Event Sourcing Commands vs Events

Question

I understand the difference between commands and events but in a lot of cases you end up with redundancy and mapping between 2 classes that are essentially the same (ThingNameUpdateCommand, ThingNameUpdatedEvent). For these simple cases can you / do you use the event also as a command? Do people serialise to a store all commands as well as all events? Just seems to be a little redundant to me.

Answer 1

All lot of this redundancy is for a reason in general and you want to avoid using the same message for two different purposes for a number of reasons:

1. Sourced events must be versioned when they change since they are stored and re-used (deserialized) when you hydrate an aggregate root. It will make things a bit awkward if the class is also being used as a message.
2. Coupling is increased, the same class is now being used by command handlers, the domain model and event handlers now. De-coupling the command side from the event can simplify life for you down the road.
3. Finally clarity. Commands are issued in a language that asks something to be done (imperative generally). Events are representations of what has happened (past-tense generally). This language gets muddled if you use the same class for both.

In the end these are just data classes, it isn't like this is "hard" code. There are ways to actually avoid some of the typing for simple scenarios like code-gen. For example, I know Greg has used XML and XSD transforms to create all the classes needed for a given domain in the past.

I'd say for a lot of simple cases you may want to question if this is really domain (i.e. modeling behavior) or just data. If it is just data consider not using event sourcing here. Below is a link to a talk by Udi Dahan about breaking up your domain model so that not all of it requires event-sourcing. I'm kind of in line with this way of thinking now myself.

http://skillsmatter.com/podcast/design-architecture/talk-from-udi-dahan