Agent Skills: API Design Patterns

API Design Patterns

Overview

RESTful and GraphQL API design patterns for building robust backend services.

REST API Design

Resource Naming

| Pattern | Example | Description | |---------|---------|-------------| | Plural nouns | /users, /orders | Collections | | Nested resources | /users/{id}/orders | Sub-resources | | No verbs in URLs | /users not /getUsers | Actions via HTTP methods | | Lowercase, hyphens | /order-items | Consistent casing |

HTTP Methods

| Method | Purpose | Idempotent | Example | |--------|---------|------------|---------| | GET | Read | Yes | GET /users/123 | | POST | Create | No | POST /users | | PUT | Replace | Yes | PUT /users/123 | | PATCH | Update | Yes | PATCH /users/123 | | DELETE | Remove | Yes | DELETE /users/123 |

Status Codes

| Code | Meaning | Usage | |------|---------|-------| | 200 | OK | Successful GET/PUT/PATCH | | 201 | Created | Successful POST | | 204 | No Content | Successful DELETE | | 400 | Bad Request | Validation error | | 401 | Unauthorized | Missing/invalid auth | | 403 | Forbidden | Insufficient permissions | | 404 | Not Found | Resource doesn't exist | | 409 | Conflict | Duplicate/conflict | | 422 | Unprocessable | Semantic error | | 500 | Server Error | Unexpected error |

Request/Response Format

// Successful response
{
  "data": {
    "id": "123",
    "name": "John Doe",
    "email": "john@example.com"
  },
  "meta": {
    "requestId": "req_abc123"
  }
}

// Error response
{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "Invalid email format",
    "details": [
      {
        "field": "email",
        "message": "Must be a valid email address"
      }
    ]
  },
  "meta": {
    "requestId": "req_abc123"
  }
}

// List response
{
  "data": [
    { "id": "1", "name": "User 1" },
    { "id": "2", "name": "User 2" }
  ],
  "pagination": {
    "total": 100,
    "page": 1,
    "pageSize": 20,
    "totalPages": 5
  }
}

Pagination

Offset Pagination

GET /users?page=2&pageSize=20

{
  "data": [...],
  "pagination": {
    "total": 100,
    "page": 2,
    "pageSize": 20,
    "totalPages": 5
  }
}

Cursor Pagination

Better for large datasets and real-time data.

GET /users?cursor=abc123&limit=20

{
  "data": [...],
  "pagination": {
    "nextCursor": "def456",
    "prevCursor": "xyz789",
    "hasMore": true
  }
}

Filtering and Sorting

Query Parameters

GET /users?status=active&role=admin    # Filtering
GET /users?sort=name&order=asc         # Sorting
GET /users?fields=id,name,email        # Field selection
GET /users?search=john                 # Search

Complex Filters

GET /orders?created_gte=2024-01-01&created_lte=2024-12-31
GET /products?price_min=10&price_max=100
GET /users?tags=premium,verified

Versioning

URL Versioning (Recommended)

/api/v1/users
/api/v2/users

Header Versioning

GET /users
Accept: application/vnd.api+json; version=2

Authentication

Bearer Token

Authorization: Bearer eyJhbGciOiJIUzI1NiIs...

API Key

X-API-Key: your-api-key
// or in query param (less secure)
GET /users?api_key=your-api-key

Rate Limiting

Response Headers

X-RateLimit-Limit: 100
X-RateLimit-Remaining: 95
X-RateLimit-Reset: 1609459200

429 Response

{
  "error": {
    "code": "RATE_LIMIT_EXCEEDED",
    "message": "Too many requests",
    "retryAfter": 60
  }
}

Endpoint Examples

User CRUD

POST   /api/v1/users              # Create user
GET    /api/v1/users              # List users
GET    /api/v1/users/:id          # Get user
PUT    /api/v1/users/:id          # Replace user
PATCH  /api/v1/users/:id          # Update user
DELETE /api/v1/users/:id          # Delete user

# Nested resources
GET    /api/v1/users/:id/orders   # User's orders
POST   /api/v1/users/:id/orders   # Create order for user

Actions (RPC-style)

For non-CRUD operations, use verbs as sub-resources:

POST /api/v1/users/:id/activate
POST /api/v1/orders/:id/cancel
POST /api/v1/payments/:id/refund

GraphQL Patterns

Schema Design

type User {
  id: ID!
  name: String!
  email: String!
  orders: [Order!]!
}

type Query {
  user(id: ID!): User
  users(filter: UserFilter, pagination: Pagination): UserConnection!
}

type Mutation {
  createUser(input: CreateUserInput!): User!
  updateUser(id: ID!, input: UpdateUserInput!): User!
  deleteUser(id: ID!): Boolean!
}

input UserFilter {
  status: UserStatus
  role: UserRole
  search: String
}

input Pagination {
  first: Int
  after: String
  last: Int
  before: String
}

Error Handling

type MutationResult {
  success: Boolean!
  errors: [Error!]
  user: User
}

type Error {
  code: String!
  message: String!
  field: String
}

type Mutation {
  createUser(input: CreateUserInput!): MutationResult!
}

API Documentation

OpenAPI (Swagger)

openapi: 3.0.0
info:
  title: User API
  version: 1.0.0

paths:
  /users:
    get:
      summary: List users
      parameters:
        - name: page
          in: query
          schema:
            type: integer
      responses:
        '200':
          description: Success
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/UserList'

components:
  schemas:
    User:
      type: object
      properties:
        id:
          type: string
        name:
          type: string

Best Practices

1. Consistency

• Same response format across all endpoints
• Consistent naming conventions
• Predictable behavior

2. Error Messages

• Clear, actionable messages
• Include error codes for programmatic handling
• Don't expose internal details

3. Idempotency

• Support idempotency keys for POST requests
• Safe to retry without side effects

POST /orders
Idempotency-Key: unique-request-id-123

4. HATEOAS (Hypermedia)

Include links to related resources:

{
  "data": {
    "id": "123",
    "name": "John"
  },
  "links": {
    "self": "/users/123",
    "orders": "/users/123/orders"
  }
}

API design patterns for RESTful and GraphQL services