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How is DI implemented in Clean Architecture?

Design PatternsMiddle~1 min read
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In Clean Architecture, dependency injection (DI) is how the Dependency Rule is implemented in practice: inner layers define abstractions, while outer layers supply concrete implementations for them.

1. Principle

Business logic (use cases, entities) does not create its own dependencies. Everything it needs is passed in from outside.

This lets inner code work with interfaces without knowing which repository, API, or service is actually behind them.

2. What it looks like by layer

Inner layer (Use Case)

Defines interfaces, but not their implementation:

python
class OrderRepository(Protocol): def save(self, order): ...

Outer layer (Adapter / Infrastructure)

Implements those interfaces:

python
class SqlOrderRepository(OrderRepository): def save(self, order): db.insert(order)

Composition Root

Somewhere outside (for example, in main.py), dependencies are wired together:

python
def main(): repo = SqlOrderRepository() use_case = CreateOrderUseCase(order_repo=repo) controller = OrderController(use_case)

This way the use case receives a ready-made dependency instead of creating it itself.

3. Where injection happens

Injection always happens at the outer layer:

  • In a web app: in the controller or a DI container (FastAPI, Spring, NestJS).
  • In a CLI: at the entry point (main()).
  • In tests: through mock objects.

4. Why it matters

  • It follows the Dependency Rule (the inner layer does not depend on the outer one).
  • It simplifies testing (mocks can be substituted in).
  • It lets you easily swap implementations (for example, switching from SQL to NoSQL).
  • It makes the architecture extensible and flexible.

Summary:

In Clean Architecture, dependencies are injected from the outside in, through constructors, DI containers, or initialization functions, so that business logic depends only on abstractions, not on concrete implementations.

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