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====== A Guide to Using Clean Architecture in Go ======

===== I. Introduction to Clean Architecture =====

Clean Architecture is a software architecture proposed by Robert C. Martin (Uncle Bob).

Its main goals are:

Clear separation of concerns

Reduced coupling between layers

Easier testing, maintenance, and evolution

This project follows Clean Architecture only.
No Domain-Driven Design (DDD) or Bounded Context concepts are required.

===== II. Core Principles =====

==== 1. Layered Separation ====

The system is divided into four logical layers:

Domain Layer
Core business rules and contracts

Application / Usecase Layer
Orchestration of business logic

Interface (Presentation) Layer
gRPC / HTTP handlers

Infrastructure Layer
Database and external service access

==== 2. Dependency Rule (Most Important Rule) ====

Dependencies always point inward.
Outer layers depend on inner layers.
Inner layers never depend on outer layers.

<code> Interface → Application → Domain Infrastructure → Domain </code>

===== III. Project Folder Structure =====

==== Overall Structure (Clean Architecture Only) ====

<code text>
internal/
├── domain/                 # Domain Layer (pure business logic)
│   ├── model/              # Core business entities
│   ├── repository/         # Repository interfaces (contracts)
│   └── service/            # Domain services (optional)
│
├── application/            # Application Layer
│   ├── usecase/            # Use cases (business orchestration)
│   └── dto/                # Input / output data structures
│
├── interface/              # Interface (Presentation) Layer
│   ├── grpc/
│   │   └── handler/        # gRPC handlers
│   └── http/               # HTTP handlers (optional)
│
├── infrastructure/         # Infrastructure Layer
│   ├── repository/         # Repository implementations
│   ├── persistence/        # Database setup & transactions
│   └── client/             # External API / service clients
│
└── shared/                 # Cross-cutting concerns
    ├── constant/
    ├── validator/
    ├── logger/
    └── database/
</code>


===== IV. Layer Responsibilities =====

^ Layer ^ Responsibility ^ Depends On ^
| Domain | Business rules and contracts | Nothing |
| Application | Usecase orchestration | Domain |
| Interface | Request / response handling | Application |
| Infrastructure | DB and external APIs | Domain |

===== V. Processing Flow (Request → Response) =====

<code text>
Client (gRPC / HTTP Request)
        ↓
Interface Layer (Handler)
  - Receive request
  - Validate input
        ↓
Application Layer (Usecase)
  - Orchestrate business logic
        ↓
Domain Layer (Repository Interface)
  - Define contracts
        ↓
Infrastructure Layer (Implementation)
  - Access DB / External API
        ↓
Database / External Service
</code>
===== VI. Detailed Layer Examples =====

==== 1. Domain Layer — Define Interfaces ====

Purpose

Define business contracts

No implementation

No framework or DB dependency

File location:
<code text>
internal/domain/repository/add_point_repository.go

<code go> type AddPointRepository interface { FetchProcessingOrders( ctx context.Context, limit, offset int, ) ([]ProcessingOrder, error)
SaveOrders(
    ctx context.Context,
    orders []ValidOrder,
) error


}
</code>

✔ No SQL
✔ No gRPC
✔ No infrastructure dependency

==== 2. Application Layer — Usecase Orchestration ====

Purpose

Coordinate business steps

Use domain interfaces only

Easy to unit test

File location:
<code>
internal/application/usecase/point_add_usecase.go

</code>

<code go>
type PointAddUsecase struct { repo domain.AddPointRepository }

func (u *PointAddUsecase) Execute(ctx context.Context) error {
orders, err := u.repo.FetchProcessingOrders(ctx, 200, 0)
if err != nil {
return err
}

// Business calculation:
// product_total - adjusted_merchandize_total_tax

return nil


}
</code>

✔ No DB logic
✔ No framework dependency
✔ Mockable for unit tests

==== 3. Interface Layer — gRPC Handler ====

Purpose

Handle incoming requests

Validate input

Delegate logic to usecase

File location:
<code>
internal/interface/grpc/handler/sso_handler.go
</code>

<code go> type SSOHandler struct { usecase application.SSOUsecase }

func (h *SSOHandler) Authenticate(
ctx context.Context,
req *pb.AuthRequest,
) (*pb.AuthResponse, error) {

if err := validator.ValidateSSO(req); err != nil {
    return nil, err
}

return h.usecase.Authenticate(ctx, req)


}
</code>

✔ Thin layer
✔ No business logic
✔ No DB access

==== 4. Infrastructure Layer — Repository Implementation ====

Purpose

Implement domain interfaces

Handle SQL and external APIs

Replaceable without affecting core logic

File location:
<code>
internal/infrastructure/repository/add_point_repository.go
</code>

<code go> type AddPointRepositoryImpl struct { db *sqlx.DB }

func (r *AddPointRepositoryImpl) FetchProcessingOrders(
ctx context.Context,
limit, offset int,
) ([]ProcessingOrder, error) {

query := `
    SELECT ...
    FROM orders
    LIMIT ? OFFSET ?
`
var orders []ProcessingOrder
err := r.db.SelectContext(ctx, &orders, query, limit, offset)
return orders, err


}
</code>

✔ Depends only on Domain
✔ Infrastructure-specific logic only

===== VII. Best Practices =====

==== 1. Dependency Injection ====

Always inject interfaces, not concrete implementations.

<code go> usecase := PointAddUsecase{ repo: addPointRepositoryImpl, } </code>

==== 2. Testing Strategy ====

Unit Tests

Test usecases with mocked repositories

No DB required

Integration Tests

Real database

Test infrastructure layer only

==== 3. Error Handling ====

Use structured errors

Log errors in:

Interface layer

Application layer

==== 4. What NOT to Do ====

❌ Business logic in handlers

❌ SQL in usecases

❌ Infrastructure importing application layer

❌ Circular dependencies

===== VIII. Example: Adding a New Feature =====

Feature: Register Customer

Steps

Domain: create CustomerRepository interface

Application: create RegisterCustomerUsecase

Interface: add gRPC handler

Infrastructure: implement repository using DB

Flow

<code> Handler → Usecase → Repository Interface → Repository Implementation → Database </code>

===== IX. Conclusion =====

By following Clean Architecture:

Dependencies always point inward

Layers are clearly separated

Business logic is framework-independent

Code is easier to:

Maintain

Test

Scale