Not necessarily append-only, but making sure that when we get a copy of something from a method - i.e. GetUser(), that the UI or its state services cannot cause mutations to propagate back down into the store without first going through another explicit method like UpdateUserName().

We don't play around with event sourcing right now, but it might be feasible at and above the UI state services if we wanted to do things like replay a user interaction with our app. The only caveat is that our domain services are very very messy in terms of side effects, so the practical utility is bounded to isolated UI testing.