CQRS - sending response by command with immediate consistency

Question

I have the following architecture: Ofc. there are ports and adapters, and everything else you can imagine...

What do you suggest, how to send a rest response by immediate consistency? Should I add another event bus and raise an event? (I guess the projection must send something about the success.)

How to handle errors in an event based system like this? (The event bus is not necessary, I can solve loose coupling with an IoC container, but I don't think sending a callback through so many objects would be a good solution.)

Answer 1

It's not hard, instead of sending a command, you can call directly the command handler from controller. Or have a service method which will handle the input and returns something. The important bit is that all these are done synchronously (i.e you need to wait until the handler finishes). The domain events handlers are unaffected, they can be async.

If you don't want to go 'hybrid' and want to always use the same workflow (as described in your pic) things are more complicated, you need the client to check often if the operation has completed. I think the better way is to be flexible so, for some tasks you can use the 'old' ways. The domain events will still be generated and handled, that part would not change. You're just changing the way a 'command' is executed.

Also, it's worth mentioning that you shouldn't expect responses from event handlers and if it makes you feel better, use the 'request-response' terminology instead of command-response.

Btw, you don't break CQRS this way, as long as your domain model isn't used to do queries i.e you have different model for writes and reads, it is CQRS.

Answer 2

Immediate consistency, at what cost? are you using DTC?

What if you later on want to have more than one subscriber for a given event in the read model, how many transactions will be involved in a DTC transaction scope? In order for you to have immediate consistency your events need to be handled sync, so what is the benefit in this architecture?

You can have immediate consistency and even immediate user notifications with client callback (signalR), but IMHO you should changes a few things in your architecture, starting with the drop of the immediate consistency bit.

Why do you think you need that btw?