What is Domain Driven Design? Basic overview

Domain-driven design is not a technology nor methodology. ~ Eric Evans

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“If you can’t explain it simply, you don’t understand it well enough.” ~ Albert Einstein

He probably didn’t say that… but the quote has some meaning…

We’re programmers, engineers, scientists - you named it. Maybe you want to become one. Although it is not important who/where you are. You will face it at some point. Making software is complex and often frustrating. Obviously, there is no silver-bullet, but you can control your ‘complexity’ with a good domain object.

[…] and if you don’t care about design… Your app will be even more incomprehensible and complicated.

So… It’s important that you understand your field. DDD is about mapping applications to the real-world. Often as programmers, we see our software through the prism of objects and methods without looking into real-world. It’s fantastic to the certain point, but not always.

Let’s go back to the beginning:

C’mon, that’s easy. We want to build database, design schema, save and read data in the most efficient way. We want to optimize it just so it works even better. Later we will find that ‘best language’ and we will rewrite whole application with it. Isn’t that the right way? Yeah… But what’s the point of staying on top of the mountain filled with gold if there are no means to mine it? The database (gold) is important, but what’s the point of having data when you don’t have meaningful information?

So… I should focus on design?

Yes! Like I said earlier we are software engineers and we have that habit of looking at the software through the prism of objects and methods… but let me ask you few questions:

Will you build a great house only with bricks, wood, and the best cement you can find?
Will you create a Porche with plate, engine and some motor oil?
Is blog only a collection of English/Polish words?
Will you create a good software only with Java/Kotlin/Python/JS and IDE?

Just like many of you… I experienced a project that was delivered lightning fast without any design. It was a great success and a customer was delighted. […] but then… Then was the second version. More features and changes in existing business logic. As a result of initial success expectations for future development were… high. Simple service may be written with no-design at all. The problem is that there are no simple-services. When new business logic comes ‘simple change’ quickly switches to ‘impossible change’. What results in high-maintenance legacy code that can’t bear business logic. Which later results in the inability to change/create features and it’s no fun - no-one likes it! (neither business nor developers).

Yeah, sure, design… everyone knows that. I should think before writing code, right? […] but I still don’t get it. What is this Domain-Driven-Design? Domain-Driven-Design- is a way of looking at the big picture (from top to down).

Domain is deeply connected with real-world. It’s banking, insurance or e-commerce.

e.g. Accountant is an expert in the domain of Accountancy. ~ Wikipedia

Let’s call himDomain Expert. He doesn’t need to be a Software Engineer instead he has some particular skills and knowledge in the area of endeavour. The development of accounting software requires knowledge in two different domains: accounting and software.

Everything is about right mindset. Let’s bring up another idea. You’re engaged to design a building. Your initial requirements are as follows:

3000m2 of land for development. The Building must have 3 floors and roof terrace. On the outside, there should be a playground with garden. On each floor, there needs to be 4-flats and 2-studios.

What’s your domain here?

Your domain might be a building, but it’s quite generic so you might miss few details of your requirements. There so many options to choose from you need to narrow down your mind-list. Let’s go with Residential Building. Now that term is more meaningful for you as a Developer and also for a business. Now we can start the search for a right house model.


Let’s go back to the basics… once again

What is Service (Application)? A bunch of code?


⬇️️ We have exe, jars, zip. Essentially already packed up and ready to deploy apps.

⬇️ Then we have - methods, objects, classes.

⬇️ Later we have principles - OOP, Design Patterns (which are reusable solutions/tips for common problems).

⬇️ Then we have modules that follow some Design Patterns.

⬇️ Next, we have Architecture. Where you organize code in some kind of layers. e.g. layered architecture.

➡️ Lastly, we have a project/application which is called a Service.

What is above that?

You may have many Services. All of those are a part of the infrastructure. They are trying to solve a BIGGER problem. Just like in case below e-commerce.

Domain has other sub-domains which serve different parts of the larger puzzle. Generally speaking, these sub-domains may have other sub-domains and so on […]


What tools Domain-Driven Design give us?

Strategic Design Tools - simplifying that is a Domain. Bigger concept. Tactical Design Tools - and that’s is just a Service. Your application.

Strategic Design Tools

Do you remember something called Object-Oriented Design? Often called interchangeable as Object-Oriented-Programming.

Which means you think in terms of Objects. Strategic Design is about thinking in Contexts. DDD is just OOD done right. Before getting to DDD patterns like Aggregates, Entities, Value-Objects, Repositories, Factories and other cool-named building blocks that are part of Tactical Tools we need to understand few concepts before that.

What is Context? Which one would you choose? TIP: the Second one is for FREE!


There is something called Bounded Context

Do you remember that statement: Accountant is an expert in the domain of Accountancy. Let’s say it differently - accountant is bounded in context of Accountancy As you can see I like to operate on examples so let me introduce to you, Luke and Lucy.


Lucy is a Software Engineer, on the other hand, Luke handles Accounting.

Accounting Department (sub-domain) - takes care of everything related to accounting.

IT Department (sub-domain) - handles everything related to IT.

They have their own internal systems and environments and they don’t go into each other business. Luke makes sure all payrolls are handled. He doesn’t mess with Lucy code. Actually, he doesn’t even have an IDE installed. When Luke finds out that there is a bug in accounting software he passes it to Lucy. He adds a detailed description of how it should work. Lucy quickly finds a broken piece of code and fixes it. […] But the entire company is huge and the same incident gets to the Security guy. Now there is third Bounded Context - a guy from security (he likes to be incognito, you know Security).

You might have Invoice. It has some meaning for Accounting guy but for a programmer - it’s just a class name. Because the name is identical it doesn’t mean it’s the same concept. An important note is that names are irrelevant. You might say, Payer and Customer. Different Bounded Context may have different naming conventions but they mean the same thing for the business […] It’s named Ubiquitous Language which is another DDD buzz-word.

Different contexts relate to each other. These relations are called context map.

Ubiquitous Language: Software Engineer vs Domain Expert

Developers and business should share a common language (Ubiquitous Language) that both understand and mean the same thing.

The idea is that you as a Software Engineer have to write down system description. How objects interact. What’s the model? […] And all that. On the other hand, there is Domain Expert that has to describe the whole system so you can map it into the software. It’s more like a cooperation than a fight.

There is a pretty cool example in Domain-Driven Design Quickly.

Developer: We want to monitor air traffic. Where do we start?

Domain Expert: Let’s start with the basics. All this traffic is made up of planes. Each plane takes off from a departure place, and lands at a destination place.

Developer: That’s easy. When it flies, the plane can just choose any air path the pilots like? Is it up to them to decide which way they should go, as long as they reach destination?

Domain Expert: Oh, no. The pilots receive a route they must follow. And they should stay on that route as close as possible.

Developer: I’m thinking of this route as a 3D path in the air. If we use a Cartesian system of coordinates, then the route is simply a series of 3D points.

Domain Expert: I don’t think so. We don’t see route that way. The route is actually the projection on the ground of the expected air path of the airplane. The route goes through a series of points on the ground determined by their latitude and longitude.

[…] and so on.

Let’s summarize everything


A Domain is a problem. We might say that DDD is Business Problem Driven Design. The domain is the business you are working with and the problems they want to solve. Model of DDD project is your solution to the problem. It’s often a simplification of the BIGGER picture. Domain Model is your organised and structured knowledge of the problem which has relationships among all of the entities within the scope of the domain.

TLTR; Domain is business. Model is your solution. Domain Model is structured knowledge of the problem.

It’s all good… But how these stories refer to the real-world?

Do you know that you’ve been using Bounded Context already without even knowing it? You know all of those layers like API, Persistence/Infrastructure, UI, Domain. They are simply implementations of separation of concerns principle. Which might be called a bounded context.