Tactical DDD Patterns

Introduction

Tactical Domain-Driven Design (DDD) patterns are essential tools that help software architects and developers create well-structured applications. These patterns guide the implementation of business logic, ensuring that the software aligns with the domain model.

Tactical Patterns

1. Entities

Entities are objects that have a distinct identity that runs through time and different states. For example, a User entity might represent a customer in an e-commerce application.


class User {
    constructor(id, name) {
        this.id = id; // Unique identifier
        this.name = name;
    }
}

2. Value Objects

Value Objects are immutable and defined by their attributes rather than a unique identity. An Address value object might include fields like street, city, and postal code.


class Address {
    constructor(street, city, postalCode) {
        this.street = street;
        this.city = city;
        this.postalCode = postalCode;
    }
}

3. Aggregates

Aggregates are clusters of domain objects that can be treated as a single unit. An aggregate contains one or more entities and value objects, enforcing consistency rules for the entire cluster.


class Order {
    constructor(id, items) {
        this.id = id;
        this.items = items; // Array of Order Items
    }
}

4. Repositories

Repositories provide a collection-like interface for accessing aggregates. They are responsible for retrieving and storing aggregates while abstracting the details of the data storage.


class OrderRepository {
    constructor() {
        this.orders = []; // In-memory storage for simplicity
    }

    add(order) {
        this.orders.push(order);
    }

    findById(id) {
        return this.orders.find(order => order.id === id);
    }
}

5. Domain Services

Domain Services contain domain logic that doesn’t naturally fit within an entity or value object. They encapsulate operations that involve multiple entities or value objects.


class PaymentService {
    processPayment(order, paymentDetails) {
        // Logic to process payment
        console.log(`Processing payment for Order ID: ${order.id}`);
    }
}

Implementation Process

Implementing Tactical DDD Patterns involves several steps:

Identify the domain and subdomains.
Define entities and their relationships.
Establish value objects and aggregates.
Implement repositories for data access.
Create domain services for business logic.

Note: Always ensure that your domain model reflects the real-world business processes accurately.

Best Practices

Keep entities focused on their identity and lifecycle.
Use value objects to represent descriptive aspects of the domain.
Design aggregates to enforce invariants and maintain consistency.
Utilize repositories to abstract data access and persistence.
Isolate domain logic within services when it spans multiple entities.

FAQ

What is the difference between an entity and a value object?

Entities have a unique identity and change over time, while value objects are immutable and defined by their attributes.

Why are aggregates important?

Aggregates ensure consistency across related entities and value objects, simplifying data management.

Can a value object contain other value objects?

Yes, value objects can contain other value objects, allowing for complex structures while maintaining immutability.