The Interview Question That Made Me Rethink My Architecture: Understanding Domain‑Driven Design

Source: Dev.to

The Interview Question

“Can you explain how you’ve used Domain‑Driven Design in your past projects?”

That one simple question stopped me cold.

It was my second interview — and also my last one for that position. After the first round, the interviewer had mentioned that a technical test would be required. So, when I got the invitation to the second interview, I spent almost a week preparing — mostly focusing on LINQ and Entity Framework in .NET Core.

My programming journey started with Visual Basic 6, then moved to VB.NET, and later to C# and Python. In my C# years, I mostly worked with ADO.NET, writing raw SQL queries and stored procedures. That’s why I wasn’t familiar with these newer methodologies — and I knew that was a weakness I had to fix.

I spent a week catching up. It only took a couple of days to get the big picture (thanks to my previous experience), and the rest of the week went into polishing and preparing for the test.

What I Thought DDD Was

During the interview, I explained my architecture like this:

• Repositories communicated with the database through Entity Framework
• Services used these repositories to handle business logic
• Controllers acted as endpoints, exchanging data with frontend or mobile apps

After I finished, the interviewer smiled and said:

“Tim, you’re smart, honestly.”

When the interview ended, I asked an AI what that architecture style was called. The answer came back: Domain‑Driven Design. That moment surprised me — I had been applying some parts of DDD all along, without even realizing it.

What I Realized Afterward

At first, I thought DDD was just about extra layers: entities, repositories, services… like a neatly organized folder structure. But I later realized DDD is not about structure — it’s about meaning. It’s about how the structure represents the real business domain.

The key idea is the ubiquitous language — code that mirrors how the business actually talks about its work.

Instead of thinking:

“This function updates a table.”

DDD makes you think:

“This function completes an order.”

The difference sounds small, but it completely changes how you design, read, and discuss your code. The interviewer hadn’t been testing my knowledge of frameworks — he’d been testing whether I could connect technical implementation with business intent. And that is what Domain‑Driven Design is all about.

What DDD Really Means (in Simple Terms)

• Domain – The real‑world problem space you’re modeling (orders, payments, customers…).
• Ubiquitous Language – A shared language between developers and domain experts.
• Entities – Objects that have identity and lifecycle (like Order, Customer).
• Value Objects – Define characteristics without identity (like Money, Address).
• Aggregates – Groups of entities that change together as one unit.
• Repositories – Abstract persistence, letting you save or load aggregates without leaking database logic.

Applying That Insight

Before that interview, my OrderService looked like this:

public void CompleteOrder(int orderId)
{
    var order = _repository.GetById(orderId);
    order.Status = "Completed";
    _repository.Update(order);
}

It works — but it’s mainly data‑driven. After learning about the domain perspective, I rewrote it to express intent, not CRUD actions:

public void CompleteOrder(Order order)
{
    order.MarkAsCompleted();
    _repository.Save(order);
}

Now, the focus is on what the business is doing — not how the database changes. That small shift made my architecture feel cleaner, more readable, and more meaningful.

Key Takeaways

• DDD isn’t about fancy patterns or layers — it’s about representing the business domain in code.
• Even if you’re not applying full DDD, thinking in domain terms makes your code clearer and more maintainable.
• The best architecture doesn’t just run — it communicates.

Closing Thoughts

That interview taught me something I didn’t expect — sometimes, you already understand good architectural principles; you just haven’t learned the name for them yet. Understanding comes in layers — just like good architecture.