API Design Principles

Design APIs that are intuitive, consistent, and a joy to use.

URL Structure

Resources as Nouns

# GOOD - nouns
GET /users
GET /users/123
GET /users/123/orders

# BAD - verbs
GET /getUsers
GET /fetchUserById/123
POST /createNewUser

Plural Resource Names

# GOOD - plural
GET /users
GET /orders
GET /products

# BAD - singular
GET /user
GET /order

Hierarchical Relationships

# Parent-child relationships
GET /users/123/orders          # Orders for user 123
GET /orders/456/items          # Items in order 456

# Avoid deep nesting (max 2 levels)
# BAD
GET /users/123/orders/456/items/789/reviews

# GOOD - flatten when needed
GET /order-items/789/reviews

HTTP Methods

GET    /users          # List users
POST   /users          # Create user
GET    /users/123      # Get user 123
PUT    /users/123      # Replace user 123
PATCH  /users/123      # Update user 123
DELETE /users/123      # Delete user 123

Request/Response Format

Consistent JSON Structure

// Success response
{
  "data": {
    "id": "123",
    "name": "John Doe",
    "email": "john@example.com"
  }
}

// Collection response
{
  "data": [
    { "id": "123", "name": "John" },
    { "id": "456", "name": "Jane" }
  ],
  "meta": {
    "total": 100,
    "page": 1,
    "perPage": 20
  }
}

// Error response
{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "Invalid email format",
    "details": [
      { "field": "email", "message": "Must be a valid email" }
    ]
  }
}

Use camelCase

// GOOD
{
  "firstName": "John",
  "lastName": "Doe",
  "emailAddress": "john@example.com"
}

// BAD - inconsistent
{
  "first_name": "John",
  "LastName": "Doe",
  "email-address": "john@example.com"
}

Status Codes

Success (2xx)

200 OK           - GET, PUT, PATCH success
201 Created      - POST success (include Location header)
204 No Content   - DELETE success

Client Errors (4xx)

400 Bad Request     - Invalid syntax, validation error
401 Unauthorized    - No/invalid authentication
403 Forbidden       - Authenticated but not allowed
404 Not Found       - Resource doesn't exist
409 Conflict        - State conflict (duplicate, version)
422 Unprocessable   - Valid syntax but semantic error
429 Too Many Requests - Rate limited

Server Errors (5xx)

500 Internal Server Error - Unexpected error
502 Bad Gateway          - Upstream service failed
503 Service Unavailable  - Temporarily unavailable
504 Gateway Timeout      - Upstream timeout

Pagination

Offset-based (simple)

GET /users?page=2&perPage=20

Response:
{
  "data": [...],
  "meta": {
    "total": 100,
    "page": 2,
    "perPage": 20,
    "totalPages": 5
  },
  "links": {
    "first": "/users?page=1&perPage=20",
    "prev": "/users?page=1&perPage=20",
    "next": "/users?page=3&perPage=20",
    "last": "/users?page=5&perPage=20"
  }
}

Cursor-based (scalable)

GET /users?cursor=eyJpZCI6MTIzfQ&limit=20

Response:
{
  "data": [...],
  "meta": {
    "hasMore": true,
    "nextCursor": "eyJpZCI6MTQzfQ"
  }
}

Filtering & Sorting

Filtering

GET /users?status=active
GET /users?role=admin&status=active
GET /users?createdAt[gte]=2024-01-01
GET /users?name[contains]=john

Sorting

GET /users?sort=name           # Ascending
GET /users?sort=-createdAt     # Descending (prefix -)
GET /users?sort=lastName,firstName

Field Selection

GET /users?fields=id,name,email
GET /users/123?fields=id,name,email

Versioning

URL Path (recommended)

GET /v1/users
GET /v2/users

Header

GET /users
Accept: application/vnd.api+json;version=2

Authentication

Bearer Token

Authorization: Bearer eyJhbGciOiJIUzI1NiIs...

API Key

X-API-Key: sk_live_abc123
# Or in query (less secure)
GET /users?api_key=sk_live_abc123

Rate Limiting

Include headers:

X-RateLimit-Limit: 100
X-RateLimit-Remaining: 95
X-RateLimit-Reset: 1640995200
Retry-After: 60

Documentation

Every endpoint needs:

• Description of what it does
• Request parameters with types
• Request body schema
• Response schema for each status code
• Example request/response
• Error cases

# OpenAPI example
/users:
  post:
    summary: Create a new user
    requestBody:
      required: true
      content:
        application/json:
          schema:
            $ref: '#/components/schemas/CreateUser'
    responses:
      '201':
        description: User created
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/User'
      '400':
        description: Validation error

Common Mistakes

Checklist

• URLs use nouns, not verbs
• HTTP methods used correctly
• Consistent response format
• Appropriate status codes
• Pagination for collections
• Filtering and sorting support
• Rate limiting implemented
• Authentication documented
• Errors are descriptive
• API is versioned