Introduction

A well-designed API is a joy to work with. A poorly designed one causes frustration, bugs, and wasted developer hours. Having built APIs consumed by mobile apps, third-party integrations, and internal services at Dr. Dangs Lab, I've learned what separates great APIs from mediocre ones.

RESTful URL Design

Resource Naming Conventions

Good: Nouns, plural, lowercase, hyphens for multi-word
GET    /users
GET    /users/123
GET    /users/123/orders
GET    /lab-results
GET    /medical-records
Bad: Verbs, singular, camelCase, underscores
GET    /getUsers
GET    /user/123
GET    /get_lab_results
POST   /createOrder

URL Structure Patterns

Collection
GET    /users                    # List all users
POST   /users                    # Create a user
Single resource
GET    /users/123               # Get user 123
PUT    /users/123               # Replace user 123
PATCH  /users/123               # Partial update user 123
DELETE /users/123               # Delete user 123
Nested resources (parent-child relationship)
GET    /users/123/orders        # User's orders
POST   /users/123/orders        # Create order for user
GET    /users/123/orders/456    # Specific order
Related resources (use query params for filtering)
GET    /orders?user_id=123      # Orders filtered by user
Actions (when CRUD doesn't fit)
POST   /orders/123/cancel       # Cancel an order
POST   /users/123/verify-email  # Trigger email verification

HTTP Methods and Status Codes

HTTP Methods

from fastapi import FastAPI, HTTPException, status
from pydantic import BaseModel
app = FastAPI()
class UserCreate(BaseModel):
    email: str
    name: str
class UserUpdate(BaseModel):
    name: str | None = None
    email: str | None = None
GET - Retrieve resource (idempotent, safe)
@app.get("/users/{user_id}")
async def get_user(user_id: int):
    user = await db.get_user(user_id)
    if not user:
        raise HTTPException(status_code=404, detail="User not found")
    return user
POST - Create resource (not idempotent)
@app.post("/users", status_code=status.HTTP_201_CREATED)
async def create_user(user: UserCreate):
    new_user = await db.create_user(user)
    return new_user
PUT - Replace entire resource (idempotent)
@app.put("/users/{user_id}")
async def replace_user(user_id: int, user: UserCreate):
    updated = await db.replace_user(user_id, user)
    if not updated:
        raise HTTPException(status_code=404, detail="User not found")
    return updated
PATCH - Partial update (idempotent)
@app.patch("/users/{user_id}")
async def update_user(user_id: int, user: UserUpdate):
    # Only update provided fields
    update_data = user.model_dump(exclude_unset=True)
    updated = await db.update_user(user_id, update_data)
    return updated
DELETE - Remove resource (idempotent)
@app.delete("/users/{user_id}", status_code=status.HTTP_204_NO_CONTENT)
async def delete_user(user_id: int):
    deleted = await db.delete_user(user_id)
    if not deleted:
        raise HTTPException(status_code=404, detail="User not found")

Status Codes Cheat Sheet

2xx Success
200 OK              # GET, PUT, PATCH successful
201 Created         # POST successful, resource created
204 No Content      # DELETE successful, no body returned
3xx Redirection
301 Moved Permanently  # Resource URL changed permanently
304 Not Modified       # Cached response still valid
4xx Client Errors
400 Bad Request     # Invalid request syntax/data
401 Unauthorized    # Authentication required
403 Forbidden       # Authenticated but not authorized
404 Not Found       # Resource doesn't exist
405 Method Not Allowed  # HTTP method not supported
409 Conflict        # Resource state conflict (e.g., duplicate)
422 Unprocessable Entity  # Validation failed
429 Too Many Requests    # Rate limit exceeded
5xx Server Errors
500 Internal Server Error  # Unexpected server error
502 Bad Gateway           # Upstream service error
503 Service Unavailable   # Server temporarily unavailable
504 Gateway Timeout       # Upstream service timeout

Request and Response Design

Consistent Response Format

from pydantic import BaseModel
from typing import Generic, TypeVar, List, Optional
from datetime import datetime
T = TypeVar('T')
class APIResponse(BaseModel, Generic[T]):
    success: bool
    data: T
    message: Optional[str] = None
    timestamp: datetime = datetime.utcnow()
class PaginatedResponse(BaseModel, Generic[T]):
    success: bool = True
    data: List[T]
    pagination: dict
    timestamp: datetime = datetime.utcnow()
Usage
@app.get("/users/{user_id}", response_model=APIResponse[User])
async def get_user(user_id: int):
    user = await db.get_user(user_id)
    return APIResponse(success=True, data=user)
Response
{
    "success": true,
    "data": {
        "id": 123,
        "email": "john@example.com",
        "name": "John Doe"
    },
    "message": null,
    "timestamp": "2024-12-12T10:30:00Z"
}

Error Response Format

from fastapi import Request
from fastapi.responses import JSONResponse
from pydantic import BaseModel
from typing import List, Optional
class ErrorDetail(BaseModel):
    field: Optional[str] = None
    message: str
    code: str
class ErrorResponse(BaseModel):
    success: bool = False
    error: str
    details: Optional[List[ErrorDetail]] = None
    request_id: str
    timestamp: datetime
@app.exception_handler(HTTPException)
async def http_exception_handler(request: Request, exc: HTTPException):
    return JSONResponse(
        status_code=exc.status_code,
        content={
            "success": False,
            "error": exc.detail,
            "request_id": request.state.request_id,
            "timestamp": datetime.utcnow().isoformat()
        }
    )
Validation error response
{
    "success": false,
    "error": "Validation failed",
    "details": [
        {
            "field": "email",
            "message": "Invalid email format",
            "code": "INVALID_EMAIL"
        },
        {
            "field": "age",
            "message": "Must be at least 18",
            "code": "MIN_VALUE"
        }
    ],
    "request_id": "req_abc123",
    "timestamp": "2024-12-12T10:30:00Z"
}

Pagination

Cursor-Based Pagination (Recommended)

from fastapi import Query
from typing import Optional
import base64
import json
@app.get("/users")
async def list_users(
    limit: int = Query(default=20, le=100),
    cursor: Optional[str] = None
):
    # Decode cursor
    if cursor:
        cursor_data = json.loads(base64.b64decode(cursor))
        after_id = cursor_data["id"]
        after_date = cursor_data["created_at"]
    else:
        after_id = 0
        after_date = None
    # Fetch data
    users = await db.get_users(
        limit=limit + 1,  # Fetch one extra to check if more exist
        after_id=after_id
    )
    has_more = len(users) > limit
    if has_more:
        users = users[:limit]
    # Create next cursor
    next_cursor = None
    if has_more and users:
        last_user = users[-1]
        cursor_data = {"id": last_user.id, "created_at": last_user.created_at.isoformat()}
        next_cursor = base64.b64encode(json.dumps(cursor_data).encode()).decode()
    return {
        "success": True,
        "data": users,
        "pagination": {
            "limit": limit,
            "has_more": has_more,
            "next_cursor": next_cursor
        }
    }

Offset-Based Pagination (Simple)

@app.get("/users")
async def list_users(
    page: int = Query(default=1, ge=1),
    per_page: int = Query(default=20, le=100)
):
    offset = (page - 1) * per_page
    users = await db.get_users(limit=per_page, offset=offset)
    total = await db.count_users()
    return {
        "success": True,
        "data": users,
        "pagination": {
            "page": page,
            "per_page": per_page,
            "total": total,
            "total_pages": (total + per_page - 1) // per_page,
            "has_next": page * per_page < total,
            "has_prev": page > 1
        }
    }

Filtering, Sorting, and Field Selection

@app.get("/orders")
async def list_orders(
    # Filtering
    status: Optional[str] = Query(None, description="Filter by status"),
    user_id: Optional[int] = Query(None, description="Filter by user"),
    min_amount: Optional[float] = Query(None, description="Minimum order amount"),
    max_amount: Optional[float] = Query(None, description="Maximum order amount"),
    created_after: Optional[datetime] = Query(None, description="Created after date"),
    # Sorting
    sort_by: str = Query("created_at", description="Field to sort by"),
    sort_order: str = Query("desc", regex="^(asc|desc)$"),
    # Field selection
    fields: Optional[str] = Query(None, description="Comma-separated fields to return"),
    # Pagination
    page: int = Query(1, ge=1),
    per_page: int = Query(20, le=100)
):
    # Build query with filters
    filters = {}
    if status:
        filters["status"] = status
    if user_id:
        filters["user_id"] = user_id
    if min_amount:
        filters["amount__gte"] = min_amount
    if max_amount:
        filters["amount__lte"] = max_amount
    # Parse fields
    selected_fields = fields.split(",") if fields else None
    orders = await db.get_orders(
        filters=filters,
        sort_by=sort_by,
        sort_order=sort_order,
        fields=selected_fields,
        limit=per_page,
        offset=(page - 1) * per_page
    )
    return {"success": True, "data": orders}
Usage examples:
GET /orders?status=pending&sort_by=amount&sort_order=desc
GET /orders?min_amount=100&max_amount=500&fields=id,status,amount
GET /orders?created_after=2024-01-01T00:00:00Z&user_id=123

API Versioning

from fastapi import APIRouter
URL versioning (most common)
v1_router = APIRouter(prefix="/api/v1")
v2_router = APIRouter(prefix="/api/v2")
@v1_router.get("/users/{user_id}")
async def get_user_v1(user_id: int):
    return {"id": user_id, "name": "John"}  # Old format
@v2_router.get("/users/{user_id}")
async def get_user_v2(user_id: int):
    return {
        "data": {"id": user_id, "name": "John"},
        "meta": {"version": "2.0"}
    }  # New format with envelope
app.include_router(v1_router)
app.include_router(v2_router)
Header versioning (alternative)
@app.get("/users/{user_id}")
async def get_user(user_id: int, api_version: str = Header(default="1.0")):
    if api_version == "2.0":
        return {"data": {...}, "meta": {...}}
    return {...}  # v1 format

Authentication and Security

from fastapi import Depends, Security
from fastapi.security import HTTPBearer, HTTPAuthorizationCredentials
import jwt
security = HTTPBearer()
async def get_current_user(
    credentials: HTTPAuthorizationCredentials = Security(security)
):
    token = credentials.credentials
    try:
        payload = jwt.decode(token, SECRET_KEY, algorithms=["HS256"])
        user_id = payload.get("sub")
        if not user_id:
            raise HTTPException(status_code=401, detail="Invalid token")
        return await db.get_user(user_id)
    except jwt.ExpiredSignatureError:
        raise HTTPException(status_code=401, detail="Token expired")
    except jwt.JWTError:
        raise HTTPException(status_code=401, detail="Invalid token")
Protected endpoint
@app.get("/users/me")
async def get_current_user_profile(user: User = Depends(get_current_user)):
    return user
Rate limiting
from slowapi import Limiter
from slowapi.util import get_remote_address
limiter = Limiter(key_func=get_remote_address)
@app.get("/api/resource")
@limiter.limit("100/minute")
async def rate_limited_endpoint(request: Request):
    return {"data": "This endpoint is rate limited"}

Documentation with OpenAPI

from fastapi import FastAPI
app = FastAPI(
    title="Healthcare API",
    description="API for managing patient data and lab results",
    version="2.0.0",
    contact={
        "name": "Tushar Agrawal",
        "email": "tusharagrawal0104@gmail.com"
    },
    license_info={
        "name": "MIT",
        "url": "https://opensource.org/licenses/MIT"
    }
)
@app.get(
    "/patients/{patient_id}",
    summary="Get patient by ID",
    description="Retrieve detailed information about a specific patient",
    response_description="Patient details",
    responses={
        200: {"description": "Patient found"},
        404: {"description": "Patient not found"},
        401: {"description": "Not authenticated"}
    },
    tags=["Patients"]
)
async def get_patient(patient_id: int):
    """
    Get a patient by their ID.
    - patient_id: The unique identifier of the patient
    """
    pass

Key Takeaways

1. Use nouns, not verbs in URLs - let HTTP methods convey the action 2. Be consistent with naming, casing, and response formats 3. Use proper status codes - they communicate intent clearly 4. Implement pagination - never return unbounded lists 5. Version your API - breaking changes will happen 6. Document thoroughly - OpenAPI/Swagger is your friend 7. Handle errors gracefully - descriptive errors help debugging 8. Secure by default - authentication, rate limiting, input validation

Conclusion

Great API design is about empathy for the developers who will use it. Follow conventions, be consistent, document well, and handle edge cases gracefully. Your future self (and your API consumers) will thank you.

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