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I have a doubt about sending/publishing NServicebus events and commands.

Commands seem pretty straightforward. According to the documentation they "They should be sent direct to their logical owner." So I guess the destination is decided either by the defined routing, or by the overriding the default routing:

using options.SetDestination("MyDestination")

So far I am correct?

But I am not sure I am understanding how events actually work:

"An event, on the other hand, is sent by one logical sender, and received by many receivers, or maybe one receiver, or even zero receivers. This makes it an announcement that something has already happened."

Does that mean that an event would be processed by ANY endpoint that implements IHandleMessages[SomethingHappened]? Regardless of the routing configuration? I mean if I have these endpoints A,B,C,D,E and A is configured to send to B can still C,D,E get the event? What If I have no explicit routing configuration because I am using options.SetDestination to send my commands? There is any way to say I want this Event to be published only to D and E?

Thank you for any light you can shed on this subject.

Art
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  • I second Sean's response. I do want to add that using `.SetDestination` usually shouldn't be done. A command usually has a fixed logical destination and [should be statically configured on the endpoint](https://docs.particular.net/nservicebus/messaging/routing#command-routing). There are exceptions but usually destinations should not be listed in code. – Ramon Smits Aug 22 '22 at 08:51

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Commands require routing. A command is always destined for a specific endpoint to be processed as there's an expectation for the outcome and knowledge of what destination can process the command at hand.

Events have no routing. Events are notifications about something that already happened. Anyone can receive and process events. It won't change the outcome of what has happened in the first place, causing the event to be raised.

An event can be analogous to a radio broadcast to understand the 'how' part and why routing is unnecessary. When a radio station is broadcasting, the radio host doesn't know who'll be tuning in to listen. Listeners can tune in (subscribe) and tune out (unsubscribe). All these listeners (subscribers) need to know the radio station frequency (a 'topic' or 'exchange') where the broadcast is taking place (events published to).

To sum it up - events are notifications, allowing loosely coupled distribution of events among endpoints. It's subscribers that choose wherever to receive events or not, not the publisher.

Sean Feldman
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