How to deal with deleted event classes in event sourcing - cqrs

Question

I am trying to implement a pure event sourced service to see where I will get problems. Now I found a problem that I can not solve so far, so I would like to open a discussion about it.
Given the following aggregate:
class User
{
public Guid Id { get; private set; }
public string Name { get; private set; }
public void Apply(UserNameChangedEvent domainEvent)
{
Name = domainEvent.NewName;
}
public void Apply(UserCreatedEvent domainEvent)
{
Name = domainEvent.Name;
Id = domainEvent.Id;
}
}
and those Domain Events
class UserCreatedEvent
{
public string NewName { get; }
public Guid Id { get; }
public UserCreatedEvent(string newName, Guid id)
{
NewName = newName;
Id = id;
}
}
class UserNameChangedEvent
{
public string NewName { get; }
public UserNameChangedEvent(string newName)
{
NewName = newName;
}
}
Lets say I create a user and change its name to "Peter" afterwards, then I have a UserCretedEvent and a UserChangedNameEvent persisted in my EventStore. Now the business says that changing a name is no longer possible and therefore I would remove the class UserChangedNameEvent and the function that handles it. But now I have the problem that I can not recreate the aggregate in its correct state, which would be with the name "Peter".
Of course I could start hacking around and mark the function and class as deprecated, so I could keep using it, but I might end up with a lot of event classes afterwards and this will be a nightmare to keep track of. I also heard you might create a new event that persists the change in the domain, but that also seems very hacky to me and not a very good style, as this is no domain event in my point of view.
So the question is, how do I deal the best with changes like this?
edit: just to clarify: I do not want to delete any event, just the class and the function where I use it, as the requirement is different now.

Answer 1

THE resource for questions related to changes in event schemas is Versioning in an Event Sourced System, by Greg Young.
So the question is, how do I deal the best with changes like this?
It depends on the real problem that you are trying to solve.
If the requirement is that users aren't allowed to change their names in the future, then you take away the logic in the domain model that creates new UserNameChangedEvents, but leave behind the correct processing where the events do appear.
If the requirement is that changes to user names should be ignored, then you also take the Apply(UserNameChanged) handler and turn it into a NoOp, just as you would for any other unrecognized event.
If the requirement is that information about name changes should be destroyed, then you migrate your event store to a new schema, that no longer includes the UserNameChanged event.
It may help to think through how you would solve the problem if you were storing your state in an RDBMS: is it enough to ignore the User Name column? do you need to delete the column? Do you need to (somehow) restore value in a column to a previously written value?
Knowing the problem in a traditional database that is analogous to the problem you want to solve in the event store should help identify the appropriate solution.
Also: pay attention to whether or not your domain model is the system of record for the data that needs changed, or if instead you are caching a representation of information published by a different authority.

Answer 2

Events captured facts about the system. If User name was changed at some point, it is a fact. Future business rule changes cannot affect past facts.
So you should not remove UserNameChanged events, and all associated handlers, events are there, and you should not rewrite past history.
In CQRS app, events are generated by command handlers. So this is a place where you specify business requirements. "Now the business says that changing a name is no longer possible" means that ChangeName command is no longer available: you can simply remove it, or just throw an error saying that you cannot change names anymore.