API Design

REST API conventions: URL structure, HTTP methods, status codes, pagination, filtering, error responses, and versioning.

URL Structure

1. Use nouns, not verbs -- the HTTP method provides the verb
2. Use plural resource names -- /users, /orders, /products
3. Use kebab-case for multi-word resources -- /order-items, not /orderItems
4. Nest resources to show relationships -- max 2 levels deep
5. Use query parameters for filtering, not path segments

# Good
GET    /api/v1/users
GET    /api/v1/users/123
GET    /api/v1/users/123/orders
POST   /api/v1/users
PATCH  /api/v1/users/123
DELETE /api/v1/users/123

# Bad
GET    /api/v1/getUsers
GET    /api/v1/user/123
POST   /api/v1/users/123/orders/456/items/789/notes   # too deeply nested

HTTP Methods

*PATCH is not guaranteed idempotent, but should be designed to be when possible.

Rules

1. GET requests must be safe -- no side effects, no state changes
2. POST for creation -- return the created resource with Location header
3. Use PATCH over PUT -- partial updates are more practical than full replacement
4. DELETE should be idempotent -- deleting a non-existent resource returns 204, not 404

Status Codes

Use the correct status code. When in doubt, refer to this table:

Success (2xx)

Client Errors (4xx)

Server Errors (5xx)

Error Responses

Use a consistent error format across all endpoints:

{
  "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 at least 18.",
        "code": "MIN_VALUE"
      }
    ]
  }
}

Rules

1. Always include a machine-readable error code -- VALIDATION_ERROR, NOT_FOUND, RATE_LIMITED
2. Include a human-readable message -- suitable for developer debugging
3. Never expose internal errors -- no stack traces, SQL queries, or file paths in production
4. Field-level errors in details array -- for validation errors, specify which field failed

Pagination

Use cursor-based pagination for large datasets, offset-based for simple cases.

Offset-based (simple)

GET /api/v1/users?page=2&per_page=25

Response:
{
  "data": [ ... ],
  "pagination": {
    "page": 2,
    "per_page": 25,
    "total": 150,
    "total_pages": 6
  }
}

Cursor-based (scalable)

GET /api/v1/users?limit=25&cursor=eyJpZCI6MTAwfQ

Response:
{
  "data": [ ... ],
  "pagination": {
    "limit": 25,
    "has_more": true,
    "next_cursor": "eyJpZCI6MTI1fQ"
  }
}

Rules

1. Default page size: 25, max: 100 -- prevent clients from requesting unlimited data
2. Always return pagination metadata -- clients need to know if there are more pages
3. Use cursor-based for real-time data or large tables -- offset-based breaks with concurrent writes

Filtering and Sorting

# Filter by field values
GET /api/v1/users?status=active&role=admin

# Date ranges
GET /api/v1/orders?created_after=2025-01-01&created_before=2025-12-31

# Search
GET /api/v1/products?q=keyboard

# Sort (prefix with - for descending)
GET /api/v1/users?sort=created_at
GET /api/v1/users?sort=-updated_at

# Combine everything
GET /api/v1/orders?status=shipped&sort=-created_at&page=1&per_page=25

Rules

1. Use snake_case for query parameter names
2. Support multiple sort fields -- ?sort=-created_at,name
3. Validate all filter parameters -- return 400 for unknown fields
4. Document allowed filter fields per endpoint

Request and Response Conventions

1. Use snake_case for all JSON keys -- created_at, first_name, order_id
2. Use ISO 8601 for dates -- 2025-06-15T14:30:00Z
3. Use UUIDs or opaque strings for IDs -- avoid exposing auto-increment integers
4. Wrap collections in a data key -- { "data": [...] }, not a bare array
5. Include created_at and updated_at in all resources
6. Use null for absent optional fields -- don't omit them entirely

{
  "data": {
    "id": "usr_a1b2c3d4",
    "email": "user@example.com",
    "first_name": "Jane",
    "last_name": "Doe",
    "role": "admin",
    "avatar_url": null,
    "created_at": "2025-06-15T14:30:00Z",
    "updated_at": "2025-06-15T14:30:00Z"
  }
}

Versioning

1. Use URL path versioning -- /api/v1/, /api/v2/
2. Increment the major version only for breaking changes
3. Support the previous version for at least 6 months after deprecation
4. Return a Deprecation header on deprecated endpoints
5. Document migration guides between versions

Authentication

1. Use Bearer tokens in the Authorization header -- Authorization: Bearer <token>
2. Never pass tokens in query parameters -- they end up in server logs
3. Return 401 for missing/invalid tokens, 403 for insufficient permissions
4. Include rate limit headers -- X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Reset

Anti-patterns

• Verbs in URLs -- use HTTP methods instead
• Returning 200 with error body -- use proper status codes
• Nested resources deeper than 2 levels -- flatten with query parameters
• Inconsistent naming -- pick snake_case or camelCase and stick with it
• Missing pagination on list endpoints -- always paginate collections
• Exposing internal IDs -- use prefixed opaque IDs like usr_, ord_, prod_