✅ Good (nouns, plural):
GET    /users           # List users
GET    /users/123       # Get user
POST   /users           # Create user
PUT    /users/123       # Update user
DELETE /users/123       # Delete user

❌ Bad (verbs, actions):
GET    /getUsers
POST   /createUser
POST   /users/123/delete

# Hierarchical relationship
GET /users/123/orders              # User's orders
GET /users/123/orders/456          # Specific order

# Alternative: Query parameter for filtering
GET /orders?userId=123             # Filter orders by user

# Rule: Nest max 2 levels deep
❌ /users/123/orders/456/items/789/reviews
✅ /order-items/789/reviews

// Response structure
interface ApiResponse<T> {
  data: T;
  meta?: {
    page: number;
    limit: number;
    total: number;
  };
}

interface ApiError {
  error: {
    code: string;        // Machine-readable
    message: string;     // Human-readable
    details?: object;    // Validation errors, etc.
  };
}

// Offset-based (simple, has issues with large datasets)
GET /users?page=2&limit=20

// Cursor-based (stable, performant)
GET /users?cursor=eyJpZCI6MTIzfQ&limit=20

// Response
{
  "data": [...],
  "meta": {
    "nextCursor": "eyJpZCI6MTQzfQ",
    "hasMore": true
  }
}

// Query parameters
GET /products?category=electronics&minPrice=100&maxPrice=500
GET /products?sort=-createdAt,name  // - prefix for descending

// Filter operators
GET /users?age[gte]=18&age[lte]=65
GET /users?status[in]=active,pending
GET /users?name[like]=john*

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!
  comments: [Comment!]!
}

# Connections for pagination (Relay spec)
type PostConnection {
  edges: [PostEdge!]!
  pageInfo: PageInfo!
}

type PostEdge {
  node: Post!
  cursor: String!
}

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

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

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

type CreateUserPayload {
  user: User
  errors: [Error!]
}

// N+1 prevention with DataLoader
const userLoader = new DataLoader(async (ids: string[]) => {
  const users = await db.users.findMany({ where: { id: { in: ids } } });
  return ids.map(id => users.find(u => u.id === id));
});

const resolvers = {
  Post: {
    author: (post, _, { loaders }) => loaders.user.load(post.authorId),
  },
};

// Field-level authorization
const resolvers = {
  User: {
    email: (user, _, { currentUser }) => {
      if (currentUser.id !== user.id && !currentUser.isAdmin) {
        return null; // Hide from other users
      }
      return user.email;
    },
  },
};

// Union types for expected errors
type CreatePostResult = Post | ValidationError | NotAuthorizedError

// Or use errors field in payload
type CreatePostPayload {
  post: Post
  errors: [CreatePostError!]
}

union CreatePostError = ValidationError | RateLimitError

syntax = "proto3";

package users.v1;

service UserService {
  rpc GetUser(GetUserRequest) returns (GetUserResponse);
  rpc ListUsers(ListUsersRequest) returns (ListUsersResponse);
  rpc CreateUser(CreateUserRequest) returns (CreateUserResponse);

  // Streaming
  rpc WatchUsers(WatchUsersRequest) returns (stream UserEvent);
  rpc BatchCreateUsers(stream CreateUserRequest) returns (BatchCreateResponse);
}

message User {
  string id = 1;
  string email = 2;
  string name = 3;
  google.protobuf.Timestamp created_at = 4;
}

message GetUserRequest {
  string id = 1;
}

message GetUserResponse {
  User user = 1;
}

message ListUsersRequest {
  int32 page_size = 1;
  string page_token = 2;
  UserFilter filter = 3;
}

message ListUsersResponse {
  repeated User users = 1;
  string next_page_token = 2;
}

# URL versioning (most common)
GET /v1/users
GET /v2/users

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

# Query parameter
GET /users?version=2

Non-Breaking (safe):
✅ Add new optional field
✅ Add new endpoint
✅ Add new optional query parameter
✅ Expand enum values (if client ignores unknown)

Breaking (requires new version):
❌ Remove field
❌ Rename field
❌ Change field type
❌ Make optional field required
❌ Change URL structure

// OpenAPI deprecation
/**
 * @deprecated Use /v2/users instead. Will be removed on 2025-06-01.
 */
app.get('/v1/users', ...);

// Response header
res.setHeader('Deprecation', 'true');
res.setHeader('Sunset', 'Sat, 01 Jun 2025 00:00:00 GMT');
res.setHeader('Link', '</v2/users>; rel="successor-version"');

// Server
wss.on('connection', (ws) => {
  ws.on('message', (data) => {
    const message = JSON.parse(data);

    switch (message.type) {
      case 'subscribe':
        subscriptions.add(ws, message.channel);
        break;
      case 'unsubscribe':
        subscriptions.remove(ws, message.channel);
        break;
    }
  });
});

// Client
const ws = new WebSocket('wss://api.example.com/ws');

ws.send(JSON.stringify({
  type: 'subscribe',
  channel: 'orders:user:123'
}));

ws.onmessage = (event) => {
  const data = JSON.parse(event.data);
  handleUpdate(data);
};

// Server
app.get('/events', (req, res) => {
  res.setHeader('Content-Type', 'text/event-stream');
  res.setHeader('Cache-Control', 'no-cache');
  res.setHeader('Connection', 'keep-alive');

  const sendEvent = (data: object) => {
    res.write(`data: ${JSON.stringify(data)}\n\n`);
  };

  // Subscribe to events
  eventEmitter.on('update', sendEvent);

  req.on('close', () => {
    eventEmitter.off('update', sendEvent);
  });
});

// Client
const source = new EventSource('/events');
source.onmessage = (event) => {
  const data = JSON.parse(event.data);
  handleUpdate(data);
};

openapi: 3.0.3
info:
  title: User API
  version: 1.0.0

paths:
  /users:
    get:
      summary: List users
      parameters:
        - name: limit
          in: query
          schema:
            type: integer
            default: 20
      responses:
        '200':
          description: Success
          content:
            application/json:
              schema:
                type: object
                properties:
                  data:
                    type: array
                    items:
                      $ref: '#/components/schemas/User'

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

// Rate limit headers
res.setHeader('X-RateLimit-Limit', '100');
res.setHeader('X-RateLimit-Remaining', '95');
res.setHeader('X-RateLimit-Reset', '1640000000');