api-design

# 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

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

### Collection Response (with Pagination)

```json
{
  "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

```json
{
  "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

```typescript
// 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
```

```json
{
  "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 equality
GET /api/v1/orders?status=active&customer_id=abc-123

# Comparison operators (use bracket notation)
GET /api/v1/products?price[gte]=10&price[lte]=100
GET /api/v1/orders?created_at[after]=2025-01-01

# Multiple values (comma-separated)
GET /api/v1/products?category=electronics,clothing

# Nested fields (dot notati