DDD – the coin drops for Marcus

Posted by Marcus Hammarberg on February 17, 2009
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I have now tried to grasp the DDD-concept without fully “getting it”. I must confess that I have thought: “What is the deal with this… I have done it before”.

But today we had a most prominent guest at Elevate (the education initiative at Avega) – Jimmy Nilsson.  And what he said got the coin to drop for me.

The heading of his presentation was something like “A new era for data access”. And the things he said that got me to understand was something like this – not a quote but rather his reasoning:

Traditionally we focus on the database design in application development. So very early we want to get the database design in place. “We need it to start to do code”. Since we did that design early in the process we often got it wrong and inherited those error through the process. And we are reluctant to change the database later on because its hard to test and overview what the change may break.

In DDD we do the opposite – we start with the domain model, as far as we know. We prepare for change – we want it to change when we get more knowledge about it. We will not even use a database for quite some time in the project, when we have got to “know” the model through test and code.

Ok – that was my interpretation of Jimmy’s reasoning. But I got it – when starting with the pure domain model you don’t need to worry how it’s stored.
And also I understand why I liked the DDD-concept from the start. It goes perfectly with SCRUM – you don’t strive to do it correct from the start but rather expect and prepare for change. I like it!

I also learned some other stuff that I will need to incorporate into my design:

  • Aggregate – use aggregate to logically divide and group your model. An aggregate root is the only way into the other objects in the aggregate. They cannot be accessed from the outside.
  • Start modeling with what you know – I have started out with the first user story and find out what I can about the model in that story, preparing for changing it down the road.
  • Behaviors – I have not put the behaviors on my entities yet. Need to do that.
  • TDD is the way to go – with DDD and TDD together you get to “feel” the model in a more direct way than to write about it.

So that was quite a lot of information to me and to the Sprint Planner Project. No code today – but quite a lot of nice input.

Here’s a last one: since I only code one hour a day I often find it hard to remember what to start with. Leave a RED test – was the very simple and resolute tip from Jimmy.

Thank you Jimmy – for everything I take with me from your presentation.



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