🚀 Day 3: Architectures Explained with My Implementation

(DDD + Clean + Onion + Microservices)

Many times in interviews, questions come like:
👉 “What’s the difference between DDD, Clean, and Onion Architecture?”
👉 “How do you apply them in a real project?”
👉 “Why Microservices over Monolith?”

Today, I’ll simplify these with my Catalog Service example (Products, Categories, Brands).

1️⃣ Domain-Driven Design (DDD)

What it is:

Focuses on the Domain Model (business rules, not just CRUD).
Uses Entities, Value Objects, Repositories, Aggregates, Aggregate Roots.
Communication happens via Ubiquitous Language (same words as business).

Layers in DDD:

Domain Layer (Inner) → Entities (Product, Category), Interfaces (IProductRepository).
Application Layer (Middle) → Services (ProductService orchestrating use cases).
Infrastructure Layer (Outer) → EF Core, DB, external APIs.

My Example:

Product is an Aggregate Root.
IProductRepository ensures access through the domain contract.
EfRepository<Product> (Infra) implements IProductRepository.
Application Layer (ProductService) coordinates actions like “Add Product to Category.”

👉 Interview Tip: DDD ensures business-first design, avoids anemic models.

2️⃣ Clean Architecture

What it is:

Robert C. Martin (Uncle Bob) → emphasis on independence of frameworks, UI, DB.
Dependency Rule: All dependencies point inward, towards the Domain.

Layers (from inner to outer):

Entities (Domain) → Business rules.
Use Cases (Application) → Application-specific business logic.
Interface Adapters → Controllers, Presenters, DTOs.
Frameworks & Drivers (Outer) → EF Core, ASP.NET Core, DB, external systems.

My Example:

Entities: Product, Category, Brand.
Use Cases: ProductService, CategoryService.
Interface Adapters: ASP.NET Core Controllers (ProductController).
Frameworks/Drivers: CatalogDbContext, EF Core repositories.

👉 Interview Tip: Clean Architecture is technology-agnostic → I can switch from EF Core to Dapper without touching Domain/Application.

3️⃣ Onion Architecture

What it is:

Similar to Clean, but visually represented as onion rings.
Inner Core = Domain, outer layers depend on inner.

Layers (from core out):

Core (Domain) → Entities (Product, Category), Interfaces (IRepository<T>).
Application Layer → Services (ProductService).
Infrastructure Layer → Repositories (EfRepository<Product>).
Presentation Layer → API Controllers.

My Example:

Inner Onion Ring = Product + IProductRepository.
Middle = ProductService.
Outer = EfRepository<Product> + ProductController.

👉 Interview Tip: Onion vs Clean = philosophy is same (dependency direction). Onion is more visual metaphor, Clean is more principle-driven.

4️⃣ Microservices

What it is:

Break monolith into independent services around business capabilities.
Each service has its own DB + bounded context.

My Example:

Catalog Service = Products, Categories, Brands.
Future Order Service = Orders, Payments.
Communication → REST / gRPC / Messaging (RabbitMQ, Kafka).
Independent DBs → CatalogDB, OrderDB.

👉 Interview Tip: Microservices solve scaling + team autonomy, but add complexity (distributed systems).

🎯 How They All Connect in My Project

DDD → defines my Domain & Repositories.
Clean Architecture → ensures dependency flow is inward.
Onion Architecture → shows layered structure visually.
Microservices → breaks Catalog, Order, Auth into separate deployable units.

✅ So when interviewers ask “Which architecture are you following?” → My answer:

“I use a combination of DDD for domain modeling, Onion/Clean for dependency flow, and Microservices for scalability and independence. For example, in my Catalog Service, Product is an Aggregate Root in the Domain Layer, EfRepository<Product> sits in Infrastructure, ProductService in Application, and ASP.NET Controllers in the Presentation Layer. This separation ensures testability, maintainability, and future scalability.”

📌 GitHub repo https://github.com/SiddharthBhamare/SmartKart-Catalog.git

Stay tuned! 🚀