NService Bus - Content based routing & auditing - is my approach ok?

Question

I have a little trouble deciding which way to go for while designing the message flow in our system.

Because the volatile nature of our business processes (i.e. calculating freight costs) we use a workflow framework to be able to change the process on the fly.

The general process should look something like this
The interface is a service which connects to the customers system via whatever interface the customer provides (webservices, tcp endpoints, database polling, files, you name it). Then a command is sent to the executor containing the received data and the id of the workflow to be executed.

The first problem comes at the point where we want to distribute load on multiple worker services.
Say we have different processes like printing parcel labels, calculating prices, sending notification mails. Printing the labels should never be delayed because a ton of mailing workflows is executed. So we want to be able to route commands to different workers based on the work they do.

Because all commands are like "execute workflow XY" we would be required to implement our own content based routing. NServicebus does not support this out of the box, most times because it's an anti pattern.

Is there a better way to do this, when you are not able to use different message types to route your messages?

The second problem comes when we want to add a monitoring. Because an endpoint can only subscribe to one queue for each message type we can not let all executors just publish a "I completed a workflow" message. The current solution would be to Bus.Send the message to a pre configured auditing endpoint. This feels a little like cheating to me ;)

Is there a better way to consolidate published messages of multiple workers into one queue again? If there would not be problem #1 I think all workers could use the same input queue however this is not possible in this scenario.

Answer 1

You can try to make your routing not content-based, but headers-based which should be much easier. You are not interested if the workflow is to print labels or not, you are interested in whether this command is priority or not. So you can probably add this information into the message header...