Before understanding Clean Architecture, let's first understand Three-Tier Architecture, and then we will understand how Clean Architecture is better compared to Three-Tier.
Three-Tier Architecture.
Three-Tier Architecture is a software design pattern that separates an application into three logical and physical layers, each with a specific responsibility.
Flow of Data in Three-Tier:
User (UI Layer)
↓
Controller → Calls Service (BLL)
↓
Service → Calls Repository (DAL)
↓
Repository → Talks to Database
Clean Architecture.
Clean Architecture is a powerful software design pattern that helps you organize your application into loosely-coupled, highly maintainable layers. It was popularized by Robert C. Martin (Uncle Bob) and is especially useful in large enterprise-level applications like those built with ASP.NET Core.Why Clean Architecture?
- Separates concerns clearly
- Promotes testability
- Enables independent development of core logic and infrastructure
- Improves scalability and flexibility
- Encourages dependency inversion (core depends on nothing)
Core Principles of Clean Architecture
UI
- Controllers, Views, View Models
- Filters, Middleware
Core
- Business Logic Services
- Business Logic Interfaces
- Data Transfer Objects (DTO)
Domain
- Repository Interfaces
- Entity Classes
Infrastructure
- DbContext, Repositories
- External API Calls
| Feature | Three-Tier Architecture | Clean Architecture |
|---|---|---|
| Layer Focus | Focuses on UI → BLL → DB separation | Focuses on core business logic and dependency flow |
| Dependency Flow | Flows UI → BLL → DAL | Always flows inward, toward the domain layer |
| Coupling | Higher coupling to frameworks (e.g., EF in BLL) | Loosely coupled; core has no external dependencies |
| Testability | Harder if BLL directly depends on infrastructure | High testability; core logic is isolated |
| Flexibility | Rigid – changing DB or UI may impact logic | Flexible – DB/UI are pluggable |
| Domain-Centric | No – business logic mixed with infrastructure | Yes – everything revolves around business rules |
| Use in Microservices | Not ideal | Highly suitable |
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