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🎨 System Design: Interface Design (API & Component Interfaces)

Interface design in system architecture defines how different parts of a system communicate—whether internal modules, external clients, or third-party services. Good interface design ensures clarity, loose coupling, and future scalability.

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🧭 1. Why Interface Design Matters

• Abstraction: Hides implementation details
• Encapsulation: Prevents misuse of internal logic
• Flexibility: Allows swapping implementations
• Testability: Simplifies mocking and testing

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🔌 2. API Interface Design (External Interfaces)

📦 RESTful API Design

• Use resource-oriented URIs: /users, /orders/{id}
• Choose appropriate HTTP verbs:
  • GET, POST, PUT, PATCH, DELETE
• Consistent status codes: 200, 201, 400, 404, 500
• Versioning: /v1/users

🧠 Semantic Rules

• Use nouns in URIs, not verbs
• Return consistent JSON response structures
• Example:

{
  "data": { "id": "123", "name": "Alice" },
  "errors": [],
  "meta": { "timestamp": "..." }
}

🧰 Tools & Practices

OpenAPI / Swagger for documentation

Use API gateways for routing, auth, rate-limiting

Design for backward compatibility

🔁 3. Internal Component Interfaces

👓 Clear Contracts

• Define input/output data structures
• Avoid leaking implementation details
• Prefer stable interfaces even if internals change

🧱 Patterns

• Interfaces in OOP (Java, C#, Go)
• Ports and Adapters (Hexagonal Architecture)
• Functional Interfaces (passing functions as arguments)

// Java Example
public interface PaymentProcessor {
    void charge(Customer customer, double amount);
}

// Go Example
type Storage interface {
    Save(data []byte) error
}

🧪 4. Designing for Testability

• Favor dependency injection
• Use mockable interfaces
• Avoid hidden state or tight coupling

🛠️ 5. Tools and Practices

• IDLs (Interface Definition Languages):
• gRPC / Protobuf for microservices
• Thrift or Avro for schema evolution
• JSON Schema or Zod (TypeScript) for validation
• Use interface segregation (keep interfaces small and focused)

🔐 6. Security & Stability

• Define clear error handling contracts
• Rate limiting and timeouts at the boundary
• Consider forward/backward compatibility of interfaces

🔄 7. Versioning Strategy

• APIs should evolve without breaking consumers
• Use URI-based versioning: /api/v1/...
• Or header-based versioning: Accept: application/vnd.company.v1+json

📋 8. Interface Review Checklist

• [ ] Does the interface abstract internal logic?
• [ ] Is the input/output well-documented and typed?
• [ ] Is it intuitive and semantically clear?
• [ ] Is it stable and backward compatible?
• [ ] Is it secure and limited in exposure?
• [ ] Can it be mocked/tested easily?

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