NEventStore CommonDomain: Why does EventStoreRepository.GetById open a new stream when an aggregate doesn't exist?

Question

Please explain the reasoning behind making EventStoreRepository.GetById create a new stream when an aggregate doesn't exist. This behavior appears to give GetById a dual responsibility which in my case could result in undesirable results. For example, my aggregates always implement a factory method for their creation so that their first event results in the setting of the aggregate's Id. If a command was handled prior to the existance of the aggregate it may succeed even though the aggregate doesn't have it's Id set or other state initialized (crash-n-burn with null reference exception is equally as likely).

I would much rather have GetById throw an exception if an aggregate doesn't exist to prevent premature creation and command handling.

Answer 1

That was a dumb design decision on my part. I went back and forth on this one. To be honest, I'm still back and forth on it.

The issue is that exceptions should really be used to indicate exceptional or unexpected scenarios. The problem is that a stream not being found can be a common operation and even an expected operation in some regards. I tinkered with the idea of throwing an exception, returning null, or returning an empty stream.

The way to determine if the aggregate is empty is to check the Revision property to see if it's zero.