Events changing state in CQRS

Question

This should be easy to follow, but after some reading I still can find an answer.

So, say that the user needs to change his mobile number, to accomplished that, we might have a command as: ChangedUserMobileNumber

holding the new number. The domain responsible for handling the command will perform the change in the aggregate and publish an event: UserMobilePhoneChanged

There is a subscriber for that event in another domain, which also holds the user mobile number in its aggregate but according to our software architect, events can not old any data so what we end up is rather stupid to say the least:

The Domain 1, receives the command to update the mobile number, the number is updated and one event is published, also, because the event cannot hold data, the command handler in the Domain 1 issues yet another command which is sent to Domain 2. The subscriber of that event lives in Domain 2 too, we then have a Saga to handle both the event and the command.

In terms of implementation we are using NServiceBus, so we have this saga to handle these message and in it we have this line of code, where the entity.IsMobilePhoneUpdated field stored in a saga entity is changed when the event is handeled.

bool isReady = (entity.IsMobilePhoneUpdated && entity.MobilePhoneNumber != null);

Effectively the Saga is started by both the command and the event raised in the Domain 1, and until this condition is met, the saga is kept alive.

If it was up to me, I would be sending the mobile number in the event itself, I just want to get a few other opinions on this.

Thanks

Answer 1

In the first event based system i've designed events also had no data. I also did enforce that rule. At the time that sounded like a clever decision. After a while i realised that it was dumb, and i was making a lot of workarounds because of it. Also this caused a lot of querying form the event subscribers, even for trivial data. I had no problem changing this "rule" after i realised i'm doing it wrong.

Events should have all the data required to make them meaningful. Also they should only have the data that makes sense for that event. ( No point in having the user address in a ChangePhoneNumber message )

If your architect imposes such a restriction, it's not going to be easy to develop a CQRS system. How are the read models updated? Since the events have no data then you either query something to get the data ( the write side ? ) of find some way of sending a command to the read model ( then what's the point of publishing events? ). To fix your problem you should try to have a professional discussion with this architect, preferably including other tech heads and without offending anybody try to get him to relax this constraint.

On argument you could use is Event Sourcing. Event Sourcing is complementary to CQRS and would not make sense without events that have data. Even more when using event sourcing, the only data you have is the data stored in the events. Even if you don't actually implement event sourcing you can use it's existence as a reason for events to have data.

There is little point in finding a technical solution to a people problem.

Answer 2

I'm not sure how a UserMobilePhoneChanged event could be useful in any way unless it contained the new phone number. User asks to change a number, the event shoots out that it has. Should be very simple indeed. Why does your architect say that events shouldn't contain any information?