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pantheon-ai/software-design-principles

Apply software design principles across architecture and implementation using deterministic decision workflows, SOLID checks, structural patterns, and anti-pattern detection; use when reviewing designs, refactoring modules, or resolving maintainability and coupling risks.

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dep-interface-ownership.mdreferences/

title:
Interfaces Belong to Clients Not Implementers
impact:
CRITICAL
impactDescription:
enables true dependency inversion
tags:
dep, interfaces, dependency-inversion, ownership

Interfaces Belong to Clients Not Implementers

Interfaces should be defined in the layer that uses them, not the layer that implements them. The client owns the abstraction; the implementation adapts to it.

Incorrect (interface defined next to implementation):

// infrastructure/persistence/UserRepository.java
public interface UserRepository {
    User findById(String id);
    void save(User user);
}

// infrastructure/persistence/PostgresUserRepository.java
public class PostgresUserRepository implements UserRepository {
    // Implementation
}

// application/usecases/CreateUserUseCase.java
import infrastructure.persistence.UserRepository;  // Use case imports from infrastructure!

public class CreateUserUseCase {
    private final UserRepository repository;
}

Correct (interface defined where it's used):

// application/ports/output/UserRepository.java
public interface UserRepository {
    User findById(String id);
    void save(User user);
}

// application/usecases/CreateUserUseCase.java
import application.ports.output.UserRepository;  // Same layer import

public class CreateUserUseCase {
    private final UserRepository repository;  // No infrastructure dependency
}

// infrastructure/persistence/PostgresUserRepository.java
import application.ports.output.UserRepository;  // Infrastructure depends on application

public class PostgresUserRepository implements UserRepository {
    // Implementation adapts to the port
}

Note: This is the essence of the Dependency Inversion Principle. The high-level module defines what it needs; low-level modules conform to that contract.

Reference: Clean Architecture - Chapter 11: DIP

Install with Tessl CLI

npx tessl i pantheon-ai/software-design-principles@0.1.4

references

adapt-anti-corruption-layer.md

adapt-controller-thin.md

adapt-gateway-abstraction.md

adapt-mapper-translation.md

adapt-presenter-formats.md

anti-patterns-and-frameworks.md

bound-boundary-cost-awareness.md

bound-defer-decisions.md

bound-humble-object.md

bound-main-component.md

bound-partial-boundaries.md

bound-service-internal-architecture.md

comp-common-closure.md

comp-common-reuse.md

comp-reuse-release-equivalence.md

comp-screaming-architecture.md

comp-stable-dependencies.md

dep-acyclic-dependencies.md

dep-data-crossing-boundaries.md

dep-interface-ownership.md

dep-inward-only.md

dep-no-framework-imports.md

dep-stable-abstractions.md

detailed-examples.md

entity-encapsulate-invariants.md

entity-no-persistence-awareness.md

entity-pure-business-rules.md

entity-rich-not-anemic.md

entity-value-objects.md

frame-di-container-edge.md

frame-domain-purity.md

frame-logging-abstraction.md

frame-orm-in-infrastructure.md

frame-web-in-infrastructure.md

test-boundary-verification.md

test-layer-isolation.md

test-testable-design.md

test-tests-are-architecture.md

usecase-explicit-dependencies.md

usecase-input-output-ports.md

usecase-no-presentation-logic.md

usecase-orchestrates-not-implements.md

usecase-single-responsibility.md

usecase-transaction-boundary.md

SKILL-FULL.md

SKILL.md

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