How do you know if you've split your domain correctly

Question

In DDD i struggle to understand whether or not my domain is split correctly into aggregate roots and then those aggregate roots are grouped correctly into bounded contexts. is there a way - like a set of rules/guidance i can use to decide whether or not i am placing things in the right AR/BC

Answer 1

I think Udi Dahan gives a fantastic answer on DDD and creation of aggregate roots: http://www.udidahan.com/2009/06/29/dont-create-aggregate-roots/

As stated by @Malvolio you need to figure out what problem you are trying to solve then decided on how the situation maps back to the real world. Once you have this then the aggregate root will become clear and obvious.

Answer 2

They say that experience is the worst teacher because you get the test first and the lesson afterwards.

Your design decisions are ratified when the problem changes (or your understanding of the problem changes) and you don't have to rip out huge chunks of code and start over.

Of course, by then it's too late.

The best you can do proactively is ask yourself two questions:

1. Does my design really represent my problem, in that basic, fundamental concepts in the problem space are represented by basic, fundamental concepts in the design and trivial, contingent features of the problem are incidental to the design as well?
2. What if I'm wrong? What if some of the assumptions I have made in the design turn out to be wrong, or stop being right? What if an Employee is managed by a Contractor? Can a Invoice be empty? Is that just a new unit test or is it a major project?

Answer 3

My suggestion: don't design the aggregation roots, keeping refactoring the model and the aggregation roots will show themselves. When developing the system, you'll notice that some related entities are always being changed together, then you know they belongs to the same aggregation root.