How to keep domain model pure using ES approach?

Question

I have flicked through few popular Event Sourcing frameworks written in a variety of different common languages. I have got the impression all of them affect the domain models to a really high degree. As far as I understand ES is just an infrastructure concern - a way of persisting aggregate state. Of course, it facilitates message driven inter-context integration but in core domain's point of view is negligible. I consider commands and events to be part of the domain itself so it looks perfectly fine that aggregate creates events (but not publishes them) or handles commands.

The problem is that all of DDD building blocks tend to be polluted by ES framework. Events must inherit from some base class. Aggregates at least are supposed to implement foreign interfaces. I wonder if domain models should be even aware of using ES approach within an application. In my opinion, even necessity of providing apply() methods indicates that other layer shapes our domain.

How you approach this issue in your projects?

Answer 1

My answer applies only when CQRS is involved (write and read models are split and they communicate using domain events).

As far as I understand ES is just an infrastructure concern - a way of persisting aggregate state

Event sourcing is indeed an infrastructure concern, a kind of repository but event-based Aggregates are not. I consider them to be an architectural style, different from the classical style.

So, the fact that an Aggregate, in reaction to an command, generates zero or more domain events that are applied onto itself in order to build its internal (private) state used to decide what events to generate in the future is just a different mode of thinking and designing an Aggregate. This is a perfect valid style, along with classical style (the one not using events but only objects) or functional programming style.

Event sourcing just means that every time a command reaches an Aggregate, its entire internal state is rebuild instead of being loaded from a flat persistence. Of course there are other huge advantages (!) but they do not affect the design of an Aggregate.

... but not publishes them ...

I like the frameworks that permit us to just return (or better yield - Aggregate's command methods are just generators!) the events.

Events must inherit from some base class

It's sad that some frameworks require that but this is not necessarily. In general, a framework needs one mean of detecting an event class. However, they can be implemented to detect an event by other means instead of using marker interfaces. For example, the client (as in YOU) could provide a filter method that rejects non-event classes.

However, there is one thing that I couldn't avoid in my framework (yes, I know, I'm guilty, I have one): the Command interface with only one method: getAggregateId.

Aggregates at least are supposed to implement foreign interfaces.

Again, like with events, this is not a necessity. A framework could be given a custom client event-applier-on-aggregates function or a convention can be used (i.e. all event-applier methods have the form applyEventClassNameOrType.

I wonder if domain models should be even aware of using ES approach within an application

Of ES not, but event-based YES, so the apply method must still exists.

Answer 2

As far as I understand ES is just an infrastructure concern - a way of persisting aggregate state.

No, events are really core to the domain model.

Technically, you could store diffs in a domain agnostic way. For example, you could look at an aggregate and say "here is the representation before the change, here is the representation after, we'll compute the difference and store that.

The difference between patches and events is the fact that you switch from a domain agnostic spelling to a domain specific spelling. Doing that is normally going to require being intimate with the domain model itself.

The problem is that all of DDD building blocks tend to be polluted by ES framework.

Yup, there's a lot of crap framework in the examples you find in the wild. Sturgeon's Law at work.

Thinking about the domain model from a functional perspective can help a lot. At it's core, the most general form of the model is a function that accepts current state as an input, and returns a list of events as the output.

List<Event> change(State current)

From there, if you want to save current state, you just wrap this function in something that knows how to do the fold

State current = ...
List<Event> events = change(current)
State updated = State.fold(current, events)

Similarly, you can get current state by folding over the previous history

List<Event> savedHistory = ...
State current = State.reduce(savedHistory)
List<Event> events = change(current)
State updated = State.fold(current, events)

Another way of saying the same thing; the "events" are already there in your (not event sourced) domain model -- they are just implicit. If there is business value in tracking those events, then you should replace the implementation of your domain model with one that makes those events explicit. Then you can decide which persisted representation to use independent of the domain model.

Core of my problem is that domain Event inherits from framework Event and aggregate implements some foreign interface (from framework). How to avoid this?

There are a couple of possibilities.

1) Roll your own: take a close look at the framework -- what is it really buying you? If your answer is "not much", then maybe you can do without it.

From what I've seen, the "win" of these frameworks tends to be in taking a heterogeneous collection of events and managing the routing for you. That's not nothing -- but it's a bit magic, and you might be happier having that code explicit, rather than relying on implicit framework magic

2) Suck it up: if the framework is unobtrusive, then it may be more practical to accept the tradeoffs that it imposes and live with them. To some degree, event frameworks are like object relational mappers or databases; sure, in theory you should be able to change them out freely. In practice? how often do you derive benefit from the investment in that flexibility

3) Interfaces: if you squint a little bit, you can see that your domain behaviors don't usually depend on in memory representations, but instead on the algebra of the domain itself.

For example, in the domain model, we deposit Money into an Account updating its Balance. We don't typically care whether those are integers, or longs, or floats, or JSON documents. We can satisfy the model with any implementation that satisfies the constraints of the algebra.

So you can use the framework to provide the implementation (which also happens to have all the hooks the framework needs); the behavior just interacts with the interface it defined itself.

In a strongly typed implementation, this can get really twisty. In Java, for instance, if you want the strong type checks you need to be comfortable with the magic of generics and type erasure.

Answer 3

The real answer to this is that DDD is overrated. It is not true that you have to have one model to rule them all. You may have different views on the state of your world, depending on your current needs. One part of the application has one view, another part - completely different view.

To put it another way, your model is not "what is", but "what happened so far". The actual data model of your application is the event stream itself. Everything else you derive from there.