API Design Skill

Use this skill when designing, implementing, or reviewing APIs.

When to Use

• Creating new API endpoints
• Refactoring existing APIs
• Designing API architecture
• Reviewing API contracts
• Writing API documentation

REST API Design

URL Structure

# Resources (plural nouns)
GET    /api/users              # List
POST   /api/users              # Create
GET    /api/users/:id          # Read
PUT    /api/users/:id          # Replace
PATCH  /api/users/:id          # Update
DELETE /api/users/:id          # Delete

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

# Actions (when CRUD doesn't fit)
POST   /api/users/:id/activate
POST   /api/orders/:id/cancel
POST   /api/payments/:id/refund

# Filtering, sorting, pagination
GET    /api/users?status=active
GET    /api/users?sort=-createdAt
GET    /api/users?page=2&limit=20
GET    /api/users?fields=id,name,email

HTTP Methods & Status Codes

Method	Use	Success	Common Errors
GET	Read	200	404
POST	Create	201	400, 409
PUT	Replace	200	400, 404
PATCH	Update	200	400, 404
DELETE	Remove	204	404

Response Format

// Success response
{
  "data": {
    "id": "user_123",
    "type": "user",
    "attributes": {
      "name": "John Doe",
      "email": "john@example.com",
      "createdAt": "2024-01-15T10:30:00Z"
    },
    "relationships": {
      "organization": {
        "id": "org_456",
        "type": "organization"
      }
    }
  },
  "meta": {
    "requestId": "req_abc123"
  }
}

// List response
{
  "data": [...],
  "meta": {
    "pagination": {
      "page": 1,
      "limit": 20,
      "total": 150,
      "totalPages": 8
    }
  },
  "links": {
    "self": "/api/users?page=1",
    "next": "/api/users?page=2",
    "prev": null,
    "first": "/api/users?page=1",
    "last": "/api/users?page=8"
  }
}

// Error response
{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "Validation failed",
    "details": [
      {
        "field": "email",
        "code": "invalid_format",
        "message": "Invalid email format"
      }
    ]
  },
  "meta": {
    "requestId": "req_abc123",
    "timestamp": "2024-01-15T10:30:00Z"
  }
}

Request Validation

import { z } from 'zod';

// Define schema
const CreateUserSchema = z.object({
  name: z.string().min(1).max(100),
  email: z.string().email(),
  password: z.string().min(8),
  role: z.enum(['user', 'admin']).default('user'),
  preferences: z.object({
    notifications: z.boolean().default(true),
    theme: z.enum(['light', 'dark']).optional(),
  }).optional(),
});

// Validation middleware
function validate<T>(schema: z.Schema<T>) {
  return (req: Request, res: Response, next: NextFunction) => {
    const result = schema.safeParse(req.body);
    
    if (!result.success) {
      return res.status(422).json({
        error: {
          code: 'VALIDATION_ERROR',
          message: 'Validation failed',
          details: result.error.issues.map(issue => ({
            field: issue.path.join('.'),
            code: issue.code,
            message: issue.message,
          })),
        },
      });
    }
    
    req.body = result.data;
    next();
  };
}

// Usage
app.post('/api/users', validate(CreateUserSchema), createUser);

Authentication

// JWT Bearer token
// Header: Authorization: Bearer <token>

async function authenticate(req: Request, res: Response, next: NextFunction) {
  const token = req.headers.authorization?.replace('Bearer ', '');
  
  if (!token) {
    return res.status(401).json({
      error: {
        code: 'UNAUTHORIZED',
        message: 'Authentication required',
      },
    });
  }

  try {
    const payload = jwt.verify(token, process.env.JWT_SECRET);
    req.user = payload;
    next();
  } catch (error) {
    return res.status(401).json({
      error: {
        code: 'UNAUTHORIZED',
        message: 'Invalid or expired token',
      },
    });
  }
}

Rate Limiting

import rateLimit from 'express-rate-limit';

const apiLimiter = rateLimit({
  windowMs: 15 * 60 * 1000, // 15 minutes
  max: 100,
  standardHeaders: true,
  legacyHeaders: false,
  message: {
    error: {
      code: 'RATE_LIMITED',
      message: 'Too many requests, please try again later',
    },
  },
});

app.use('/api/', apiLimiter);

Versioning

// URL versioning (recommended)
app.use('/api/v1', v1Routes);
app.use('/api/v2', v2Routes);

// Header versioning
// Accept: application/vnd.myapi.v2+json

function versionMiddleware(req, res, next) {
  const accept = req.headers.accept || '';
  const match = accept.match(/application\/vnd\.myapi\.v(\d+)\+json/);
  req.apiVersion = match ? parseInt(match[1]) : 1;
  next();
}

GraphQL API Design

Schema Design

# Types
type User {
  id: ID!
  name: String!
  email: String!
  createdAt: DateTime!
  orders(first: Int, after: String): OrderConnection!
}

type Order {
  id: ID!
  total: Float!
  status: OrderStatus!
  items: [OrderItem!]!
  user: User!
}

enum OrderStatus {
  PENDING
  PROCESSING
  SHIPPED
  DELIVERED
  CANCELLED
}

# Connections for pagination
type OrderConnection {
  edges: [OrderEdge!]!
  pageInfo: PageInfo!
  totalCount: Int!
}

type OrderEdge {
  node: Order!
  cursor: String!
}

type PageInfo {
  hasNextPage: Boolean!
  hasPreviousPage: Boolean!
  startCursor: String
  endCursor: String
}

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

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

# Inputs
input CreateUserInput {
  name: String!
  email: String!
  password: String!
}

# Payloads (for error handling)
type CreateUserPayload {
  user: User
  errors: [UserError!]!
}

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

Resolvers

const resolvers = {
  Query: {
    user: async (_, { id }, context) => {
      return context.dataSources.users.findById(id);
    },
    users: async (_, args, context) => {
      return context.dataSources.users.findMany(args);
    },
  },
  
  Mutation: {
    createUser: async (_, { input }, context) => {
      try {
        const user = await context.dataSources.users.create(input);
        return { user, errors: [] };
      } catch (error) {
        return {
          user: null,
          errors: [{ message: error.message, code: 'CREATE_FAILED' }],
        };
      }
    },
  },
  
  User: {
    orders: async (user, args, context) => {
      return context.dataSources.orders.findByUser(user.id, args);
    },
  },
};

OpenAPI Documentation

openapi: 3.0.3
info:
  title: My API
  version: 1.0.0
  description: API for managing users and orders

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

paths:
  /users:
    get:
      summary: List users
      tags: [Users]
      parameters:
        - name: page
          in: query
          schema:
            type: integer
            default: 1
        - name: limit
          in: query
          schema:
            type: integer
            default: 20
            maximum: 100
      responses:
        '200':
          description: Success
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/UserList'
    
    post:
      summary: Create user
      tags: [Users]
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/CreateUser'
      responses:
        '201':
          description: Created
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/User'
        '422':
          description: Validation error
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Error'

components:
  schemas:
    User:
      type: object
      properties:
        id:
          type: string
        name:
          type: string
        email:
          type: string
          format: email
      required: [id, name, email]
    
    CreateUser:
      type: object
      properties:
        name:
          type: string
          minLength: 1
          maxLength: 100
        email:
          type: string
          format: email
        password:
          type: string
          minLength: 8
      required: [name, email, password]
    
    Error:
      type: object
      properties:
        error:
          type: object
          properties:
            code:
              type: string
            message:
              type: string
            details:
              type: array
              items:
                type: object

API Design Checklist

Naming & Structure

• Resources are plural nouns
• URLs are lowercase with hyphens
• Consistent naming across endpoints
• Logical resource hierarchy

Request/Response

• Appropriate HTTP methods used
• Correct status codes returned
• Consistent response format
• Proper error responses

Security

• Authentication implemented
• Authorization checked
• Input validation present
• Rate limiting configured

Documentation

• OpenAPI/Swagger spec complete
• Examples provided
• Error codes documented
• Authentication explained