concurrency level. I manage 5 different commands (handlers). All the commands are stored in the same queue, mixed. In this case the endpoint is able to take 10 commands per time from the queue without considering the type, correct?
2. In a second scenario i have 5 satellites which manage the 5 message types, 1 dedicated queue per type. In this case each satellite is able to take 10 messages per time from its queue?

Answer 1

Satellites are an advanced feature for the raw processing of messages without all the benefits of the NServiceBus message processing pipeline. It's not normal to use them—they're most often used when implementing a message transport. For example, the RabbitMQ transport uses a Satellite for a feature that makes an endpoint instance individually addressable, so you have a QueueName queue plus a QueueName-InstanceName queue on the broker, so that another endpoint can do context.Reply() and have the reply go to the specific server that sent the original command. In any case, each satellite manages its concurrency separately, as it's a more low-level construct.

So yes, the concurrency through the main queue is for the endpoint instance, not per message type, because there's a 1:1 relationship between endpoint and queue, and you can't selectively pull messages off the queue by type.

As a result, the endpoint is your unit of scalability, both scaling up (by increasing the concurrency) or out (by adding more endpoint instances on different servers).

This means you should be careful about what message types you process in the same endpoint. They should generally have the same SLA. You wouldn't want a bunch of messages that take 50ms to process held up behind a glut of messages that process for 20 seconds.

Some people will take this to an extreme and go with one endpoint per message type. This level of complexity is usually not necessary, but it does give you the ultimate control over scalability for every single message type.