REST API Design

REST (Representational State Transfer) is an architectural style for designing networked applications. This skill covers best practices for designing clean, consistent, and scalable REST APIs.

Core Principles

1. Resources as Nouns

URLs represent resources (nouns), not actions (verbs).

# Good
GET /users
GET /users/123
GET /users/123/orders

# Bad
GET /getUsers
GET /getUserById?id=123
POST /createUser

2. HTTP Methods for Actions

Method	Purpose	Idempotent	Safe
GET	Read resource	Yes	Yes
POST	Create resource	No	No
PUT	Replace resource	Yes	No
PATCH	Update resource	Yes	No
DELETE	Delete resource	Yes	No

3. Statelessness

Each request contains all information needed. No server-side session state.

// Good - Token in header
Authorization: Bearer eyJhbGciOiJIUzI1NiIs...

// Bad - Relies on session
Cookie: session_id=abc123

URL Design

Resource Naming

# Collections (plural nouns)
/users
/products
/orders

# Single resource
/users/123
/products/abc

# Nested resources
/users/123/orders
/orders/456/items

# Avoid deep nesting (max 2 levels)
# Bad: /users/123/orders/456/items/789
# Good: /orders/456/items/789

Query Parameters

# Filtering
GET /products?category=electronics&status=active

# Sorting
GET /products?sort=price&order=desc
GET /products?sort=-price  # Prefix with - for descending

# Pagination
GET /products?page=2&limit=20
GET /products?offset=20&limit=20
GET /products?cursor=abc123&limit=20

# Field selection
GET /users/123?fields=id,name,email

# Search
GET /products?q=laptop
GET /products?search=laptop

Versioning

# URL versioning (most common)
/api/v1/users
/api/v2/users

# Header versioning
Accept: application/vnd.api+json;version=1

# Query parameter
/users?version=1

HTTP Status Codes

Success (2xx)

// 200 OK - Successful GET, PUT, PATCH, DELETE
res.status(200).json({ user })

// 201 Created - Successful POST (with Location header)
res.status(201)
   .header('Location', `/users/${user.id}`)
   .json({ user })

// 204 No Content - Successful DELETE (no body)
res.status(204).send()

Client Errors (4xx)

// 400 Bad Request - Invalid input
res.status(400).json({
  error: 'VALIDATION_ERROR',
  message: 'Invalid request body',
  details: [
    { field: 'email', message: 'Invalid email format' }
  ]
})

// 401 Unauthorized - Missing/invalid auth
res.status(401).json({
  error: 'UNAUTHORIZED',
  message: 'Authentication required'
})

// 403 Forbidden - Authenticated but not allowed
res.status(403).json({
  error: 'FORBIDDEN',
  message: 'Insufficient permissions'
})

// 404 Not Found - Resource doesn't exist
res.status(404).json({
  error: 'NOT_FOUND',
  message: 'User not found'
})

// 409 Conflict - State conflict (duplicate, etc.)
res.status(409).json({
  error: 'CONFLICT',
  message: 'Email already registered'
})

// 422 Unprocessable Entity - Semantic errors
res.status(422).json({
  error: 'UNPROCESSABLE_ENTITY',
  message: 'Cannot process request'
})

// 429 Too Many Requests - Rate limited
res.status(429)
   .header('Retry-After', '60')
   .json({
     error: 'RATE_LIMITED',
     message: 'Too many requests',
     retryAfter: 60
   })

Server Errors (5xx)

// 500 Internal Server Error
res.status(500).json({
  error: 'INTERNAL_ERROR',
  message: 'An unexpected error occurred',
  requestId: req.id
})

// 503 Service Unavailable
res.status(503)
   .header('Retry-After', '300')
   .json({
     error: 'SERVICE_UNAVAILABLE',
     message: 'Service temporarily unavailable'
   })

Error Response Format

Standard Error Structure

interface ApiError {
  error: string        // Machine-readable code
  message: string      // Human-readable message
  details?: any[]      // Additional details
  requestId?: string   // For debugging
  documentation?: string // Link to docs
}

// Example
{
  "error": "VALIDATION_ERROR",
  "message": "Request validation failed",
  "details": [
    { "field": "email", "message": "Invalid email format" },
    { "field": "age", "message": "Must be at least 18" }
  ],
  "requestId": "req-abc123",
  "documentation": "https://api.example.com/docs/errors#validation"
}

Problem Details (RFC 7807)

{
  "type": "https://api.example.com/errors/validation",
  "title": "Validation Error",
  "status": 400,
  "detail": "One or more fields failed validation",
  "instance": "/users",
  "errors": [
    { "pointer": "/email", "detail": "Invalid email format" }
  ]
}

Pagination

Offset-Based

// Request
GET /products?page=2&limit=20

// Response
{
  "data": [...],
  "pagination": {
    "page": 2,
    "limit": 20,
    "total": 150,
    "totalPages": 8
  }
}

Cursor-Based (Recommended)

// Request
GET /products?cursor=eyJpZCI6MTAwfQ&limit=20

// Response
{
  "data": [...],
  "pagination": {
    "limit": 20,
    "hasMore": true,
    "nextCursor": "eyJpZCI6MTIwfQ"
  }
}

Link Headers (RFC 5988)

Link: <https://api.example.com/products?cursor=abc>; rel="next",
      <https://api.example.com/products?cursor=xyz>; rel="prev"

Filtering & Sorting

Filtering Patterns

# Exact match
GET /products?status=active

# Multiple values
GET /products?status=active,pending

# Range
GET /products?price_min=10&price_max=100
GET /products?price[gte]=10&price[lte]=100

# Date range
GET /orders?created_after=2024-01-01&created_before=2024-12-31

# Full-text search
GET /products?q=laptop

# Nested filtering
GET /products?category.name=electronics

Sorting Patterns

# Single field
GET /products?sort=price
GET /products?sort=-price  # Descending

# Multiple fields
GET /products?sort=category,-price
GET /products?sort=category:asc,price:desc

# Default sort
GET /products  # Defaults to created_at desc

Request/Response Conventions

Consistent Response Envelope

// Success response
{
  "data": { ... },
  "meta": {
    "requestId": "req-123",
    "timestamp": "2024-01-15T10:30:00Z"
  }
}

// Collection response
{
  "data": [...],
  "pagination": { ... },
  "meta": { ... }
}

// Error response
{
  "error": { ... }
}

Timestamps

// Use ISO 8601
{
  "createdAt": "2024-01-15T10:30:00Z",
  "updatedAt": "2024-01-15T14:45:00Z"
}

IDs

// String IDs (preferred for external APIs)
{ "id": "usr_abc123" }

// Numeric IDs
{ "id": 12345 }

HATEOAS (Hypermedia)

Include links for discoverability:

{
  "data": {
    "id": "usr_123",
    "name": "John Doe",
    "links": {
      "self": "/users/usr_123",
      "orders": "/users/usr_123/orders",
      "profile": "/users/usr_123/profile"
    }
  }
}

Rate Limiting

Headers

X-RateLimit-Limit: 100
X-RateLimit-Remaining: 95
X-RateLimit-Reset: 1640000000
Retry-After: 60

Implementation

import rateLimit from 'express-rate-limit'

const limiter = rateLimit({
  windowMs: 60 * 1000, // 1 minute
  max: 100,
  standardHeaders: true,
  legacyHeaders: false,
  handler: (req, res) => {
    res.status(429).json({
      error: 'RATE_LIMITED',
      message: 'Too many requests',
      retryAfter: Math.ceil(req.rateLimit.resetTime / 1000)
    })
  }
})

Security Headers

// CORS
app.use(cors({
  origin: ['https://app.example.com'],
  methods: ['GET', 'POST', 'PUT', 'DELETE'],
  allowedHeaders: ['Content-Type', 'Authorization']
}))

// Security headers
app.use(helmet())

// Content-Type enforcement
app.use((req, res, next) => {
  if (req.method !== 'GET' && !req.is('application/json')) {
    return res.status(415).json({
      error: 'UNSUPPORTED_MEDIA_TYPE',
      message: 'Content-Type must be application/json'
    })
  }
  next()
})

Idempotency

Idempotency Keys

// Client sends
POST /payments
Idempotency-Key: unique-request-id-123

// Server tracks and returns same response for duplicate requests

Implementation

const idempotencyCache = new Map()

app.post('/payments', async (req, res) => {
  const key = req.headers['idempotency-key']

  if (key && idempotencyCache.has(key)) {
    return res.json(idempotencyCache.get(key))
  }

  const result = await processPayment(req.body)

  if (key) {
    idempotencyCache.set(key, result)
  }

  res.status(201).json(result)
})

Best Practices Summary

1. Use plural nouns for collections
2. Use HTTP methods correctly - GET read, POST create, PUT replace, PATCH update, DELETE remove
3. Return appropriate status codes - 2xx success, 4xx client error, 5xx server error
4. Version your API - URL path versioning is most common
5. Use cursor pagination - More reliable than offset
6. Consistent error format - Machine-readable codes + human messages
7. Rate limit endpoints - Protect against abuse
8. Add request IDs - For debugging and support
9. Use HTTPS - Always encrypt in transit
10. Document everything - OpenAPI/Swagger

References

• Resource Patterns
• Bulk Operations