npm create cloudflare@latest my-durable-app -- --template=cloudflare/durable-objects-template --ts

// src/counter.ts - Durable Object class
import { DurableObject } from 'cloudflare:workers';

export class Counter extends DurableObject {
  async increment(): Promise<number> {
    let value = (await this.ctx.storage.get<number>('value')) || 0;
    await this.ctx.storage.put('value', ++value);
    return value;
  }
}
export default Counter;  // CRITICAL: Export required

// wrangler.jsonc - Configuration
{
  "durable_objects": {
    "bindings": [{ "name": "COUNTER", "class_name": "Counter" }]
  },
  "migrations": [
    { "tag": "v1", "new_sqlite_classes": ["Counter"] }  // SQLite backend (10GB limit)
  ]
}

// src/index.ts - Worker
import { Counter } from './counter';
export { Counter };

export default {
  async fetch(request: Request, env: { COUNTER: DurableObjectNamespace<Counter> }) {
    const stub = env.COUNTER.getByName('global-counter');  // Aug 2025: getByName() shortcut
    return new Response(`Count: ${await stub.increment()}`);
  }
};

import { DurableObject } from 'cloudflare:workers';

export class MyDO extends DurableObject {
  constructor(ctx: DurableObjectState, env: Env) {
    super(ctx, env);  // REQUIRED first line

    // Load state before requests (optional)
    ctx.blockConcurrencyWhile(async () => {
      this.value = await ctx.storage.get('key') || defaultValue;
    });
  }

  // RPC methods (recommended)
  async myMethod(): Promise<string> { return 'Hello'; }

  // HTTP fetch handler (optional)
  async fetch(request: Request): Promise<Response> { return new Response('OK'); }
}

export default MyDO;  // CRITICAL: Export required

// Worker must export DO class too
import { MyDO } from './my-do';
export { MyDO };

{ "migrations": [{ "tag": "v1", "new_sqlite_classes": ["MyDO"] }] }

export class MyDO extends DurableObject {
  sql: SqlStorage;

  constructor(ctx: DurableObjectState, env: Env) {
    super(ctx, env);
    this.sql = ctx.storage.sql;

    this.sql.exec(`
      CREATE TABLE IF NOT EXISTS messages (id INTEGER PRIMARY KEY, text TEXT, created_at INTEGER);
      CREATE INDEX IF NOT EXISTS idx_created ON messages(created_at);
      PRAGMA optimize;  // Feb 2025: Query performance optimization
    `);
  }

  async addMessage(text: string): Promise<number> {
    const cursor = this.sql.exec('INSERT INTO messages (text, created_at) VALUES (?, ?) RETURNING id', text, Date.now());
    return cursor.one<{ id: number }>().id;
  }

  async getMessages(limit = 50): Promise<any[]> {
    return this.sql.exec('SELECT * FROM messages ORDER BY created_at DESC LIMIT ?', limit).toArray();
  }
}

// Single operations
await this.ctx.storage.put('key', value);
const value = await this.ctx.storage.get<T>('key');
await this.ctx.storage.delete('key');

// Batch operations
await this.ctx.storage.put({ key1: val1, key2: val2 });
const map = await this.ctx.storage.get(['key1', 'key2']);
await this.ctx.storage.delete(['key1', 'key2']);

// List and delete all
const map = await this.ctx.storage.list({ prefix: 'user:', limit: 100 });
await this.ctx.storage.deleteAll();  // Atomic on SQLite only

// Transactions
await this.ctx.storage.transaction(async (txn) => {
  await txn.put('key1', val1);
  await txn.put('key2', val2);
});

export class ChatRoom extends DurableObject {
  sessions: Map<WebSocket, { userId: string; username: string }>;

  constructor(ctx: DurableObjectState, env: Env) {
    super(ctx, env);
    this.sessions = new Map();

    // CRITICAL: Restore WebSocket metadata after hibernation
    ctx.getWebSockets().forEach((ws) => {
      this.sessions.set(ws, ws.deserializeAttachment());
    });
  }

  async fetch(request: Request): Promise<Response> {
    const pair = new WebSocketPair();
    const [client, server] = Object.values(pair);

    const url = new URL(request.url);
    const metadata = { userId: url.searchParams.get('userId'), username: url.searchParams.get('username') };

    // CRITICAL: Use ctx.acceptWebSocket(), NOT ws.accept()
    this.ctx.acceptWebSocket(server);
    server.serializeAttachment(metadata);  // Persist across hibernation
    this.sessions.set(server, metadata);

    return new Response(null, { status: 101, webSocket: client });
  }

  async webSocketMessage(ws: WebSocket, message: string | ArrayBuffer): Promise<void> {
    const session = this.sessions.get(ws);
    // Handle message (max 32 MiB since Oct 2025)
  }

  async webSocketClose(ws: WebSocket, code: number, reason: string, wasClean: boolean): Promise<void> {
    this.sessions.delete(ws);
    ws.close(code, 'Closing');
  }

  async webSocketError(ws: WebSocket, error: any): Promise<void> {
    this.sessions.delete(ws);
  }
}

export class Batcher extends DurableObject {
  async addItem(item: string): Promise<void> {
    // Add to buffer
    const buffer = await this.ctx.storage.get<string[]>('buffer') || [];
    buffer.push(item);
    await this.ctx.storage.put('buffer', buffer);

    // Schedule alarm if not set
    if ((await this.ctx.storage.getAlarm()) === null) {
      await this.ctx.storage.setAlarm(Date.now() + 10000);  // 10 seconds
    }
  }

  async alarm(info: { retryCount: number; isRetry: boolean }): Promise<void> {
    if (info.retryCount > 3) return;  // Give up after 3 retries

    const buffer = await this.ctx.storage.get<string[]>('buffer') || [];
    await this.processBatch(buffer);
    await this.ctx.storage.put('buffer', []);
    // Alarm auto-deleted after success
  }
}

// DO class
export class Counter extends DurableObject {
  async increment(): Promise<number> {
    let value = (await this.ctx.storage.get<number>('count')) || 0;
    await this.ctx.storage.put('count', ++value);
    return value;
  }
}

// Worker calls
const stub = env.COUNTER.getByName('my-counter');
const count = await stub.increment();  // Type-safe!

// DO class
export class Counter extends DurableObject {
  async fetch(request: Request): Promise<Response> {
    const url = new URL(request.url);
    if (url.pathname === '/increment') {
      let value = (await this.ctx.storage.get<number>('count')) || 0;
      await this.ctx.storage.put('count', ++value);
      return new Response(JSON.stringify({ count: value }));
    }
    return new Response('Not found', { status: 404 });
  }
}

// Worker calls
const stub = env.COUNTER.getByName('my-counter');
const response = await stub.fetch('https://fake-host/increment', { method: 'POST' });
const data = await response.json();

const stub = env.CHAT_ROOM.getByName('room-123');  // Aug 2025: Shortcut for idFromName + get
// Use for: chat rooms, user sessions, per-tenant logic, singletons

const id = env.MY_DO.newUniqueId({ jurisdiction: 'eu' });  // Optional: EU compliance
const idString = id.toString();  // Save to KV/D1 for later

const id = env.MY_DO.idFromString(await env.KV.get('session:123'));
const stub = env.MY_DO.get(id);

const stub = env.MY_DO.get(id, { locationHint: 'enam' });  // wnam, enam, sam, weur, eeur, apac, oc, afr, me

const id = env.MY_DO.newUniqueId({ jurisdiction: 'eu' });  // Options: 'eu', 'fedramp'
// Cannot combine with location hints, higher latency outside jurisdiction

{ "migrations": [{ "tag": "v1", "new_sqlite_classes": ["Counter"] }] }  // SQLite 10GB
// Or: "new_classes": ["Counter"]  // KV 128MB (legacy)

{ "migrations": [
  { "tag": "v1", "new_sqlite_classes": ["OldName"] },
  { "tag": "v2", "renamed_classes": [{ "from": "OldName", "to": "NewName" }] }
]}

{ "migrations": [
  { "tag": "v1", "new_sqlite_classes": ["Counter"] },
  { "tag": "v2", "deleted_classes": ["Counter"] }  // Immediate deletion, cannot undo
]}

{ "migrations": [{ "tag": "v1", "transferred_classes": [
  { "from": "OldClass", "from_script": "old-worker", "to": "NewClass" }
]}]}

async checkLimit(userId: string, limit: number, window: number): Promise<boolean> {
  const requests = (await this.ctx.storage.get<number[]>(`rate:${userId}`)) || [];
  const valid = requests.filter(t => Date.now() - t < window);
  if (valid.length >= limit) return false;
  valid.push(Date.now());
  await this.ctx.storage.put(`rate:${userId}`, valid);
  return true;
}

async set(key: string, value: any, ttl?: number): Promise<void> {
  const expiresAt = ttl ? Date.now() + ttl : null;
  this.sql.exec('INSERT OR REPLACE INTO session (key, value, expires_at) VALUES (?, ?, ?)',
    key, JSON.stringify(value), expiresAt);
}

async alarm(): Promise<void> {
  this.sql.exec('DELETE FROM session WHERE expires_at < ?', Date.now());
  await this.ctx.storage.setAlarm(Date.now() + 3600000);  // Hourly cleanup
}

async electLeader(workerId: string): Promise<boolean> {
  try {
    this.sql.exec('INSERT INTO leader (id, worker_id, elected_at) VALUES (1, ?, ?)', workerId, Date.now());
    return true;
  } catch { return false; }  // Already has leader
}

// Coordinator delegates to child DOs
const gameRoom = env.GAME_ROOM.getByName(gameId);
await gameRoom.initialize();
await this.ctx.storage.put(`game:${gameId}`, { created: Date.now() });

export class MyDO extends DurableObject { }
export default MyDO;  // Required

constructor(ctx: DurableObjectState, env: Env) {
  super(ctx, env);  // Required first line
}

{ "tag": "v1", "new_sqlite_classes": ["MyDO"] }

this.ctx.acceptWebSocket(server);  // Enables hibernation

await this.ctx.storage.put('important', value);

await this.ctx.storage.setAlarm(Date.now() + 60000);

this.sql.exec('SELECT * FROM table WHERE id = ?', id);

constructor(ctx, env) {
  super(ctx, env);
  // Minimal initialization only
  ctx.blockConcurrencyWhile(async () => {
    // Load from storage
  });
}

// Missing migrations array = error

class MyDO extends DurableObject { }
// Missing: export default MyDO;

setTimeout(() => {}, 1000);  // Prevents hibernation

// ❌ WRONG: this.sessions will be lost on hibernation
// ✅ CORRECT: Use serializeAttachment()

undefined

❌ **Enable SQLite on existing KV-backed DO**
```jsonc
// Not supported - must create new DO class instead

ws.accept();  // ❌ No hibernation
this.ctx.acceptWebSocket(ws);  // ✅ Hibernation enabled

// Location hints are best-effort only

export class MyDO extends DurableObject { }
export default MyDO;  // ← Required

{
  "migrations": [
    { "tag": "v1", "new_sqlite_classes": ["MyDO"] }
  ]
}

constructor(ctx, env) {
  super(ctx, env);
  ctx.blockConcurrencyWhile(async () => {
    // Load from storage
  });
}

// ❌ WRONG
setTimeout(() => {}, 1000);

// ✅ CORRECT
await this.ctx.storage.setAlarm(Date.now() + 1000);

ws.serializeAttachment({ userId, username });

// Restore in constructor
ctx.getWebSockets().forEach(ws => {
  const metadata = ws.deserializeAttachment();
  this.sessions.set(ws, metadata);
});

async alarm(info: { retryCount: number }): Promise<void> {
  await this.ctx.storage.deleteAlarm();  // ← Call first
  await this.ctx.storage.deleteAll();    // Then delete all
}

{ "bindings": [{ "name": "MY_DO", "class_name": "MyDO" }] }

env.MY_DO.getByName('instance');  // Must match binding name

async alarm(info: { retryCount: number }): Promise<void> {
  if (info.retryCount > 3) {
    console.error('Giving up after 3 retries');
    return;
  }
  // Idempotent operation
}