DDD is not architecture

While reviewing some résumés last week, my eyes were caught by this guy writing:

  • I’ve recently been focused on migrating to a DDD architecture

And some lines further:

  • Architecture redesign: Solution shifted first to MVC then DDD, multi-layered architecture implementation.

Apparently this guy tried to do some MVC but seemed not to be happy with it, so he decided to shift to DDD. This technique must certainly come from a secret chapter of Eric EvansDomain-Driven Design book that only this guy knows about! ;-)

Unfortunately this is not the first time I hear and see people confusing architecture and DDD.

A couple of weeks ago I went to a meetup where someone proposed to do a DDD Kata. I was quite enthusiast to participate and I started to imagine that we were going to do some Event Storming. Well, I must admit that I was quickly disappointed seeing the guy starting the kata by opening his favorite IDE. His idea was to do some code refactoring using DDD principles. To tell you the truth I was quite curious to see how he would manage to do so! The kata was similar to the Guilded Rose one; there was a lot of unreadable legacy code that you had to refactor to be able, at the end, to add a new feature without breaking the whole stuff. The guy quickly stated things like “as DDD tells us, let’s extract some interfaces so that we can put their implementations in another layer” Ouch! It took them some time (and a few remarks made by a couple of attendees) to realize that they were not doing any DDD at all.

So why do people keep on confusing DDD and architecture?

What I find as being the most likely explanation is that it’s due to the presence of the “design” term in Domain-Driven Design. But as Grady Booch says “All architecture is design, but not all design is architecture”.

DDD is not a technology; it’s a set of principles that will help you to deal with the complexity of an application domain by modeling it. DDD is focused on the domain and its logic where “Software architecture introduces structure and guidelines into a software system, leading to consistency and clarity” to quote Simon Brown.

But keep in mind that principles do impact your software architecture. Going with DDD will obviously lead you to choose the software architecture that will welcome it warmly. Architecture styles like Hexagonal Architecture or Onion Architecture will definitely help you to accomplish that as it will isolate the domain and avoid the business logic to be tightly coupled with any infrastructure related technology.

Should you go with DDD? Well, as it’s often the case, it depends! From my point of view it mainly depends on your domain complexity: the more complex it is the more you might benefit from DDD. But every choice comes with a cost; it will be up to you to evaluate the need and as Simon Brown says: “Principles are good, but make sure they’re realistic and don’t have a negative impact”!

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