Domain-Driven Design: From Theory to Code — A Practitioner’s Guide

“) When I first encountered Domain-Driven Design, I felt like Alice chasing the White Rabbit down a rabbit hole. “Target the business core! Ubiquitous language! Strategic design layers!” the books cried. But how do these concepts translate into actual code? You’re here because you’ve scanned endless tutorials and still wonder where to start. Let’s cut through the jargon with a practical, code-first approach.

The Three-Legged Stool of DDD Implementation

Before we dive into code, let’s visualize the foundation. Think of DDD as three interdependent pillars supporting your application’s architecture:

Step 1: Discover Your Core Domain

Your domain model isn’t just “business logic” – it’s the heart of why your application exists. Let’s say you’re building an e-commerce platform. The core domain isn’t processing payments or managing user accounts; it’s optimizing inventory turnover and pricing strategies that adapt to market demands. To find your core domain:

1. Conduct an “Impact Hunt”: Ask stakeholders, “Which feature directly impacts revenue?”
2. Identify Constraints: What processes have rigid business rules? (e.g., “A product’s inventory count must never go negative”)
3. Map Competing Pressures: Use Event Storming techniques to visualize domain events

Step 2: Build Bounded Contexts

Not all features deserve first-class representation in your domain model. Bounded contexts partition complex systems into manageable pieces.

Step 3: Implement Tactical Patterns

Now we get to the code. DDD’s tactical patterns provide concrete implementation strategies.

1. Entities vs Value Objects

Entities:

public class Product {
    private final String sku;
    private Version version; // Entity tracks lifecycle
    public Product(String sku) {
        this.sku = sku;
        this.version = new Version();
    }
    public void updatePrice(BigDecimal newPrice) {
        version.increment();
        this.price = newPrice;
    }
}

Value Objects:

public class Money {
    private final BigDecimal amount;
    private final Currency currency;
    public Money(BigDecimal amount, Currency currency) {
        this.amount = amount.abs(); // Enforce business invariants
        this.currency = currency;
    }
    public boolean isHigherThan(Money other) {
        return this.amount.compareTo(other.amount) > 0;
    }
}

Step 4: Create An Implementation Setup

Layered Architecture Pattern

Step 5: Add Context Mapping Services

Implementing boundaries between contexts isn’t just about networking – it’s about data responsibility.

// Anti-Corruption Layer: Convert external system data to domain model
public class ThirdPartyPriceServiceProvider {
    public ProductPrice getExternalPrice(Stringsku) {
        ExternalPriceResponse response = restTemplate...
        return new ProductPrice(
            response.getCurrency(),
            response.getAmount(),
            response.isPromotional()
        );
    }
}

Step 6: Establish The Development Cycle

When to Stop Complicating Things

Not every project needs DDD. If you’re building:

• A proof-of-concept prototype
• A simple CRUD application
• A manage(with care) legacy system … Resist the urge to force DDD patterns.

Common Implementation Pitfalls

1. The Anemic Model Trap: Domain objects that are just data containers
2. Over-Engineering: Creating aggregates for simple data structures
3. Context Bleed: Tightly coupling bounded contexts
4. Event Storming Fatigue: Over-ceremonial event modeling sessions

Real-World Example: The ‘Steakhouse’ Pitfall

Suppose you’re building a restaurant management system. A well-intentioned team might create:

// Bad example
public class Steak {
    public CookingMethod cook(CookingMethod method) {
        // DTO with some calculations
    }
}

But in reality, the core domain is likely price optimization based on ingredient costs and customer preferences, not the cooking process itself.

Final Checklist: Are You DDD-Ready?

1. Do domain experts participate in modeling sessions?
2. Is there a clear distinction between core and supporting subdomains?
3. Can you articulate the value driven by each domain service?
4. Does your team maintain a glossary of key business terms?

Now, go forth and model domains with purpose. Remember: DDD is about making targeted Trade-offs, not creating architecture museums. As the great software philosopher* once said, “The best domain model is the one that makes your next feature easier to build.” (I might be making that up, but it sounds cool.)