How to Approach Any API Design Interview

This closing module works through four full, unfamiliar prompts end to end - the API design equivalent of LLD Fundamentals' mock interview capstone. Before diving in, it's worth deliberately re-grounding in the framework from this course's very first module, since a strong answer to any of these prompts depends on applying it consistently, not on having memorized these four specific scenarios.

• -Clarify use cases and clients - who calls this API, and for what?
• -Model the resources or schema - what are the core nouns and their relationships?
• -Choose the protocol - REST, GraphQL, or gRPC, justified by the client landscape.
• -Design the contract - endpoints/methods, request/response shapes, status/error handling, auth boundaries.
• -Discuss evolution - how does this API change without breaking existing clients?

Every earlier chapter in this course isolated one specific technique - pagination, idempotency, streaming, circuit breakers - against a small, focused example. A real system design prompt is a mixing problem: "design a payment API" needs idempotency AND webhooks AND careful error handling AND versioning, combined and prioritized correctly against each other in one 45-minute answer, not addressed as isolated topics one at a time.

• -Design a Payment API - idempotency, webhooks, PCI-relevant design constraints, and versioning combined in a domain where correctness genuinely matters most.
• -Design a Chat API - a real-time, WebSocket-shaped protocol choice, presence, delivery guarantees, and read receipts.
• -Design a Food Delivery API - geospatial queries, real-time order tracking, and a multi-actor state machine (customer, restaurant, driver).
• -The Grand Unified API - a capstone combining REST, GraphQL, and gRPC deliberately in one system, mirroring the API Design Lab's own final level.