Aggregates and Bounded Contexts in DDD-Based Architecture

Introduction

Domain-Driven Design (DDD) provides a systematic approach to software development that focuses on the core domain of the application. Two critical concepts in DDD are Aggregates and Bounded Contexts. This lesson will explore these concepts and how they relate to building robust software architectures.

What are Aggregates?

An Aggregate is a cluster of domain objects that can be treated as a single unit. An aggregate will have a root entity and may contain other entities and value objects. The root entity is responsible for managing the lifecycle of the aggregate and enforcing business rules.

Example of an Aggregate

class Order {
    constructor(id, customer) {
        this.id = id;
        this.customer = customer;
        this.items = [];
    }

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

    // Business logic related to the Order
}
            

What are Bounded Contexts?

A Bounded Context is a conceptual boundary within which a specific model is defined and applicable. It helps teams to collaborate effectively by providing a shared understanding of the domain model, reducing ambiguity, and clarifying terminology.

Example of Bounded Contexts

In an e-commerce system, you might have two bounded contexts: Ordering and Inventory. Each would have its own models and can evolve independently.

Relationship Between Aggregates and Bounded Contexts

Aggregates exist within Bounded Contexts. Each bounded context may contain multiple aggregates, and each aggregate should only enforce its own invariants. This relationship ensures that models remain cohesive and that domain logic is appropriately encapsulated.

graph TD;
    A[Bounded Context 1] -->|contains| B[Aggregate 1]
    A -->|contains| C[Aggregate 2]
    D[Bounded Context 2] -->|contains| E[Aggregate 3]
    D -->|contains| F[Aggregate 4]
            

Best Practices

Tips for Working with Aggregates and Bounded Contexts:

1. Identify the core domain and separate it from other concerns.
2. Define clear boundaries for each bounded context.
3. Ensure aggregates encapsulate all invariants within their boundaries.
4. Use domain events for communication between aggregates in different bounded contexts.

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