/api-design

name: api-design
description: REST API design patterns including resource naming, status codes, pagination, filtering, error responses, versioning, and rate limiting for production APIs. Use when designing or reviewing REST endpoints, resource names, status codes, pagination, or versioning.
metadata:
  origin: ECC

API Design Patterns

Conventions and best practices for designing consistent, developer-friendly REST APIs.

When to Activate

• Designing new API endpoints
• Reviewing existing API contracts
• Adding pagination, filtering, or sorting
• Implementing error handling for APIs
• Planning API versioning strategy
• Building public or partner-facing APIs

Resource Design

URL Structure

# Resources are nouns, plural, lowercase, kebab-case
GET    /api/v1/users
GET    /api/v1/users/:id
POST   /api/v1/users
PUT    /api/v1/users/:id
PATCH  /api/v1/users/:id
DELETE /api/v1/users/:id

# Sub-resources for relationships
GET    /api/v1/users/:id/orders
POST   /api/v1/users/:id/orders

# Actions that don't map to CRUD (use verbs sparingly)
POST   /api/v1/orders/:id/cancel
POST   /api/v1/auth/login
POST   /api/v1/auth/refresh

Naming Rules

# GOOD
/api/v1/team-members          # kebab-case for multi-word resources
/api/v1/orders?status=active  # query params for filtering
/api/v1/users/123/orders      # nested resources for ownership

# BAD
/api/v1/getUsers              # verb in URL
/api/v1/user                  # singular (use plural)
/api/v1/team_members          # snake_case in URLs
/api/v1/users/123/getOrders   # verb in nested resource

HTTP Methods and Status Codes

Method Semantics

| Method | Idempotent | Safe | Use For | |--------|-----------|------|---------| | GET | Yes | Yes | Retrieve resources | | POST | No | No | Create resources, trigger actions | | PUT | Yes | No | Full replacement of a resource | | PATCH | No* | No | Partial update of a resource | | DELETE | Yes | No | Remove a resource |

*PATCH can be made idempotent with proper implementation

Status Code Reference

# Success
200 OK                    — GET, PUT, PATCH (with response body)
201 Created               — POST (include Location header)
204 No Content            — DELETE, PUT (no response body)

# Client Errors
400 Bad Request           — Validation failure, malformed JSON
401 Unauthorized          — Missing or invalid authentication
403 Forbidden             — Authenticated but not authorized
404 Not Found             — Resource doesn't exist
409 Conflict              — Duplicate entry, state conflict
422 Unprocessable Entity  — Semantically invalid (valid JSON, bad data)
429 Too Many Requests     — Rate limit exceeded

# Server Errors
500 Internal Server Error — Unexpected failure (never expose details)
502 Bad Gateway           — Upstream service failed
503 Service Unavailable   — Temporary overload, include Retry-After

Common Mistakes

# BAD: 200 for everything
{ "status": 200, "success": false, "error": "Not found" }

# GOOD: Use HTTP status codes semantically
HTTP/1.1 404 Not Found
{ "error": { "code": "not_found", "message": "User not found" } }

# BAD: 500 for validation errors
# GOOD: 400 or 422 with field-level details

# BAD: 200 for created resources
# GOOD: 201 with Location header
HTTP/1.1 201 Created
Location: /api/v1/users/abc-123

Response Format

Success Response

{
  "data": {
    "id": "abc-123",
    "email": "alice@example.com",
    "name": "Alice",
    "created_at": "2025-01-15T10:30:00Z"
  }
}

Collection Response (with Pagination)

{
  "data": [
    { "id": "abc-123", "name": "Alice" },
    { "id": "def-456", "name": "Bob" }
  ],
  "meta": {
    "total": 142,
    "page": 1,
    "per_page": 20,
    "total_pages": 8
  },
  "links": {
    "self": "/api/v1/users?page=1&per_page=20",
    "next": "/api/v1/users?page=2&per_page=20",
    "last": "/api/v1/users?page=8&per_page=20"
  }
}

Error Response

{
  "error": {
    "code": "validation_error",
    "message": "Request validation failed",
    "details": [
      {
        "field": "email",
        "message": "Must be a valid email address",
        "code": "invalid_format"
      },
      {
        "field": "age",
        "message": "Must be between 0 and 150",
        "code": "out_of_range"
      }
    ]
  }
}

Response Envelope Variants

// Option A: Envelope with data wrapper (recommended for public APIs)
interface ApiResponse<T> {
  data: T;
  meta?: PaginationMeta;
  links?: PaginationLinks;
}

interface ApiError {
  error: {
    code: string;
    message: string;
    details?: FieldError[];
  };
}

// Option B: Flat response (simpler, common for internal APIs)
// Success: just return the resource directly
// Error: return error object
// Distinguish by HTTP status code

Pagination

Offset-Based (Simple)

GET /api/v1/users?page=2&per_page=20

# Implementation
SELECT * FROM users
ORDER BY created_at DESC
LIMIT 20 OFFSET 20;

**Pros:** Easy to implement, supports "jump to page N" **Cons:** Slow on large offsets (OFFSET 100000), inconsistent with concurrent inserts

Cursor-Based (Scalable)

GET /api/v1/users?cursor=eyJpZCI6MTIzfQ&limit=20

# Implementation
SELECT * FROM users
WHERE id > :cursor_id
ORDER BY id ASC
LIMIT 21;  -- fetch one extra to determine has_next

{
  "data": [...],
  "meta": {
    "has_next": true,
    "next_cursor": "eyJpZCI6MTQzfQ"
  }
}

**Pros:** Consistent performance regardless of position, stable with concurrent inserts **Cons:** Cannot jump to arbitrary page, cursor is opaque

When to Use Which

| Use Case | Pagination Type | |----------|----------------| | Admin dashboards, small datasets (<10K) | Offset | | Infinite scroll, feeds, large datasets | Cursor | | Public APIs | Cursor (default) with offset (optional) | | Search results | Offset (users expect page numbers) |

Filtering, Sorting, and Search

Filtering

# Simple