API Design Patterns

RESTful API design principles, error handling, pagination, versioning, and security best practices. Guidelines for building consistent, developer-friendly APIs.

When to Apply

Reference these guidelines when:

• Designing new API endpoints
• Reviewing existing API structure
• Implementing error handling
• Setting up pagination/filtering
• Planning API versioning

Rule Categories by Priority

Priority	Category	Impact	Prefix
1	Resource Design	CRITICAL	rest-
2	Error Handling	CRITICAL	error-
3	Security	CRITICAL	sec-
4	Pagination & Filtering	HIGH	page-
5	Versioning	HIGH	ver-
6	Response Format	MEDIUM	resp-
7	Documentation	MEDIUM	doc-

Quick Reference

1. Resource Design (CRITICAL)

• rest-nouns-not-verbs - Use nouns for endpoints
• rest-plural-resources - Use plural resource names
• rest-http-methods - Correct HTTP method usage
• rest-nested-resources - Proper resource nesting
• rest-status-codes - Appropriate status codes
• rest-idempotency - Idempotent operations
• rest-hateoas - Hypermedia links

2. Error Handling (CRITICAL)

• error-consistent-format - Consistent error structure
• error-meaningful-messages - Helpful error messages
• error-validation-details - Field-level validation errors
• error-codes - Machine-readable error codes
• error-stack-traces - Never expose in production

3. Security (CRITICAL)

• sec-authentication - Proper auth implementation
• sec-authorization - Resource-level permissions
• sec-rate-limiting - Prevent abuse
• sec-input-validation - Validate all input
• sec-cors - CORS configuration
• sec-sensitive-data - Protect sensitive data

4. Pagination & Filtering (HIGH)

• page-cursor-based - Cursor pagination for large datasets
• page-offset-based - Offset pagination for simple cases
• page-consistent-params - Consistent parameter names
• page-metadata - Include pagination metadata
• filter-query-params - Filter via query parameters
• sort-flexible - Flexible sorting options

5. Versioning (HIGH)

• ver-url-path - Version in URL path
• ver-header-based - Version in headers
• ver-backward-compatible - Maintain compatibility
• ver-deprecation - Deprecation strategy

6. Response Format (MEDIUM)

• resp-consistent-structure - Consistent response envelope
• resp-json-conventions - JSON naming conventions
• resp-partial-responses - Field selection
• resp-compression - Response compression

7. Documentation (MEDIUM)

• doc-openapi - OpenAPI/Swagger spec
• doc-examples - Request/response examples
• doc-changelog - API changelog

Essential Guidelines

Resource Naming

# ❌ Verbs in URLs - RPC style
GET    /getUsers
POST   /createUser
PUT    /updateUser/123
DELETE /deleteUser/123

# ✅ Nouns with HTTP methods - REST style
GET    /users          # List users
POST   /users          # Create user
GET    /users/123      # Get user
PUT    /users/123      # Update user (full)
PATCH  /users/123      # Update user (partial)
DELETE /users/123      # Delete user

Nested Resources

# ✅ Logical nesting (max 2 levels)
GET    /users/123/orders              # User's orders
GET    /users/123/orders/456          # Specific order
POST   /users/123/orders              # Create order for user

# ❌ Too deeply nested
GET    /users/123/orders/456/items/789/reviews

# ✅ Flatten when appropriate
GET    /order-items/789/reviews       # Direct access

HTTP Methods & Status Codes

# GET - Retrieve resource(s)
GET /users              → 200 OK + array
GET /users/123          → 200 OK + object
GET /users/999          → 404 Not Found

# POST - Create resource
POST /users             → 201 Created + object + Location header
POST /users (invalid)   → 400 Bad Request + errors

# PUT - Full update (replace)
PUT /users/123          → 200 OK + updated object
PUT /users/999          → 404 Not Found

# PATCH - Partial update
PATCH /users/123        → 200 OK + updated object

# DELETE - Remove resource
DELETE /users/123       → 204 No Content
DELETE /users/999       → 404 Not Found

# Other common status codes
401 Unauthorized        # Not authenticated
403 Forbidden           # Authenticated but not authorized
409 Conflict            # Resource state conflict
422 Unprocessable       # Validation failed
429 Too Many Requests   # Rate limited
500 Internal Error      # Server error
503 Service Unavailable # Maintenance/overload

Error Response Format

// ❌ Inconsistent error formats
{ "error": "Not found" }
{ "message": "Invalid email" }
{ "errors": ["Error 1", "Error 2"] }

// ✅ Consistent error envelope
{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "The request contains invalid data",
    "details": [
      {
        "field": "email",
        "code": "INVALID_FORMAT",
        "message": "Please provide a valid email address"
      },
      {
        "field": "password",
        "code": "TOO_SHORT",
        "message": "Password must be at least 8 characters"
      }
    ],
    "request_id": "req_abc123"
  }
}

// ✅ Not found error
{
  "error": {
    "code": "NOT_FOUND",
    "message": "User with ID 123 not found",
    "request_id": "req_def456"
  }
}

// ✅ Server error (no sensitive details)
{
  "error": {
    "code": "INTERNAL_ERROR",
    "message": "An unexpected error occurred. Please try again.",
    "request_id": "req_ghi789"
  }
}

Pagination

// ✅ Offset-based pagination
GET /users?page=2&per_page=20

{
  "data": [...],
  "meta": {
    "current_page": 2,
    "per_page": 20,
    "total_pages": 10,
    "total_count": 195
  },
  "links": {
    "first": "/users?page=1&per_page=20",
    "prev": "/users?page=1&per_page=20",
    "next": "/users?page=3&per_page=20",
    "last": "/users?page=10&per_page=20"
  }
}

// ✅ Cursor-based pagination (for large datasets)
GET /users?cursor=eyJpZCI6MTIzfQ&limit=20

{
  "data": [...],
  "meta": {
    "has_more": true,
    "next_cursor": "eyJpZCI6MTQzfQ"
  },
  "links": {
    "next": "/users?cursor=eyJpZCI6MTQzfQ&limit=20"
  }
}

Filtering & Sorting

# ✅ Query parameter filtering
GET /users?status=active
GET /users?role=admin&status=active
GET /users?created_after=2024-01-01

# ✅ Sorting
GET /users?sort=created_at        # Ascending (default)
GET /users?sort=-created_at       # Descending (prefix with -)
GET /users?sort=last_name,-created_at  # Multiple fields

# ✅ Field selection (sparse fieldsets)
GET /users?fields=id,name,email
GET /users/123?fields=id,name,orders

# ✅ Search
GET /users?q=john
GET /users?search=john@example

Consistent Response Structure

// ✅ Single resource
GET /users/123
{
  "data": {
    "id": "123",
    "type": "user",
    "attributes": {
      "name": "John Doe",
      "email": "john@example.com",
      "created_at": "2024-01-15T10:30:00Z"
    },
    "relationships": {
      "orders": {
        "links": {
          "related": "/users/123/orders"
        }
      }
    }
  }
}

// ✅ Collection
GET /users
{
  "data": [
    { "id": "123", "type": "user", ... },
    { "id": "124", "type": "user", ... }
  ],
  "meta": {
    "total_count": 100
  },
  "links": {
    "self": "/users?page=1",
    "next": "/users?page=2"
  }
}

// ✅ Simpler envelope (also acceptable)
{
  "user": {
    "id": "123",
    "name": "John Doe"
  }
}

{
  "users": [...],
  "pagination": {...}
}

Versioning

# ✅ URL path versioning (recommended - explicit)
GET /api/v1/users
GET /api/v2/users

# ✅ Header versioning
GET /api/users
Accept: application/vnd.myapi.v2+json

# ✅ Query parameter (simple, but less clean)
GET /api/users?version=2

Rate Limiting

# ✅ Include rate limit headers
HTTP/1.1 200 OK
X-RateLimit-Limit: 1000
X-RateLimit-Remaining: 998
X-RateLimit-Reset: 1640995200

# ✅ Rate limited response
HTTP/1.1 429 Too Many Requests
Retry-After: 60

{
  "error": {
    "code": "RATE_LIMITED",
    "message": "Too many requests. Please retry after 60 seconds.",
    "retry_after": 60
  }
}

Actions on Resources

# For non-CRUD actions, use sub-resources or actions
# ✅ Sub-resource style
POST /users/123/activate
POST /orders/456/cancel
POST /posts/789/publish

# ✅ Or controller-style for complex operations
POST /auth/login
POST /auth/logout
POST /auth/refresh
POST /password/reset
POST /password/reset/confirm

Request Validation

// ✅ Validate and return all errors at once
POST /users
{
  "email": "invalid",
  "password": "short"
}

// Response: 422 Unprocessable Entity
{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "Validation failed",
    "details": [
      {
        "field": "email",
        "code": "INVALID_FORMAT",
        "message": "Must be a valid email address"
      },
      {
        "field": "password",
        "code": "TOO_SHORT",
        "message": "Must be at least 8 characters",
        "meta": {
          "min_length": 8,
          "actual_length": 5
        }
      }
    ]
  }
}

HATEOAS (Hypermedia)

// ✅ Include links for discoverability
{
  "data": {
    "id": "123",
    "status": "pending",
    "total": 99.99
  },
  "links": {
    "self": "/orders/123",
    "cancel": "/orders/123/cancel",
    "pay": "/orders/123/pay",
    "items": "/orders/123/items"
  },
  "actions": {
    "cancel": {
      "method": "POST",
      "href": "/orders/123/cancel"
    },
    "pay": {
      "method": "POST",
      "href": "/orders/123/pay",
      "fields": [
        { "name": "payment_method", "type": "string", "required": true }
      ]
    }
  }
}

Bulk Operations

// ✅ Bulk create
POST /users/bulk
{
  "users": [
    { "name": "User 1", "email": "user1@example.com" },
    { "name": "User 2", "email": "user2@example.com" }
  ]
}

// Response with partial success
{
  "data": {
    "succeeded": [
      { "id": "123", "name": "User 1" }
    ],
    "failed": [
      {
        "index": 1,
        "error": {
          "code": "DUPLICATE_EMAIL",
          "message": "Email already exists"
        }
      }
    ]
  },
  "meta": {
    "total": 2,
    "succeeded": 1,
    "failed": 1
  }
}

Output Format

When auditing APIs, output findings in this format:

endpoint - [category] Description of issue

Example:

GET /getUsers - [rest] Use noun '/users' instead of verb '/getUsers'
POST /users - [error] Missing validation error details in 400 response
GET /users - [page] Missing pagination metadata in list response

How to Use

Read individual rule files for detailed explanations:

rules/rest-http-methods.md
rules/error-consistent-format.md
rules/page-cursor-based.md