API Design and Architecture: A Complete Developer's Guide to Building Scalable APIs

API design is the foundation of modern software architecture. Whether you're building microservices, mobile applications, or integrating third-party services, well-designed APIs determine the success of your software ecosystem. This comprehensive guide covers everything developers need to know about API design and architecture, from fundamental principles to advanced patterns.

Key Takeaways:

• Learn proven API design principles that scale
• Master REST, GraphQL, and modern API patterns
• Implement security, versioning, and documentation best practices
• Build APIs that developers love to use

What is API Design?

API design is the process of creating application programming interfaces that are intuitive, maintainable, and scalable. It involves defining endpoints, request/response formats, authentication mechanisms, and error handling strategies before writing implementation code.

Why API Design Matters

Good API design delivers:

• Developer Experience (DX): Intuitive interfaces that reduce integration time
• Maintainability: Clear contracts that evolve without breaking changes
• Performance: Efficient data transfer and minimal over-fetching
• Security: Robust authentication and authorization mechanisms
• Scalability: Architecture that handles growth gracefully

Design-First vs Code-First Approach

Design-First (Recommended):

# OpenAPI 3.0 Specification Example
openapi: 3.0.0
info:
  title: User Management API
  version: 1.0.0
  description: API for managing user resources

paths:
  /api/v1/users:
    get:
      summary: Retrieve all users
      parameters:
        - name: page
          in: query
          schema:
            type: integer
            default: 1
        - name: limit
          in: query
          schema:
            type: integer
            default: 20
      responses:
        "200":
          description: Successful response
          content:
            application/json:
              schema:
                type: object
                properties:
                  data:
                    type: array
                    items:
                      $ref: "#/components/schemas/User"
                  pagination:
                    $ref: "#/components/schemas/Pagination"

components:
  schemas:
    User:
      type: object
      required:
        - id
        - email
        - username
      properties:
        id:
          type: string
          format: uuid
        email:
          type: string
          format: email
        username:
          type: string
        createdAt:
          type: string
          format: date-time

Benefits of Design-First:

• Early validation and stakeholder feedback
• Automatic documentation generation
• Mock server creation before implementation
• Contract testing between frontend and backend teams

API Design Principles

1. Consistency

Maintain consistent naming conventions, response structures, and error formats across all endpoints.

// Good: Consistent naming and structure
GET    /api/v1/users
POST   /api/v1/users
GET    /api/v1/users/{id}
PUT    /api/v1/users/{id}
DELETE /api/v1/users/{id}

GET    /api/v1/products
POST   /api/v1/products
GET    /api/v1/products/{id}
PUT    /api/v1/products/{id}
DELETE /api/v1/products/{id}

// Bad: Inconsistent naming
GET    /api/v1/users
POST   /api/v1/createUser
GET    /api/v1/user/{id}
PUT    /api/v1/updateUser/{id}
DELETE /api/v1/user/delete/{id}

2. Simplicity and Intuitive Design

APIs should be self-explanatory. Developers should understand endpoints without extensive documentation.

// Express.js Example: Simple, Intuitive Routes
const express = require("express");
const router = express.Router();

// Clear, self-documenting endpoints
router.get("/users", async (req, res) => {
  const { page = 1, limit = 20, search = "" } = req.query;

  try {
    const users = await User.find({
      username: new RegExp(search, "i"),
    })
      .limit(limit)
      .skip((page - 1) * limit)
      .select("-password");

    const total = await User.countDocuments();

    res.json({
      data: users,
      pagination: {
        page: parseInt(page),
        limit: parseInt(limit),
        total,
        pages: Math.ceil(total / limit),
      },
    });
  } catch (error) {
    res.status(500).json({
      error: "Internal server error",
      message: error.message,
    });
  }
});

module.exports = router;

3. Resource-Oriented Design

Design APIs around resources (nouns) rather than actions (verbs).

# FastAPI Example: Resource-Oriented Design
from fastapi import FastAPI, HTTPException, Query
from pydantic import BaseModel, EmailStr
from typing import List, Optional
from datetime import datetime
from uuid import UUID, uuid4

app = FastAPI()

class User(BaseModel):
    id: UUID
    email: EmailStr
    username: str
    created_at: datetime

class UserCreate(BaseModel):
    email: EmailStr
    username: str
    password: str

class UserUpdate(BaseModel):
    email: Optional[EmailStr] = None
    username: Optional[str] = None

# Resource: Users
@app.get("/api/v1/users", response_model=List[User])
async def list_users(
    page: int = Query(1, ge=1),
    limit: int = Query(20, ge=1, le=100)
):
    """Retrieve paginated list of users"""
    # Implementation
    return users

@app.post("/api/v1/users", response_model=User, status_code=201)
async def create_user(user: UserCreate):
    """Create a new user"""
    # Implementation
    return new_user

@app.get("/api/v1/users/{user_id}", response_model=User)
async def get_user(user_id: UUID):
    """Retrieve a specific user by ID"""
    # Implementation
    if not user:
        raise HTTPException(status_code=404, detail="User not found")
    return user

@app.put("/api/v1/users/{user_id}", response_model=User)
async def update_user(user_id: UUID, user_update: UserUpdate):
    """Update a user's information"""
    # Implementation
    return updated_user

@app.delete("/api/v1/users/{user_id}", status_code=204)
async def delete_user(user_id: UUID):
    """Delete a user"""
    # Implementation
    return None

4. Proper HTTP Method Usage

Use HTTP methods according to their semantic meaning:

// Go (Gin Framework) Example: Proper HTTP Methods
package main

import (
    "net/http"
    "github.com/gin-gonic/gin"
    "github.com/google/uuid"
)

type User struct {
    ID        string    `json:"id"`
    Email     string    `json:"email" binding:"required,email"`
    Username  string    `json:"username" binding:"required"`
    CreatedAt time.Time `json:"created_at"`
}

func main() {
    router := gin.Default()

    v1 := router.Group("/api/v1")
    {
        users := v1.Group("/users")
        {
            // Safe, Idempotent - retrieves without side effects
            users.GET("", listUsers)
            users.GET("/:id", getUser)

            // Non-idempotent - creates new resource each time
            users.POST("", createUser)

            // Idempotent - replaces entire resource
            users.PUT("/:id", replaceUser)

            // Partial update
            users.PATCH("/:id", updateUser)

            // Idempotent - multiple deletions have same effect
            users.DELETE("/:id", deleteUser)
        }
    }

    router.Run(":8080")
}

func createUser(c *gin.Context) {
    var user User

    if err := c.ShouldBindJSON(&user); err != nil {
        c.JSON(http.StatusBadRequest, gin.H{
            "error": "Validation failed",
            "details": err.Error(),
        })
        return
    }

    user.ID = uuid.New().String()
    user.CreatedAt = time.Now()

    // Save to database

    c.JSON(http.StatusCreated, user)
}

func updateUser(c *gin.Context) {
    id := c.Param("id")

    var updates map[string]interface{}
    if err := c.ShouldBindJSON(&updates); err != nil {
        c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
        return
    }

    // Apply partial updates

    c.JSON(http.StatusOK, updatedUser)
}

5. Meaningful Status Codes

Return appropriate HTTP status codes that clearly communicate the result.

// TypeScript (Express) Example: Comprehensive Status Codes
import express, { Request, Response, NextFunction } from "express";

const app = express();

// Success Responses
app.get("/api/v1/users", async (req: Request, res: Response) => {
  const users = await fetchUsers();
  res.status(200).json({ data: users }); // 200 OK
});

app.post("/api/v1/users", async (req: Request, res: Response) => {
  const user = await createUser(req.body);
  res.status(201).json({ data: user }); // 201 Created
});

app.delete("/api/v1/users/:id", async (req: Request, res: Response) => {
  await deleteUser(req.params.id);
  res.status(204).send(); // 204 No Content
});

// Client Error Responses
app.get("/api/v1/users/:id", async (req: Request, res: Response) => {
  const user = await findUser(req.params.id);

  if (!user) {
    return res.status(404).json({
      // 404 Not Found
      error: "User not found",
      code: "USER_NOT_FOUND",
    });
  }

  res.status(200).json({ data: user });
});

app.post("/api/v1/users", async (req: Request, res: Response) => {
  const { email, username, password } = req.body;

  // Validation error
  if (!email || !username || !password) {
    return res.status(400).json({
      // 400 Bad Request
      error: "Validation failed",
      code: "VALIDATION_ERROR",
      details: {
        email: !email ? "Email is required" : null,
        username: !username ? "Username is required" : null,
        password: !password ? "Password is required" : null,
      },
    });
  }

  // Conflict - user already exists
  const existingUser = await findUserByEmail(email);
  if (existingUser) {
    return res.status(409).json({
      // 409 Conflict
      error: "User with this email already exists",
      code: "USER_EXISTS",
    });
  }

  const user = await createUser({ email, username, password });
  res.status(201).json({ data: user });
});

// Authorization and Authentication
app.get(
  "/api/v1/admin/users",
  authenticateToken,
  async (req: Request, res: Response) => {
    if (!req.user) {
      return res.status(401).json({
        // 401 Unauthorized
        error: "Authentication required",
        code: "AUTH_REQUIRED",
      });
    }

    if (!req.user.isAdmin) {
      return res.status(403).json({
        // 403 Forbidden
        error: "Insufficient permissions",
        code: "FORBIDDEN",
      });
    }

    const users = await fetchAllUsers();
    res.status(200).json({ data: users });
  }
);

// Server Errors
app.use((err: Error, req: Request, res: Response, next: NextFunction) => {
  console.error(err.stack);

  res.status(500).json({
    // 500 Internal Server Error
    error: "An unexpected error occurred",
    code: "INTERNAL_ERROR",
    ...(process.env.NODE_ENV === "development" && {
      message: err.message,
      stack: err.stack,
    }),
  });
});

// Rate Limiting
app.use((req: Request, res: Response, next: NextFunction) => {
  if (rateLimitExceeded(req)) {
    return res.status(429).json({
      // 429 Too Many Requests
      error: "Rate limit exceeded",
      code: "RATE_LIMIT_EXCEEDED",
      retryAfter: 60,
    });
  }
  next();
});

Common HTTP Status Codes Reference

2xx Success:

• 200 OK - Request succeeded
• 201 Created - Resource created successfully
• 204 No Content - Success with no response body

4xx Client Errors:

• 400 Bad Request - Invalid request format
• 401 Unauthorized - Authentication required
• 403 Forbidden - Insufficient permissions
• 404 Not Found - Resource doesn't exist
• 409 Conflict - Resource conflict (e.g., duplicate)
• 422 Unprocessable Entity - Validation errors
• 429 Too Many Requests - Rate limit exceeded

5xx Server Errors:

• 500 Internal Server Error - Unexpected server error
• 502 Bad Gateway - Invalid upstream response
• 503 Service Unavailable - Server temporarily unavailable

API Architecture Patterns

1. RESTful Architecture

REST (Representational State Transfer) remains the most popular API architecture pattern.

REST Constraints:

1. Client-Server separation
2. Stateless communication
3. Cacheable responses
4. Uniform interface
5. Layered system
6. Code on demand (optional)

// Spring Boot REST API Example
@RestController
@RequestMapping("/api/v1/products")
@Validated
public class ProductController {

    @Autowired
    private ProductService productService;

    @GetMapping
    public ResponseEntity<PagedResponse<ProductDTO>> getProducts(
        @RequestParam(defaultValue = "0") int page,
        @RequestParam(defaultValue = "20") int size,
        @RequestParam(required = false) String category,
        @RequestParam(required = false) BigDecimal minPrice,
        @RequestParam(required = false) BigDecimal maxPrice
    ) {
        ProductFilterDTO filter = ProductFilterDTO.builder()
            .category(category)
            .minPrice(minPrice)
            .maxPrice(maxPrice)
            .build();

        Page<Product> products = productService.findAll(filter,
            PageRequest.of(page, size, Sort.by("createdAt").descending()));

        List<ProductDTO> productDTOs = products.getContent()
            .stream()
            .map(this::convertToDTO)
            .collect(Collectors.toList());

        PagedResponse<ProductDTO> response = new PagedResponse<>(
            productDTOs,
            products.getNumber(),
            products.getSize(),
            products.getTotalElements(),
            products.getTotalPages(),
            products.isLast()
        );

        return ResponseEntity.ok()
            .cacheControl(CacheControl.maxAge(5, TimeUnit.MINUTES))
            .body(response);
    }

    @GetMapping("/{id}")
    public ResponseEntity<ProductDTO> getProduct(@PathVariable UUID id) {
        Product product = productService.findById(id)
            .orElseThrow(() -> new ResourceNotFoundException("Product not found"));

        return ResponseEntity.ok()
            .eTag(String.valueOf(product.getVersion()))
            .body(convertToDTO(product));
    }

    @PostMapping
    @PreAuthorize("hasRole('ADMIN')")
    public ResponseEntity<ProductDTO> createProduct(
        @Valid @RequestBody ProductCreateDTO productDTO
    ) {
        Product product = productService.create(convertToEntity(productDTO));
        URI location = ServletUriComponentsBuilder
            .fromCurrentRequest()
            .path("/{id}")
            .buildAndExpand(product.getId())
            .toUri();

        return ResponseEntity.created(location)
            .body(convertToDTO(product));
    }

    @PutMapping("/{id}")
    @PreAuthorize("hasRole('ADMIN')")
    public ResponseEntity<ProductDTO> updateProduct(
        @PathVariable UUID id,
        @Valid @RequestBody ProductUpdateDTO productDTO,
        @RequestHeader("If-Match") String ifMatch
    ) {
        Product product = productService.update(id, productDTO, ifMatch);
        return ResponseEntity.ok()
            .eTag(String.valueOf(product.getVersion()))
            .body(convertToDTO(product));
    }

    @DeleteMapping("/{id}")
    @PreAuthorize("hasRole('ADMIN')")
    public ResponseEntity<Void> deleteProduct(@PathVariable UUID id) {
        productService.delete(id);
        return ResponseEntity.noContent().build();
    }

    private ProductDTO convertToDTO(Product product) {
        return modelMapper.map(product, ProductDTO.class);
    }

    private Product convertToEntity(ProductCreateDTO dto) {
        return modelMapper.map(dto, Product.class);
    }
}

2. GraphQL Architecture

GraphQL offers flexible querying, allowing clients to request exactly the data they need.

// GraphQL Schema Definition
const { ApolloServer, gql } = require("apollo-server-express");

const typeDefs = gql`
  type User {
    id: ID!
    email: String!
    username: String!
    posts: [Post!]!
    followers: [User!]!
    following: [User!]!
    createdAt: DateTime!
  }

  type Post {
    id: ID!
    title: String!
    content: String!
    author: User!
    comments: [Comment!]!
    likes: Int!
    createdAt: DateTime!
    updatedAt: DateTime!
  }

  type Comment {
    id: ID!
    content: String!
    author: User!
    post: Post!
    createdAt: DateTime!
  }

  type Query {
    user(id: ID!): User
    users(limit: Int = 10, offset: Int = 0): [User!]!
    post(id: ID!): Post
    posts(authorId: ID, limit: Int = 10, offset: Int = 0): [Post!]!
    searchPosts(query: String!): [Post!]!
  }

  type Mutation {
    createUser(email: String!, username: String!, password: String!): User!
    updateUser(id: ID!, email: String, username: String): User!
    deleteUser(id: ID!): Boolean!

    createPost(title: String!, content: String!): Post!
    updatePost(id: ID!, title: String, content: String): Post!
    deletePost(id: ID!): Boolean!

    createComment(postId: ID!, content: String!): Comment!
    likePost(postId: ID!): Post!
  }

  type Subscription {
    postCreated(authorId: ID): Post!
    commentAdded(postId: ID!): Comment!
  }

  scalar DateTime
`;

const resolvers = {
  Query: {
    user: async (_, { id }, { dataSources }) => {
      return await dataSources.userAPI.getUserById(id);
    },

    posts: async (_, { authorId, limit, offset }, { dataSources }) => {
      return await dataSources.postAPI.getPosts({ authorId, limit, offset });
    },

    searchPosts: async (_, { query }, { dataSources }) => {
      return await dataSources.postAPI.searchPosts(query);
    },
  },

  Mutation: {
    createUser: async (_, { email, username, password }, { dataSources }) => {
      return await dataSources.userAPI.createUser({
        email,
        username,
        password,
      });
    },

    createPost: async (_, { title, content }, { user, dataSources }) => {
      if (!user) throw new Error("Authentication required");
      return await dataSources.postAPI.createPost({
        title,
        content,
        authorId: user.id,
      });
    },

    likePost: async (_, { postId }, { user, dataSources }) => {
      if (!user) throw new Error("Authentication required");
      return await dataSources.postAPI.likePost(postId, user.id);
    },
  },

  User: {
    posts: async (user, _, { dataSources }) => {
      return await dataSources.postAPI.getPostsByAuthor(user.id);
    },

    followers: async (user, _, { dataSources }) => {
      return await dataSources.userAPI.getFollowers(user.id);
    },
  },

  Post: {
    author: async (post, _, { dataSources }) => {
      return await dataSources.userAPI.getUserById(post.authorId);
    },

    comments: async (post, _, { dataSources }) => {
      return await dataSources.commentAPI.getCommentsByPost(post.id);
    },
  },
};

// DataLoader for N+1 problem prevention
const DataLoader = require("dataloader");

class UserAPI {
  constructor() {
    this.loader = new DataLoader(async (ids) => {
      const users = await User.find({ _id: { $in: ids } });
      return ids.map((id) => users.find((user) => user.id === id));
    });
  }

  async getUserById(id) {
    return this.loader.load(id);
  }
}

3. Event-Driven Architecture

For real-time updates and asynchronous processing.

# Python (FastAPI + WebSockets) Event-Driven API
from fastapi import FastAPI, WebSocket, WebSocketDisconnect
from typing import List, Dict
import asyncio
import json
from datetime import datetime

app = FastAPI()

class ConnectionManager:
    def __init__(self):
        self.active_connections: Dict[str, List[WebSocket]] = {}

    async def connect(self, websocket: WebSocket, channel: str):
        await websocket.accept()
        if channel not in self.active_connections:
            self.active_connections[channel] = []
        self.active_connections[channel].append(websocket)

    def disconnect(self, websocket: WebSocket, channel: str):
        self.active_connections[channel].remove(websocket)

    async def broadcast(self, message: dict, channel: str):
        if channel in self.active_connections:
            for connection in self.active_connections[channel]:
                await connection.send_json(message)

manager = ConnectionManager()

@app.websocket("/ws/notifications/{user_id}")
async def websocket_endpoint(websocket: WebSocket, user_id: str):
    channel = f"user:{user_id}"
    await manager.connect(websocket, channel)

    try:
        while True:
            data = await websocket.receive_text()
            # Process incoming messages if needed

    except WebSocketDisconnect:
        manager.disconnect(websocket, channel)

@app.post("/api/v1/events/publish")
async def publish_event(event: dict):
    """Publish event to specific channel"""
    channel = event.get('channel')
    message = {
        'type': event.get('type'),
        'data': event.get('data'),
        'timestamp': datetime.utcnow().isoformat()
    }

    await manager.broadcast(message, channel)
    return {'status': 'published'}

# Message Queue Integration (RabbitMQ/Redis)
import aio_pika
import asyncio

async def process_events():
    connection = await aio_pika.connect_robust("amqp://guest:guest@localhost/")

    async with connection:
        channel = await connection.channel()
        queue = await channel.declare_queue('events', durable=True)

        async with queue.iterator() as queue_iter:
            async for message in queue_iter:
                async with message.process():
                    event_data = json.loads(message.body)

                    # Broadcast to WebSocket clients
                    await manager.broadcast(
                        event_data,
                        event_data.get('channel')
                    )

RESTful API Design Best Practices

1. Pagination

Always paginate list endpoints to prevent performance issues.

# Ruby on Rails API Pagination Example
class Api::V1::UsersController < ApplicationController
  def index
    page = params[:page]&.to_i || 1
    per_page = [params[:per_page]&.to_i || 20, 100].min

    users = User.page(page).per(per_page)

    render json: {
      data: users.map { |user| UserSerializer.new(user).as_json },
      pagination: {
        current_page: users.current_page,
        per_page: users.limit_value,
        total_pages: users.total_pages,
        total_count: users.total_count,
        next_page: users.next_page,
        prev_page: users.prev_page
      },
      links: {
        self: api_v1_users_url(page: page, per_page: per_page),
        first: api_v1_users_url(page: 1, per_page: per_page),
        last: api_v1_users_url(page: users.total_pages, per_page: per_page),
        next: users.next_page ? api_v1_users_url(page: users.next_page, per_page: per_page) : nil,
        prev: users.prev_page ? api_v1_users_url(page: users.prev_page, per_page: per_page) : nil
      }
    }
  end
end

2. Filtering, Sorting, and Searching

// Node.js Advanced Query Builder
const express = require("express");
const router = express.Router();

router.get("/api/v1/products", async (req, res) => {
  const {
    // Filtering
    category,
    minPrice,
    maxPrice,
    inStock,

    // Searching
    search,

    // Sorting
    sortBy = "createdAt",
    order = "desc",

    // Pagination
    page = 1,
    limit = 20,
  } = req.query;

  // Build query
  let query = Product.find();

  // Apply filters
  if (category) {
    query = query.where("category").equals(category);
  }

  if (minPrice || maxPrice) {
    query = query.where("price");
    if (minPrice) query = query.gte(parseFloat(minPrice));
    if (maxPrice) query = query.lte(parseFloat(maxPrice));
  }

  if (inStock !== undefined) {
    query = query.where("stock").gt(0);
  }

  // Apply search
  if (search) {
    query = query.where({
      $or: [
        { name: new RegExp(search, "i") },
        { description: new RegExp(search, "i") },
        { sku: new RegExp(search, "i") },
      ],
    });
  }

  // Apply sorting
  const sortOrder = order === "desc" ? "-" : "";
  query = query.sort(`${sortOrder}${sortBy}`);

  // Apply pagination
  const skip = (page - 1) * limit;
  query = query.skip(skip).limit(parseInt(limit));

  // Execute query
  const products = await query.exec();
  const total = await Product.countDocuments(query.getFilter());

  res.json({
    data: products,
    pagination: {
      page: parseInt(page),
      limit: parseInt(limit),
      total,
      pages: Math.ceil(total / limit),
    },
    filters: {
      category,
      minPrice,
      maxPrice,
      inStock,
      search,
    },
  });
});

module.exports = router;

3. HATEOAS (Hypermedia as the Engine of Application State)

Include links to related resources in responses.

// C# ASP.NET Core HATEOAS Implementation
[ApiController]
[Route("api/v1/[controller]")]
public class OrdersController : ControllerBase
{
    [HttpGet("{id}")]
    public async Task<ActionResult<OrderResponse>> GetOrder(Guid id)
    {
        var order = await _orderService.GetOrderAsync(id);

        if (order == null)
            return NotFound();

        var response = new OrderResponse
        {
            Id = order.Id,
            CustomerId = order.CustomerId,
            Total = order.Total,
            Status = order.Status,
            CreatedAt = order.CreatedAt,

            // HATEOAS Links
            Links = new List<Link>
            {
                new Link(
                    Url.Action(nameof(GetOrder), new { id = order.Id }),
                    "self",
                    "GET"
                ),
                new Link(
                    Url.Action(nameof(UpdateOrder), new { id = order.Id }),
                    "update",
                    "PUT"
                ),
                new Link(
                    Url.Action("GetCustomer", "Customers", new { id = order.CustomerId }),
                    "customer",
                    "GET"
                ),
                new Link(
                    Url.Action("GetOrderItems", new { orderId = order.Id }),
                    "items",
                    "GET"
                )
            }
        };

        // Conditional links based on order status
        if (order.Status == OrderStatus.Pending)
        {
            response.Links.Add(new Link(
                Url.Action(nameof(CancelOrder), new { id = order.Id }),
                "cancel",
                "POST"
            ));
        }

        if (order.Status == OrderStatus.Paid)
        {
            response.Links.Add(new Link(
                Url.Action(nameof(ShipOrder), new { id = order.Id }),
                "ship",
                "POST"
            ));
        }

        return Ok(response);
    }
}

public class Link
{
    public string Href { get; set; }
    public string Rel { get; set; }
    public string Method { get; set; }

    public Link(string href, string rel, string method)
    {
        Href = href;
        Rel = rel;
        Method = method;
    }
}

API Versioning Strategies

1. URI Versioning (Recommended)

GET /api/v1/users
GET /api/v2/users

2. Header Versioning

GET /api/users
Accept: application/vnd.myapi.v1+json

3. Query Parameter Versioning

GET /api/users?version=1

Implementation Example:

// PHP Laravel API Versioning
// routes/api.php
Route::prefix('v1')->group(function () {
    Route::apiResource('users', 'Api\V1\UserController');
    Route::apiResource('products', 'Api\V1\ProductController');
});

Route::prefix('v2')->group(function () {
    Route::apiResource('users', 'Api\V2\UserController');
    Route::apiResource('products', 'Api\V2\ProductController');
});

// app/Http/Controllers/Api/V1/UserController.php
namespace App\Http\Controllers\Api\V1;

class UserController extends Controller
{
    public function index()
    {
        // V1 implementation
        return response()->json([
            'data' => User::all(),
            'version' => '1.0'
        ]);
    }
}

// app/Http/Controllers/Api/V2/UserController.php
namespace App\Http\Controllers\Api\V2;

class UserController extends Controller
{
    public function index()
    {
        // V2 implementation with enhanced features
        return response()->json([
            'data' => User::with('profile')->get(),
            'version' => '2.0',
            'features' => ['includes_profile', 'enhanced_pagination']
        ]);
    }
}

API Security and Authentication

1. JWT Authentication

// JWT Authentication Middleware (Node.js)
const jwt = require("jsonwebtoken");
const { promisify } = require("util");

const signToken = (payload) => {
  return jwt.sign(payload, process.env.JWT_SECRET, {
    expiresIn: process.env.JWT_EXPIRES_IN || "1h",
  });
};

const verifyToken = promisify(jwt.verify);

// Authentication Middleware
const authenticate = async (req, res, next) => {
  try {
    // 1. Get token from header
    const authHeader = req.headers.authorization;

    if (!authHeader || !authHeader.startsWith("Bearer ")) {
      return res.status(401).json({
        error: "No token provided",
        code: "NO_TOKEN",
      });
    }

    const token = authHeader.split(" ")[1];

    // 2. Verify token
    const decoded = await verifyToken(token, process.env.JWT_SECRET);

    // 3. Check if user still exists
    const user = await User.findById(decoded.id);

    if (!user) {
      return res.status(401).json({
        error: "User no longer exists",
        code: "USER_NOT_FOUND",
      });
    }

    // 4. Check if user changed password after token was issued
    if (user.passwordChangedAfter(decoded.iat)) {
      return res.status(401).json({
        error: "Password recently changed. Please log in again.",
        code: "PASSWORD_CHANGED",
      });
    }

    // 5. Grant access
    req.user = user;
    next();
  } catch (error) {
    if (error.name === "JsonWebTokenError") {
      return res.status(401).json({
        error: "Invalid token",
        code: "INVALID_TOKEN",
      });
    }

    if (error.name === "TokenExpiredError") {
      return res.status(401).json({
        error: "Token expired",
        code: "TOKEN_EXPIRED",
      });
    }

    res.status(500).json({
      error: "Authentication failed",
      code: "AUTH_ERROR",
    });
  }
};

// Login endpoint
router.post("/auth/login", async (req, res) => {
  const { email, password } = req.body;

  // 1. Check if email and password exist
  if (!email || !password) {
    return res.status(400).json({
      error: "Please provide email and password",
      code: "MISSING_CREDENTIALS",
    });
  }

  // 2. Check if user exists && password is correct
  const user = await User.findOne({ email }).select("+password");

  if (!user || !(await user.correctPassword(password))) {
    return res.status(401).json({
      error: "Incorrect email or password",
      code: "INVALID_CREDENTIALS",
    });
  }

  // 3. Generate token
  const token = signToken({ id: user._id });

  res.json({
    token,
    user: {
      id: user._id,
      email: user.email,
      username: user.username,
    },
  });
});

module.exports = { authenticate, signToken };

2. OAuth 2.0 Implementation

# Python FastAPI OAuth2 Implementation
from fastapi import FastAPI, Depends, HTTPException, status
from fastapi.security import OAuth2PasswordBearer, OAuth2PasswordRequestForm
from jose import JWTError, jwt
from passlib.context import CryptContext
from datetime import datetime, timedelta
from typing import Optional

app = FastAPI()

# Configuration
SECRET_KEY = "your-secret-key-here"
ALGORITHM = "HS256"
ACCESS_TOKEN_EXPIRE_MINUTES = 30

pwd_context = CryptContext(schemes=["bcrypt"], deprecated="auto")
oauth2_scheme = OAuth2PasswordBearer(tokenUrl="token")

class TokenData:
    username: Optional[str] = None

def verify_password(plain_password, hashed_password):
    return pwd_context.verify(plain_password, hashed_password)

def get_password_hash(password):
    return pwd_context.hash(password)

def create_access_token(data: dict, expires_delta: Optional[timedelta] = None):
    to_encode = data.copy()

    if expires_delta:
        expire = datetime.utcnow() + expires_delta
    else:
        expire = datetime.utcnow() + timedelta(minutes=15)

    to_encode.update({"exp": expire})
    encoded_jwt = jwt.encode(to_encode, SECRET_KEY, algorithm=ALGORITHM)

    return encoded_jwt

async def get_current_user(token: str = Depends(oauth2_scheme)):
    credentials_exception = HTTPException(
        status_code=status.HTTP_401_UNAUTHORIZED,
        detail="Could not validate credentials",
        headers={"WWW-Authenticate": "Bearer"},
    )

    try:
        payload = jwt.decode(token, SECRET_KEY, algorithms=[ALGORITHM])
        username: str = payload.get("sub")

        if username is None:
            raise credentials_exception

        token_data = TokenData(username=username)

    except JWTError:
        raise credentials_exception

    user = get_user(username=token_data.username)

    if user is None:
        raise credentials_exception

    return user

@app.post("/token")
async def login(form_data: OAuth2PasswordRequestForm = Depends()):
    user = authenticate_user(form_data.username, form_data.password)

    if not user:
        raise HTTPException(
            status_code=status.HTTP_401_UNAUTHORIZED,
            detail="Incorrect username or password",
            headers={"WWW-Authenticate": "Bearer"},
        )

    access_token_expires = timedelta(minutes=ACCESS_TOKEN_EXPIRE_MINUTES)
    access_token = create_access_token(
        data={"sub": user.username},
        expires_delta=access_token_expires
    )

    return {"access_token": access_token, "token_type": "bearer"}

@app.get("/api/v1/users/me")
async def read_users_me(current_user: User = Depends(get_current_user)):
    return current_user

3. API Rate Limiting

// Go Rate Limiting with Redis
package middleware

import (
    "context"
    "fmt"
    "net/http"
    "time"

    "github.com/gin-gonic/gin"
    "github.com/go-redis/redis/v8"
)

type RateLimiter struct {
    client *redis.Client
    ctx    context.Context
}

func NewRateLimiter(redisClient *redis.Client) *RateLimiter {
    return &RateLimiter{
        client: redisClient,
        ctx:    context.Background(),
    }
}

// Sliding Window Rate Limiter
func (rl *RateLimiter) Limit(requestsPerWindow int, window time.Duration) gin.HandlerFunc {
    return func(c *gin.Context) {
        // Get identifier (IP or user ID)
        identifier := getIdentifier(c)
        key := fmt.Sprintf("rate_limit:%s", identifier)

        // Current timestamp
        now := time.Now().UnixNano()
        windowStart := now - int64(window)

        // Remove old entries
        rl.client.ZRemRangeByScore(rl.ctx, key, "0", fmt.Sprintf("%d", windowStart))

        // Count current requests
        count, err := rl.client.ZCard(rl.ctx, key).Result()
        if err != nil {
            c.JSON(http.StatusInternalServerError, gin.H{
                "error": "Rate limit check failed",
            })
            c.Abort()
            return
        }

        // Check if limit exceeded
        if count >= int64(requestsPerWindow) {
            c.Header("X-RateLimit-Limit", fmt.Sprintf("%d", requestsPerWindow))
            c.Header("X-RateLimit-Remaining", "0")

            // Get oldest request time to calculate retry-after
            oldest, _ := rl.client.ZRange(rl.ctx, key, 0, 0).Result()
            if len(oldest) > 0 {
                retryAfter := window - time.Duration(now-parseInt64(oldest[0]))
                c.Header("X-RateLimit-Reset", fmt.Sprintf("%d", time.Now().Add(retryAfter).Unix()))
                c.Header("Retry-After", fmt.Sprintf("%d", int(retryAfter.Seconds())))
            }

            c.JSON(http.StatusTooManyRequests, gin.H{
                "error": "Rate limit exceeded",
                "code":  "RATE_LIMIT_EXCEEDED",
            })
            c.Abort()
            return
        }

        // Add current request
        rl.client.ZAdd(rl.ctx, key, &redis.Z{
            Score:  float64(now),
            Member: fmt.Sprintf("%d", now),
        })

        // Set expiry
        rl.client.Expire(rl.ctx, key, window)

        // Set rate limit headers
        c.Header("X-RateLimit-Limit", fmt.Sprintf("%d", requestsPerWindow))
        c.Header("X-RateLimit-Remaining", fmt.Sprintf("%d", requestsPerWindow-int(count)-1))
        c.Header("X-RateLimit-Reset", fmt.Sprintf("%d", time.Now().Add(window).Unix()))

        c.Next()
    }
}

func getIdentifier(c *gin.Context) string {
    // Try to get user ID from context (if authenticated)
    if userID, exists := c.Get("userID"); exists {
        return fmt.Sprintf("user:%v", userID)
    }

    // Fall back to IP address
    return fmt.Sprintf("ip:%s", c.ClientIP())
}

API Documentation and Standards

OpenAPI/Swagger Documentation

# Complete OpenAPI 3.0 Specification
openapi: 3.0.3
info:
  title: E-Commerce API
  version: 1.0.0
  description: |
    RESTful API for e-commerce platform.

    ## Authentication
    All endpoints require JWT authentication via Bearer token in Authorization header.

    ## Rate Limiting
    - 100 requests per minute for authenticated users
    - 20 requests per minute for unauthenticated requests

  contact:
    name: API Support
    email: api@example.com
    url: https://api.example.com/support
  license:
    name: MIT
    url: https://opensource.org/licenses/MIT

servers:
  - url: https://api.example.com/v1
    description: Production server
  - url: https://staging-api.example.com/v1
    description: Staging server
  - url: http://localhost:3000/v1
    description: Development server

tags:
  - name: Authentication
    description: User authentication endpoints
  - name: Users
    description: User management
  - name: Products
    description: Product catalog
  - name: Orders
    description: Order management

paths:
  /auth/login:
    post:
      summary: User login
      tags: [Authentication]
      requestBody:
        required: true
        content:
          application/json:
            schema:
              type: object
              required:
                - email
                - password
              properties:
                email:
                  type: string
                  format: email
                  example: user@example.com
                password:
                  type: string
                  format: password
                  example: SecurePass123!
      responses:
        "200":
          description: Login successful
          content:
            application/json:
              schema:
                type: object
                properties:
                  token:
                    type: string
                    example: eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
                  user:
                    $ref: "#/components/schemas/User"
        "401":
          $ref: "#/components/responses/UnauthorizedError"

  /products:
    get:
      summary: List products
      tags: [Products]
      parameters:
        - $ref: "#/components/parameters/PageParam"
        - $ref: "#/components/parameters/LimitParam"
        - name: category
          in: query
          schema:
            type: string
        - name: minPrice
          in: query
          schema:
            type: number
        - name: maxPrice
          in: query
          schema:
            type: number
      responses:
        "200":
          description: Success
          content:
            application/json:
              schema:
                type: object
                properties:
                  data:
                    type: array
                    items:
                      $ref: "#/components/schemas/Product"
                  pagination:
                    $ref: "#/components/schemas/Pagination"

components:
  securitySchemes:
    BearerAuth:
      type: http
      scheme: bearer
      bearerFormat: JWT

  schemas:
    User:
      type: object
      properties:
        id:
          type: string
          format: uuid
        email:
          type: string
          format: email
        username:
          type: string
        createdAt:
          type: string
          format: date-time

    Product:
      type: object
      required:
        - name
        - price
      properties:
        id:
          type: string
          format: uuid
        name:
          type: string
        description:
          type: string
        price:
          type: number
          format: decimal
        category:
          type: string
        stock:
          type: integer
        createdAt:
          type: string
          format: date-time

    Pagination:
      type: object
      properties:
        page:
          type: integer
        limit:
          type: integer
        total:
          type: integer
        pages:
          type: integer

    Error:
      type: object
      properties:
        error:
          type: string
        code:
          type: string
        details:
          type: object

  parameters:
    PageParam:
      name: page
      in: query
      schema:
        type: integer
        default: 1
        minimum: 1

    LimitParam:
      name: limit
      in: query
      schema:
        type: integer
        default: 20
        minimum: 1
        maximum: 100

  responses:
    UnauthorizedError:
      description: Authentication required
      content:
        application/json:
          schema:
            $ref: "#/components/schemas/Error"

security:
  - BearerAuth: []

Testing and Monitoring APIs

1. Unit Testing

// Jest API Testing Example
const request = require("supertest");
const app = require("../app");
const User = require("../models/User");

describe("User API Endpoints", () => {
  let authToken;
  let userId;

  beforeAll(async () => {
    // Setup test database
    await setupTestDatabase();
  });

  afterAll(async () => {
    // Cleanup
    await cleanupTestDatabase();
  });

  describe("POST /api/v1/users", () => {
    it("should create a new user", async () => {
      const userData = {
        email: "test@example.com",
        username: "testuser",
        password: "SecurePass123!",
      };

      const response = await request(app)
        .post("/api/v1/users")
        .send(userData)
        .expect("Content-Type", /json/)
        .expect(201);

      expect(response.body.data).toHaveProperty("id");
      expect(response.body.data.email).toBe(userData.email);
      expect(response.body.data).not.toHaveProperty("password");

      userId = response.body.data.id;
    });

    it("should return 400 for invalid email", async () => {
      const response = await request(app)
        .post("/api/v1/users")
        .send({
          email: "invalid-email",
          username: "testuser",
          password: "password123",
        })
        .expect(400);

      expect(response.body).toHaveProperty("error");
      expect(response.body.code).toBe("VALIDATION_ERROR");
    });

    it("should return 409 for duplicate email", async () => {
      const response = await request(app)
        .post("/api/v1/users")
        .send({
          email: "test@example.com",
          username: "anotheruser",
          password: "password123",
        })
        .expect(409);

      expect(response.body.code).toBe("USER_EXISTS");
    });
  });

  describe("GET /api/v1/users/:id", () => {
    it("should return user by ID", async () => {
      const response = await request(app)
        .get(`/api/v1/users/${userId}`)
        .expect(200);

      expect(response.body.data.id).toBe(userId);
    });

    it("should return 404 for non-existent user", async () => {
      const fakeId = "550e8400-e29b-41d4-a716-446655440000";

      await request(app).get(`/api/v1/users/${fakeId}`).expect(404);
    });
  });
});

2. Integration Testing

# pytest API Integration Testing
import pytest
from httpx import AsyncClient
from app.main import app
from app.database import get_db

@pytest.fixture
async def client():
    async with AsyncClient(app=app, base_url="http://test") as ac:
        yield ac

@pytest.fixture
async def auth_token(client):
    response = await client.post(
        "/api/v1/auth/login",
        json={
            "email": "test@example.com",
            "password": "testpassword"
        }
    )
    return response.json()["token"]

@pytest.mark.asyncio
async def test_create_product_authenticated(client, auth_token):
    response = await client.post(
        "/api/v1/products",
        headers={"Authorization": f"Bearer {auth_token}"},
        json={
            "name": "Test Product",
            "price": 29.99,
            "category": "Electronics"
        }
    )

    assert response.status_code == 201
    data = response.json()["data"]
    assert data["name"] == "Test Product"
    assert data["price"] == 29.99

@pytest.mark.asyncio
async def test_create_product_unauthorized(client):
    response = await client.post(
        "/api/v1/products",
        json={
            "name": "Test Product",
            "price": 29.99
        }
    )

    assert response.status_code == 401

3. API Monitoring

// API Health Check and Monitoring Endpoints
const express = require("express");
const router = express.Router();
const mongoose = require("mongoose");
const redis = require("redis");

router.get("/health", async (req, res) => {
  const health = {
    uptime: process.uptime(),
    timestamp: Date.now(),
    status: "OK",
    checks: {},
  };

  try {
    // Database check
    const dbState = mongoose.connection.readyState;
    health.checks.database = {
      status: dbState === 1 ? "healthy" : "unhealthy",
      responseTime: 0,
    };

    // Redis check
    const redisStart = Date.now();
    await redisClient.ping();
    health.checks.redis = {
      status: "healthy",
      responseTime: Date.now() - redisStart,
    };

    // Check if all services are healthy
    const allHealthy = Object.values(health.checks).every(
      (check) => check.status === "healthy"
    );

    if (!allHealthy) {
      health.status = "DEGRADED";
      return res.status(503).json(health);
    }

    res.json(health);
  } catch (error) {
    health.status = "ERROR";
    health.error = error.message;
    res.status(503).json(health);
  }
});

router.get("/metrics", (req, res) => {
  // Prometheus-compatible metrics
  res.set("Content-Type", "text/plain");
  res.send(`
# HELP api_requests_total Total number of API requests
# TYPE api_requests_total counter
api_requests_total{method="GET",endpoint="/api/v1/users"} 1523
api_requests_total{method="POST",endpoint="/api/v1/users"} 342

# HELP api_request_duration_seconds API request duration
# TYPE api_request_duration_seconds histogram
api_request_duration_seconds_bucket{le="0.1"} 856
api_request_duration_seconds_bucket{le="0.5"} 1432
api_request_duration_seconds_bucket{le="1.0"} 1789
  `);
});

module.exports = router;

Conclusion

Effective API design and architecture is crucial for building scalable, maintainable software systems. By following the principles and best practices outlined in this guide, you can create APIs that:

1. Provide excellent developer experience through intuitive, consistent interfaces
2. Scale efficiently with proper pagination, caching, and rate limiting
3. Remain secure with robust authentication and authorization
4. Evolve gracefully through versioning and backward compatibility
5. Stay maintainable with comprehensive documentation and testing

Key Takeaways

• Design First: Define your API contract before implementation
• Be Consistent: Maintain uniform naming, structure, and error handling
• Think RESTful: Use HTTP methods and status codes correctly
• Secure by Default: Implement authentication, authorization, and rate limiting
• Document Everything: Provide clear, up-to-date API documentation
• Test Thoroughly: Write comprehensive unit and integration tests
• Monitor Continuously: Track performance and errors in production

Next Steps

1. Choose your API architecture pattern (REST, GraphQL, gRPC)
2. Define your API specification using OpenAPI or GraphQL schema
3. Implement security and authentication mechanisms
4. Create comprehensive documentation
5. Set up testing and monitoring infrastructure
6. Iterate based on developer feedback

Remember: Great APIs are built iteratively. Start with core functionality, gather feedback from developers, and continuously improve based on real-world usage.

Tools and Resources:

• Postman - API development and testing
• Swagger/OpenAPI - API documentation
• Insomnia - API client
• GraphiQL - GraphQL IDE
• Postman Learning Center - API tutorials

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