API Design Best Practices for Interviews

API Design Best Practices for Technical Interviews

API design is tested in system design interviews, backend interviews, and product engineering rounds. According to Postman's 2025 State of APIs Report, 89% of developers consider API design quality a top factor in software architecture decisions. Understanding REST, GraphQL, and gRPC trade-offs is essential.

Well-designed APIs follow consistent naming, versioning, and error handling conventions that make systems maintainable at scale.

REST API Design Principles

• Resource-based URLs — Use nouns, not verbs: /users/{id} not /getUser
• HTTP Methods — GET (read), POST (create), PUT (full update), PATCH (partial update), DELETE
• Status Codes — 200 OK, 201 Created, 400 Bad Request, 401 Unauthorized, 404 Not Found, 429 Too Many Requests, 500 Server Error
• Pagination — Cursor-based (scalable) vs offset-based (simple). Cursor-based is preferred for large datasets
• Versioning — URL path (/v2/users) or header (Accept: application/vnd.api.v2+json)
• Rate Limiting — Token bucket or sliding window. See rate limiting system design

REST vs GraphQL vs gRPC

Feature	REST	GraphQL	gRPC
Protocol	HTTP/JSON	HTTP/JSON	HTTP/2 + Protobuf
Flexibility	Fixed endpoints	Client-defined queries	Strict contract
Performance	Good	Good (no over-fetching)	Excellent (binary)
Use Case	Public APIs, CRUD	Complex data graphs	Internal microservices

Authentication & Security

Every API must implement authentication. The three patterns interviewers expect you to know:

1. JWT (JSON Web Tokens) — Stateless, self-contained. Best for microservices. Expires in 15-60 minutes
2. OAuth 2.0 — Delegated authorization for third-party access. Authorization code flow is the most secure
3. API Keys — Simple but less secure. Use for server-to-server communication only

For real-time API design guidance during interviews, use AissenceAI's desktop app with its Private private overlay.