Domain Driven Design (DDD) for Beginners

Domain-Driven Design (DDD) is often misunderstood as a complex architecture or a framework.
In reality, DDD is a way of thinking and designing software around business problems.

This article answers four essential questions:

• What is Domain-Driven Design?

• Why was it invented

• What problem does it solve?

• What is Ubiquitous Language?

• What are bounded contexts and strategic design principles?

What Is Domain-Driven Design?

Domain-Driven Design is an approach, not an architecture.

DDD focuses on:

• Understanding the business domain

• Modeling software around real business concepts

• Aligning developers and domain experts using a shared language

Instead of organizing your project by technical layers (Controllers, Services, Repositories), DDD encourages organizing it by business capabilities.

Simple definition:
DDD is about structuring your software to reflect how the business actually works.

Why Was DDD Invented?

DDD was introduced by Eric Evans to solve a common problem in software projects:

The growing gap between business knowledge and technical implementation

Common problems before DDD:

• Business rules scattered across the codebase

• Developers and domain experts use different terminology

• Large, monolithic models that try to represent everything

DDD addresses these problems by:

• Creating clear boundaries

• Defining explicit models

• Using business language directly in code

Strategic Design vs Tactical Design

DDD is divided into two complementary parts:

1️⃣ Strategic Design (The big picture)

• Defines boundaries

• Defines relationships between domains

• Helps manage complexity at scale

2️⃣ Tactical Design (Inside a boundary)

• Entities

• Value Objects

• Aggregates

• Repositories

• Domain Services

Ubiquitous Language

Ubiquitous Language is a shared vocabulary used by everyone, domain, and developers.

This language is used in meetings, appears in documentation, and is reflected directly in code

Without a shared language, developers guess the business meaning, business people misunderstand technical constraints, and we are lost.

Example

In an e-commerce system, the word Order might mean different things:

• To a customer: "What I bought"

• To accounting: "An invoice"

• To shipping: "A package with weight and address"

DDD says: don't force one meaning everywhere.

Bounded Context

A Bounded Context defines a clear and explicit boundary around a specific domain scope, with its own domain model and its own vocabulary. Inside a bounded context, every term has one precise meaning, and the model remains consistent and unambiguous. However, across different bounded contexts, the same word may carry different meanings, depending on the business perspective and rules of that context.

E-commerce Example

Context	Meaning of "Order"
Ordering	Items, quantities, prices
Shipping	Dimensions, address, weight
Billing	Invoice, taxes, payment status

We can't merge all of these into one Order class.

Context Map

A Context Map describes:

• Which bounded contexts exist

• How they communicate

• The direction of dependencies

It is usually visualized as a diagram.

Example: Banking System (Why Context Maps Matter)

Imagine a banking system with:

Context 1: Banking

Account
- AccountNumber
- Balance

Context 2: Payee Management

Account
- AccountNumber
- Notes

At first glance, both have an Account class.

In a classic UML only approach, a developer might say:

"We already have Account. Let's reuse it."

This creates tight coupling and wrong assumptions:

• Payee accounts don't need balances

• Banking rules shouldn't affect payee management

How Context Maps Fix This

The context map makes boundaries explicit:

• These Account models live in different contexts

• They evolve independently

Anti-Corruption Layer (ACL)

An Anti-Corruption Layer protects one domain from another.

Example

Instead of using an external ExternalCustomerDto directly:

public class CustomerTranslator
{
    public Customer Translate(ExternalCustomerDto dto)
    {
        return new Customer(
            new CustomerId(dto.Id),
            dto.FullName
        );
    }
}

Your domain stays clean and consistent.

Shared Kernel

Sometimes, two bounded contexts must share a small part of the model.

This shared part is called a Shared Kernel.

Example:

• Shared Money value object

• Shared Currency enum

These items may be saved in a common project between different contexts.

Tactical Design (Inside a Bounded Context)

Once boundaries are clear, we design the internals.

Entities

• Have identity

• Change over time

• Equality based on ID

class Customer
{
    public CustomerId Id { get; }
    public string Name { get; private set; }
}

Value Objects

• Immutable

• Identified by value

• No identity

public record Money(decimal Amount, string Currency);

Aggregates

• A consistency boundary

• Root entity controls access

Example:

• Order aggregate

• Contains OrderItems, entities, and Moneyvalue objects

Benefits of Domain-Driven Design

• Better communication with domain experts

• Code reflects real business rules

• Easier to evolve with business changes

• Clear separation of concerns

• Reduced accidental complexity

Real Life Steps to Apply DDD

1. Understand the business domains

2. Define a Ubiquitous Language

3. Identify bounded contexts

4. Draw a context map

5. Design aggregates inside each context