commands should be atomic

[lukaszb]

One way we can accomplish that is to add version field to events and make unique constraint on 2 fields that identify entity at particular moment in time (entity's id and version).

There are two possible outcomes:

1. one write would fail, leaving up to the client to decide if it want's to retry
2. both would succeed, however this would make the client to need to re-fetch the entity to make sure it's state is in a desired shape

We are going to implement 1) approach with a custom exception.