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

Method	Purpose	Idempotent	Request Body	Success Code
GET	Read resource(s)	Yes	No	200
POST	Create resource	No	Yes	201
PUT	Replace resource entirely	Yes	Yes	200
PATCH	Partial update	No*	Yes	200
DELETE	Remove resource	Yes	No	204

*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)

Code	Meaning	When to Use
200	OK	Successful GET, PUT, PATCH
201	Created	Successful POST that creates a resource
204	No Content	Successful DELETE, or PUT/PATCH with no response body

Client Errors (4xx)

Code	Meaning	When to Use
400	Bad Request	Malformed JSON, invalid field values, validation errors
401	Unauthorized	Missing or invalid authentication
403	Forbidden	Authenticated but lacks permission
404	Not Found	Resource does not exist
409	Conflict	Duplicate resource, state conflict
422	Unprocessable Entity	Valid JSON but fails business rules
429	Too Many Requests	Rate limit exceeded

Server Errors (5xx)

Code	Meaning	When to Use
500	Internal Server Error	Unexpected server failure
502	Bad Gateway	Upstream service failure
503	Service Unavailable	Server overloaded or in maintenance

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": "[email protected]",
    "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_