Domain Driven Design

DDD is a software design approach that prioritizes the business domain at the core of development.

Coined by Eric Evans in his book Domain-Driven Design: Tackling Complexity in the Heart of Software, DDD helps teams structure code based on real-world business concepts, making applications easier to understand and evolve.

Key Goals of DDD

• Align software with real business problems
• Improve communication between developers and domain experts
• Reduce complexity by organizing code around business concepts
• Facilitate maintainability and scalability

"In order to create good software, you have to know what that software is all about. You cannot create a banking software system unless you have a good understanding of what banking is all about, one must understand the domain of banking."

— Domain-Driven Design by Eric Evans

The Core Concepts of DDD

DDD introduces key concepts to help structure applications. Let’s break them down:

1. Domain & Subdomains

A domain is the sphere of knowledge or activity your software operates in (e.g., e-commerce, banking, healthcare).

Within a domain, you may have multiple subdomains, such as payments, inventory, and customer management.

2. Bounded Contexts

A bounded context defines a clear boundary around a specific part of the domain, ensuring that models remain independent and cohesive within their context.

For instance, in an e-commerce system, the "Order Processing" and "Product Catalog" contexts operate separately with their own data models.

3. Entities & Value Objects

• Entities have unique identities and change over time (e.g., User, Order).
• Value Objects are immutable and defined by their attributes (e.g., Money, Address).

4. Aggregates & Aggregate Roots

An aggregate is a cluster of domain objects treated as a single unit.

The aggregate root is the main entity that ensures consistency within the aggregate.

For example, an Order entity (aggregate root) may contain OrderItem entities but enforces rules such as "an order must have at least one item."

5. Domain Events

Domain events represent significant occurrences in the business domain (e.g., OrderPlaced, PaymentReceived).

These help decouple business logic and improve maintainability.

6. Repositories, Services & Factories

• Repositories manage the retrieval and persistence of domain objects, abstracting data access concerns.
• Domain Services encapsulate domain-specific operations that don't naturally fit within entities or value objects.
• Factories streamline object creation, ensuring complex instances are constructed correctly.

7. Ubiquitous Language

A shared language between developers and domain experts ensures clarity and alignment, avoiding miscommunication.

The idea is to create a conceptual description of the system that both developers and domain experts can understand.

For example, in a betting system, the language would include terms like 'race', 'bet', and 'odds'.

Addressing Complexity with DDD

What one person finds complex, another might find straightforward.

However, in software development, complexity often arises due to interconnected systems, multiple data sources, and diverse business objectives.

DDD is designed to tackle these challenges by structuring software around core business concepts.

In contrast, when dealing with straightforward applications, an emergent design approach may suffice.

But as a system evolves, its complexity naturally increases, making a domain-driven approach more beneficial in the long run.

Modern business environments demand precision, as poor architectural decisions can have significant repercussions.

DDD provides a framework to handle intricate domain logic while keeping software aligned with business goals.

Implementing DDD in a Real-World Project

Now that we understand the basics, let's see how DDD fits into a real-world application.

1. Identify the Domain and Subdomains

For an e-commerce platform, the main domain is "Online Shopping." Subdomains might include:

• Order Management (core domain)
• Customer Support (supporting domain)
• Product Recommendations (generic domain)

2. Define Bounded Contexts

• The Order Context handles orders, payments, and invoices.
• The Product Context manages product catalog and inventory.

3. Design Aggregates & Entities

For the Order Context, you might define:

class Order {
  constructor(
    private orderId: string,
    private items: OrderItem[],
    private status: OrderStatus
  ) {}

  addItem(item: OrderItem) {
    this.items.push(item);
  }

  completeOrder() {
    this.status = 'COMPLETED';
  }
}

4. Use Domain Events for Decoupling

Instead of directly modifying the payment system from Order, we fire a domain event:

class OrderPlaced {
  constructor(public readonly orderId: string) {}
}

A handler listens for this event to trigger payment processing.

5. Apply Repositories for Data Persistence

class OrderRepository {
  private orders: Map<string, Order> = new Map();

  save(order: Order) {
    this.orders.set(order.orderId, order);
  }

  findById(orderId: string): Order | undefined {
    return this.orders.get(orderId);
  }
}

When to Use DDD

DDD is powerful but isn’t for every project. Here’s when it makes sense:

1. Complex business logic with evolving requirements
2. Large-scale applications with multiple teams
3. Projects needing strong alignment between business and development teams

Avoid DDD for:

1. Simple CRUD applications
2. Prototypes or MVPs with minimal domain complexity

Do You Really Need DDD?

If you don’t recognize the problems that DDD solves, then maybe you don’t need it.

DDD is particularly beneficial for long-term, complex projects (typically >6 months).

If your project lacks significant business complexity, using DDD might introduce unnecessary overhead.

Additionally, before diving into DDD, it's crucial to be familiar with design patterns and enterprise design principles.

Concepts like repositories, factories, and aggregates can be challenging without prior experience in object-oriented design.

Final Thoughts

Domain-Driven Design isn’t just a set of rules—it’s a mindset that helps developers build software that aligns with real business problems.

By focusing on the domain, defining clear boundaries, and leveraging domain events, you can create scalable and maintainable applications.

If you’re working on a complex system, consider adopting DDD principles to bring clarity and long-term stability to your codebase!

What’s next?

• Dive deeper into Event Sourcing and CQRS as they often complement DDD.
• Explore tools like NestJS or Spring Boot that support DDD patterns.
• Join communities discussing DDD best practices.

Got thoughts on DDD? Drop a comment below!

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Comments

[Luca Guadagnini]

Nice! I truly encourage embracing Domain-Driven Design (DDD) for any project. Complexity can arise even in simple CRUD applications, and DDD helps clarify your goals. By diving into it, you'll elevate your skills as a developer, architect, and analyst. Enjoy it!