Top 38 DDD Interview Questions And Answers

Question 1

What are the fundamental _components_ of _Domain-Driven Design_?

Accepted Answer

**Ubiquitous language and Unified Model Language (`UML`)**:

* _Ubiquitous language_ is a common language to communicate within a project.
* It's because _designing a model_ is a collaborative effort of _software designers_, _domain experts_, and _developers_ that it requires a common language to communicate with.
* It removes **misunderstandings**, and **misinterpretations**.
* _Communication gaps so often lead to bad software_ - _ubiquitous language_ minimizes these gaps.

**Multilayered architecture** is a common solution for Domain-Driven Design and contains four layers:

* _Presentation_ layer or (UI)
* _Application_ layer
* _Domain_ layer
* _Infrastructure_ layer

**_Artifacts_** used in Domain-Driven Design to **express**, **create** and **retrieve** domain models mapping business domain concepts into software artefacts. It’s about organizing code artifacts in alignment with business problems, using the same common, ubiquitous language. Some of the **artefacts** are:
* DAO interface and implementation class
* Factories
* Repositories
* Domain Delegate (if needed)
* Facade
* DTO's
* Unit Tests for the above classes (including test class and test data)
* Configuration files

![https://miro.medium.com/max/1290/1*x4O5ae2f8UzCXncyvqprMg.jpeg](https://miro.medium.com/max/1290/1*x4O5ae2f8UzCXncyvqprMg.jpeg)

Question 2

List some advantages of _Domain-Driven Design_. Why developers shall use it?

Accepted Answer

**Simpler communication:**

* Thanks to the _Ubiquitous Language_, _communication between developers and teams become much easier_.
* As the _Ubiquitous language_ is likely to contain simpler terms developers refer to, _there's no need for complicated technical terms_.

**More flexibility:**

* As DDD is object-oriented, everything about the domain is _based on and object is modular and caged_.
* Thanks to this, the _entire system can be modified and improved regularly_

**The domain is more important than `UI/UX`:**

* As the domain is the central concept, developers will _bild applications suited for the particular domain_.
* This won't be another _interface-focused application_.
* Although you shouldn't leave out `UX`, using the DDD approach means that the _product targets exactly the users_ that are directly connected to the domain.

Question 3

What is a _Specification_ in DDD?

Accepted Answer

* A **Specification** represents a _business rule that needs to be satisfied_ by at least part of the domain model,

* You can also use **Specification** for query _criteria_
    
  * For example, you can query for all objects that satisfy a given _specification_.

Question 4

How would you describe what is _Domain Driven Design_ from the developer point of view?

Accepted Answer

**Domain Driven Design** is a methodology and process prescription for the development of complex systems whose focus is mapping **activities**, **tasks**, **events**, and **data** within a problem domain into the technology **artifacts** of a solution domain.

The emphasis of Domain Driven Design is to understand the problem domain in order to create an abstract model of the problem domain which can then be implemented in a particular set of technologies. Domain Driven Design as a methodology provides guidelines for how this model development and technology development can result in a system that meets the needs of the people using it while also being robust in the face of change in the problem domain.

The process side of Domain Driven Design involves the collaboration between domain experts, people who know the problem domain, and the design/architecture experts, people who know the solution domain. The idea is to have a shared model with shared language so that as people from these two different domains with their two different perspectives discuss the solution they are actually discussing a shared knowledge base with shared concepts.

Question 5

What is a _Domain Model_ in DDD?

Accepted Answer

The **Domain Model** is a _software model_ of the very specific business domain you are working in. Often it's implemented as an _object model_, where those objects have _both data and behavior_ with literal and accurate business meaning.

Creating a _unique_, carefully crafted _domain model_ at the heart of a core, strategic application or subsystems essential to practicing DDD. 
With _DDD your domain models_ will tend to be smallish, very focused.
Using DDD, you never try to model the whole business enterprise with a single, large domain model.

Question 6

What is _Domain_ in DDD?

Accepted Answer

**Domain** is the field for which a system is built. Airport management, insurance sales, coffee shops, orbital flight, you name it.

It's not unusual for an application to span several different domains. For example, an online retail system might be working in the domains of shipping (picking appropriate ways to deliver, depending on items and destination), pricing (including promotions and user-specific pricing by, say, location), and recommendations (calculating related products by purchase history).

Question 7

What is the distinction between _Value Types_ and _Entities_ in DDD?

Accepted Answer

* Classical implementation of DDD differentiates between **Value Types** and **Entity Types**, based on their _mutability and notion of identity_.
* **Value types** are _immutable_ and do not convey enough information on their own

For example:

* `Color` could be a **value type** where _color type does not hold any meaning in itself_ but when attached to an _entity_ like a `shirt` or a `car` (for example a `red shirt` or a `black shirt`) which mean something in the domain.

On the contrary, an **entity** has a _lifecycle_. These are the types that are _mutable_ and go through changes through different lifecycle events.
* `Order` could be an **entity** that goes through different lifecycle events such as _item added to order or item removed from order_.
_Each lifecycle event mutates the entity_.

Question 8

What is Domain Driven Design?

Accepted Answer

**Domain Driven Design** is a methodology and process prescription for the development of complex systems whose focus is mapping activities, tasks, events, and data within a problem domain into the technology artifacts of a solution domain.

It is all about trying to make your software a model of a real-world system or process.

Question 9

What types of issues does an _Aggregate_ solve in DDD?

Accepted Answer

Basically _Aggregates_ help:

* _Simplify_ the models when they start getting out of hand
* _Isolate_ complex business rules
* _Deal with performance issues_ when loading large object graphs into memory
* _Allow flexibility_ to more easily deal with future unexpected business requirements

Question 10

What is the difference between DTOs and ViewModels in DDD?

Accepted Answer

* The canonical definition of a **DTO** is the data shape of an object _without any behavior_. Generally DTOs are used to ship data from one layer to another layer across process boundries.

* **ViewModels** are the model of the view. ViewModels typically are full or _partial_ data from one or _more_ objects (or DTOs) plus any additional members specific to the view's behavior (methods that can be executed by the view, properties to indicate how toggle view elements etc...). In the MVVM pattern the ViewModel is used to isolate the Model from the View.

Top 38 DDD Interview Questions

What is Domain Driven Design?

What is Domain in DDD?

How would you describe what is Domain Driven Design from the developer point of view?

What is a Domain Model in DDD?

What is a Specification in DDD?

What are the fundamental components of Domain-Driven Design?

List some advantages of Domain-Driven Design. Why developers shall use it?

What is the distinction between Value Types and Entities in DDD?

What is the purpose of Service in Domain-Driven Design (DDD)

What is a Model in DDD?

Explain the concept of Repository in the context of DDD

What is the difference between Value vs Entity Objects in DDD?

What is the difference between Domain Objects, POCO, Services, Repositories and Entities?

What is the difference between Behavior-Driven Development (BDD) vs Domain-Driven Design (DDD)?

What is Event Storming in DDD?

What is the Command and Query Responsibility Segregation (CQRS) Pattern?

Name some benefits of CQRS Pattern

Describe what is the Event Sourcing Pattern

What's an Aggregate Root in DDD?

What is the differences between Strategic Patterns and Tactical Patterns?

What is Domain in DDD?

What is the difference between DTOs and ViewModels in DDD?

Explain the different layers in DDD

What types of issues does an Aggregate solve in DDD?

What does Bounded Context mean in DDD?

Mention how can you give objects an Unique Identity in DDD?

What does it mean to focus on Problem Space rather than the Solution Space?

What is the difference between DAO and Repository in DDD?

What is the difference between Infrastructure Service and Repository in DDD?

What is the difference between Factory and Repository in DDD?

What is Context Mapping's purpose in DDD?

What is Core Domain, Supporting Subdomain, and Generic Subdomain in DDD?

What exactly are the Anti-Corruption layers in DDD? Provide an example.

Provide some examples of Infrastructural Services in DDD

What are Aggregates in Domain-Driven Design?

What is main difference between Domain vs Application vs Infrastructure Services?

Can we use the CQRS without the Event Sourcing?

Where DTO should be implemented, in a Domain Layer or in an Application Service Layer? Explain.