API Design Principles

API Design Principles Skill

Practical reference for consistent, production-ready API design decisions.

URL & Method Conventions

✅ Plural nouns, kebab-case, resource hierarchy max 2 levels
   GET    /users/{id}
   POST   /orders
   PATCH  /orders/{id}
   DELETE /orders/{id}
   POST   /orders/{id}/cancel      ← actions as sub-resource verbs

❌ Verbs in base path
   POST /createOrder
   GET  /getUser?id=123

Standard Error Contract

Every error must follow the same shape — never return raw exception messages.

{
  "error": {
    "code": "ORDER_NOT_FOUND",
    "message": "Order ord_123 not found",
    "details": [{ "field": "items[0].quantity", "issue": "must be > 0" }],
    "request_id": "req_abc123"
  }
}

# FastAPI
raise HTTPException(
    status_code=404,
    detail={"code": "ORDER_NOT_FOUND", "message": f"Order {id} not found",
            "request_id": request.state.request_id}
)

Pagination

Cursor-based — preferred for live/large datasets:

class PaginatedResponse(BaseModel, Generic[T]):
    items: List[T]
    next_cursor: Optional[str] = None   # base64-encoded, opaque to client

def encode_cursor(last_id: int) -> str:
    return base64.b64encode(str(last_id).encode()).decode()

Offset-based — only for small static datasets where total count is cheap.

Versioning

URL versioning for breaking changes:   /api/v1/orders → /api/v2/orders
Header for minor variations:           Accept: application/vnd.myapi.v2+json

Rules:
- v1 stays alive ≥ 6 months after v2 launch
- Deprecated: return  Deprecation: true  +  Sunset: <date>  headers
- Never remove a field without a major version bump

Idempotency (POST mutations)

@router.post("/orders", status_code=201)
async def create_order(
    body: CreateOrderRequest,
    idempotency_key: Optional[str] = Header(None, alias="X-Idempotency-Key"),
):
    if idempotency_key:
        cached = await redis.get(f"idempotency:{idempotency_key}")
        if cached:
            return JSONResponse(json.loads(cached), status_code=200)

    order = await order_service.create(body)

    if idempotency_key:
        await redis.setex(f"idempotency:{idempotency_key}", 86400, order.model_dump_json())
    return order

Security Checklist

• Auth middleware applied before handler — never inside handler
• Ownership check: if resource.owner_id != current_user.id: raise 403
• All inputs validated via Pydantic/Zod before use
• Rate limiting on public + auth endpoints
• No secrets or PII in URL parameters
• CORS: no allow_origins=["*"] in production
• Response never leaks internal IDs, stack traces, or system paths

Response Design Checklist

• Consistent envelope: { data: T } or flat — choose one, never mix
• Dates always ISO 8601 with timezone: 2024-03-15T14:30:00Z
• Monetary values: string decimal or integer cents — never float
• Nullable fields explicit in schema (not absent)
• List responses always return array, never null when empty