API Design Patterns

API Design Patterns

Build APIs that developers love to use.

Design Principles

The Three Laws of API Design

1. Predictable: Consistent patterns throughout
2. Discoverable: Self-documenting, intuitive naming
3. Evolvable: Can change without breaking clients

API-First Mindset

Design → Document → Mock → Build → Test → Deploy

NOT: Build → Document (maybe) → Hope it works

REST Patterns

Resource Naming

# Collection
GET    /users              # List users
POST   /users              # Create user

# Item
GET    /users/{id}         # Get user
PUT    /users/{id}         # Replace user
PATCH  /users/{id}         # Update user
DELETE /users/{id}         # Delete user

# Nested resources
GET    /users/{id}/posts   # User's posts
POST   /users/{id}/posts   # Create post for user

# Actions (when CRUD doesn't fit)
POST   /users/{id}/activate
POST   /orders/{id}/cancel

Naming Conventions

HTTP Methods

*PATCH can be idempotent if designed carefully

Status Codes

Request/Response Patterns

Request Body

{
  "email": "user@example.com",
  "name": "John Doe",
  "preferences": {
    "newsletter": true
  }
}

Response: Single Resource

{
  "data": {
    "id": "usr_123",
    "type": "user",
    "attributes": {
      "email": "user@example.com",
      "name": "John Doe",
      "createdAt": "2024-01-15T10:30:00Z"
    },
    "relationships": {
      "organization": {
        "id": "org_456",
        "type": "organization"
      }
    }
  }
}

Response: Collection

{
  "data": [
    { "id": "usr_123", "type": "user", ... },
    { "id": "usr_124", "type": "user", ... }
  ],
  "meta": {
    "total": 150,
    "page": 1,
    "perPage": 20
  },
  "links": {
    "self": "/users?page=1",
    "next": "/users?page=2",
    "last": "/users?page=8"
  }
}

Error Response

{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "Invalid request parameters",
    "details": [
      {
        "field": "email",
        "code": "INVALID_FORMAT",
        "message": "Must be a valid email address"
      }
    ],
    "requestId": "req_abc123"
  }
}

Pagination Patterns

Offset-Based

GET /users?page=2&perPage=20
GET /users?offset=20&limit=20

Pros: Simple, familiar Cons: Inconsistent with real-time data, slow on large datasets

Cursor-Based

GET /users?cursor=eyJpZCI6MTIzfQ&limit=20

Response:

{
  "data": [...],
  "cursors": {
    "before": "eyJpZCI6MTAzfQ",
    "after": "eyJpZCI6MTIzfQ"
  },
  "hasMore": true
}

Pros: Consistent, performant Cons: Can't jump to page N

Keyset-Based

GET /users?after_id=123&limit=20

Pros: Very performant Cons: Requires sortable unique field

Filtering, Sorting, Search

Filtering

# Simple
GET /users?status=active

# Multiple values
GET /users?status=active,pending

# Operators
GET /users?created_at[gte]=2024-01-01
GET /users?name[contains]=john

# Nested
GET /users?organization.name=Acme

Sorting

# Single field
GET /users?sort=createdAt

# Descending
GET /users?sort=-createdAt

# Multiple fields
GET /users?sort=-createdAt,name

Field Selection

GET /users?fields=id,name,email
GET /users?fields[user]=id,name&fields[posts]=title

Search

GET /users?q=john
GET /users?search=john+doe

Versioning Strategies

URL Path (Recommended for major versions)

GET /v1/users
GET /v2/users

Pros: Explicit, cacheable Cons: URL pollution

Header

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

Pros: Clean URLs Cons: Harder to test, less visible

Query Parameter

GET /users?version=2

Pros: Easy to test Cons: Breaks caching, URL pollution

Deprecation Pattern

Deprecation: true
Sunset: Sat, 31 Dec 2024 23:59:59 GMT
Link: </v2/users>; rel="successor-version"

Authentication Patterns

API Keys

# Header
Authorization: Api-Key sk_live_abc123

# Query (avoid - logged in URLs)
GET /users?api_key=sk_live_abc123 <!-- allow-secret -->

Bearer Tokens (OAuth2/JWT)

Authorization: Bearer eyJhbGciOiJIUzI1NiIs...

Signing Requests (AWS-style)

Authorization: HMAC-SHA256 Credential=key/date/region/service,
               SignedHeaders=host;x-date,
               Signature=abc123

Rate Limiting

Headers

X-RateLimit-Limit: 1000
X-RateLimit-Remaining: 999
X-RateLimit-Reset: 1640000000
Retry-After: 60

Response (429)

{
  "error": {
    "code": "RATE_LIMITED",
    "message": "Rate limit exceeded",
    "retryAfter": 60
  }
}

Strategies

GraphQL Patterns

Schema Design

type User {
  id: ID!
  email: String!
  name: String
  posts(first: Int, after: String): PostConnection!
  createdAt: DateTime!
}

type Post {
  id: ID!
  title: String!
  content: String!
  author: User!
}

type PostConnection {
  edges: [PostEdge!]!
  pageInfo: PageInfo!
}

type Query {
  user(id: ID!): User
  users(filter: UserFilter, first: Int, after: String): UserConnection!
}

type Mutation {
  createUser(input: CreateUserInput!): CreateUserPayload!
  updateUser(id: ID!, input: UpdateUserInput!): UpdateUserPayload!
}

Naming Conventions

Error Handling

{
  "data": null,
  "errors": [
    {
      "message": "User not found",
      "locations": [{ "line": 2, "column": 3 }],
      "path": ["user"],
      "extensions": {
        "code": "NOT_FOUND",
        "timestamp": "2024-01-15T10:30:00Z"
      }
    }
  ]
}

OpenAPI Documentation

Basic Structure

openapi: 3.0.3
info:
  title: My API
  version: 1.0.0
  description: API description

servers:
  - url: https://api.example.com/v1
    description: Production

paths:
  /users:
    get:
      summary: List users
      operationId: listUsers
      tags: [Users]
      parameters:
        - name: page
          in: query
          schema:
            type: integer
            default: 1
      responses:
        '200':
          description: Successful response
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/UserList'

components:
  schemas:
    User:
      type: object
      required: [id, email]
      properties:
        id:
          type: string
          format: uuid
        email:
          type: string
          format: email

Related Skills

Complementary Skills (Use Together)

• backend-implementation-patterns - Implement the APIs you design
• testing-patterns - Test your API endpoints
• deployment-cicd - Deploy and version your APIs

Alternative Skills (Similar Purpose)

• mcp-builder - If building MCP servers instead of REST/GraphQL APIs

Prerequisite Skills (Learn First)

• None required - this is a foundational design skill

References

• references/openapi-template.md - Full OpenAPI template
• references/error-codes.md - Standard error code catalog
• references/sdk-patterns.md - Client SDK design patterns