Event Sourcing and Read Model generation

Question

Assuming Stack Overflow domain problem and the following definition of events:

UserRegistered(UserId, Name, Email) UserNameChanged(UserId, Name) QuestionAsked(UserId, QuestionId, Title, Question) 

Assuming the following state of event store (in the order of appearance):

1) UserRegistered(1, "John", "[email protected]") 2) UserNameChanged(1, "SuperJohn") 3) UserNameChanged(1, "John007") 4) QuestionAsked(1, 1, "Help!", "Please!") 

Assuming the following denormalized read model for questions listing (for the first page of SO):

QuestionItem(UserId, QuestionId, QuestionTitle, Question, UserName) 

And the following event handler (which builds denormalized read model):

public class QuestionEventsHandler {     public void Handle(QuestionAsked question)     {         var item = new QuestionItem(             question.UserId,              question.QuestionId,              question.Title,              question.Question,              ??? /* how should i get name of the user? */);         ...     } } 

My question is how can i find the name of the user who asked a question? Or more common: how should i handle events if my denormalized read model requires additional data which is not exists in the particular event?

I've examined existing samples of CQRS including SimpleSQRS of Greg Young and Fohjin sample of Mark Nijhof. But it seems to me that they are operate only with data that is included in events.

Answer 1

Personally I think there's nothing wrong with looking up the user's name from within the event handler. But if you're in a position where you can't query the name from the User's read model then I'd introduce an additional event handler to QuestionEventsHandler, to handle the UserRegistered event.

That way the QuestionEventsHandler could maintain its own repository of user names (you wouldn't need to store the users email). The QuestionAsked handler can then query the user's name direct from it's own repository (as Rinat Abdullin said storage is cheap!).

Besides since your QuestionItem read model holds the user's name, you would need to handle the UserNameChanged event within the QuestionEventsHandler as well to ensure the name field within the QuestionItem is up-to-date.

To me this seems less effort than 'enriching the events' and has the benefit of not building dependencies on other parts of the system and their read models.