Cloudflare Durable Objects

Cloudflare Durable Objects

Status: Production Ready ✅ Last Updated: 2025-11-23 Dependencies: cloudflare-worker-base (recommended) Latest Versions: wrangler@4.50.0, @cloudflare/workers-types@4.20251121.0 Official Docs: https://developers.cloudflare.com/durable-objects/

Recent Updates (2025):

• Oct 2025: WebSocket message size 1 MiB → 32 MiB, Data Studio UI for SQLite DOs (view/edit storage in dashboard)
• Aug 2025: getByName() API shortcut for named DOs
• June 2025: @cloudflare/actors library (beta) - recommended SDK with migrations, alarms, Actor class pattern
• May 2025: Python Workers support for Durable Objects
• April 2025: SQLite GA with 10GB storage (beta → GA, 1GB → 10GB), Free tier access
• Feb 2025: PRAGMA optimize support, improved error diagnostics with reference IDs

Quick Start

Scaffold new DO project:

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

Or add to existing Worker:

// 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()}`);
  }
};

DO Class Essentials

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 };

Constructor Rules:

• ✅ Call super(ctx, env) first
• ✅ Keep minimal - heavy work blocks hibernation wake
• ✅ Use ctx.blockConcurrencyWhile() for storage initialization
• ❌ Never setTimeout/setInterval (use alarms)
• ❌ Don't rely on in-memory state with WebSockets (persist to storage)

Storage API

Two backends available:

• SQLite (recommended): 10GB storage, SQL queries, atomic operations, PITR
• KV: 128MB storage, key-value only

Enable SQLite in migrations:

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

SQL API (SQLite backend)

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();
  }
}

SQL Methods:

• sql.exec(query, ...params) → cursor
• cursor.one<T>() → single row (throws if none)
• cursor.one<T>({ allowNone: true }) → row or null
• cursor.toArray<T>() → all rows
• ctx.storage.transactionSync(() => { ... }) → atomic multi-statement

Rules: Always use ? placeholders, create indexes, use PRAGMA optimize after schema changes

Key-Value API (both backends)

// 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);
});

Storage Limits: SQLite 10GB (April 2025 GA) | KV 128MB

WebSocket Hibernation API

Capabilities:

• Thousands of WebSocket connections per instance
• Hibernate when idle (~10s no activity) to save costs
• Auto wake-up when messages arrive
• Message size limit: 32 MiB (Oct 2025, up from 1 MiB)

How it works:

1. Active → handles messages
2. Idle → ~10s no activity
3. Hibernation → in-memory state cleared, WebSockets stay connected
4. Wake → message arrives → constructor runs → handler called

CRITICAL: In-memory state is lost on hibernation. Use serializeAttachment() to persist per-WebSocket metadata.

Hibernation-Safe Pattern

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);
  }
}

Hibernation Rules:

• ✅ ctx.acceptWebSocket(ws) - enables hibernation
• ✅ ws.serializeAttachment(data) - persist metadata
• ✅ ctx.getWebSockets().forEach() - restore in constructor
• ✅ Use alarms instead of setTimeout/setInterval
• ❌ ws.accept() - standard API, no hibernation
• ❌ setTimeout/setInterval - prevents hibernation
• ❌ In-progress fetch() - blocks hibernation

Alarms API

Schedule DO to wake at future time. Use for: batching, cleanup, reminders, periodic tasks.

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
  }
}

API Methods:

• await ctx.storage.setAlarm(Date.now() + 60000) - set alarm (overwrites existing)
• await ctx.storage.getAlarm() - get timestamp or null
• await ctx.storage.deleteAlarm() - cancel alarm
• async alarm(info) - handler called when alarm fires

Behavior:

• ✅ At-least-once execution, auto-retries (up to 6x, exponential backoff)
• ✅ Survives hibernation/eviction
• ✅ Auto-deleted after success
• ⚠️ One alarm per DO (new alarm overwrites)

RPC vs HTTP Fetch

RPC (Recommended): Direct method calls, type-safe, simple

// 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!

HTTP Fetch: Request/response pattern, required for WebSocket upgrades

// 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();

When to use: RPC for new projects (simpler), HTTP Fetch for WebSocket upgrades or complex routing

Getting DO Stubs

Three ways to get IDs:

1. idFromName(name) - Consistent routing (same name = same DO)

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

2. newUniqueId() - Random unique ID (must store for reuse)

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

3. idFromString(idString) - Recreate from saved ID

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

Location hints (best-effort):

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

Jurisdiction (strict enforcement):

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

Migrations

Required for: create, rename, delete, transfer DO classes

1. Create:

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

2. Rename:

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

3. Delete:

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

4. Transfer:

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

Migration Rules:

• ❌ Atomic (all instances migrate at once, no gradual rollout)
• ❌ Tags are unique and append-only
• ❌ Cannot enable SQLite on existing KV-backed DOs
• ✅ Code changes don't need migrations (only schema changes)
• ✅ Class names globally unique per account

Common Patterns

Rate Limiting:

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;
}

Session Management with TTL:

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
}

Leader Election:

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
}

Multi-DO Coordination:

// 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() });

Critical Rules

Always Do

✅ Export DO class from Worker

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

✅ Call super(ctx, env) in constructor

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

✅ Use new_sqlite_classes for new DOs

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

✅ Use ctx.acceptWebSocket() for hibernation

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

✅ Persist critical state to storage (not just memory)

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

✅ Use alarms instead of setTimeout/setInterval

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

✅ Use parameterized SQL queries

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

✅ Minimize constructor work

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

Never Do

❌ Create DO without migration

// Missing migrations array = error

❌ Forget to export DO class

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

❌ Use setTimeout or setInterval

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

❌ Rely only on in-memory state with WebSockets

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

❌ Deploy migrations gradually

# Migrations are atomic - cannot use gradual rollout

❌ Enable SQLite on existing KV-backed DO

// Not supported - must create new DO class instead

❌ Use standard WebSocket API expecting hibernation

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

❌ Assume location hints are guaranteed

// Location hints are best-effort only

Known Issues Prevention

This skill prevents 15+ documented issues:

Issue #1: Class Not Exported

Error: "binding not found" or "Class X not found" Source: https://developers.cloudflare.com/durable-objects/get-started/ Why It Happens: DO class not exported from Worker Prevention:

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

Issue #2: Missing Migration

Error: "migrations required" or "no migration found for class" Source: https://developers.cloudflare.com/durable-objects/reference/durable-objects-migrations/ Why It Happens: Created DO class without migration entry Prevention: Always add migration when creating new DO class

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

Issue #3: Wrong Migration Type (KV vs SQLite)

Error: Schema errors, storage API mismatch Source: https://developers.cloudflare.com/durable-objects/api/sqlite-storage-api/ Why It Happens: Used new_classes instead of new_sqlite_classes Prevention: Use new_sqlite_classes for SQLite backend (recommended)

Issue #4: Constructor Overhead Blocks Hibernation Wake

Error: Slow hibernation wake-up times Source: https://developers.cloudflare.com/durable-objects/best-practices/access-durable-objects-storage/ Why It Happens: Heavy work in constructor Prevention: Minimize constructor, use blockConcurrencyWhile()

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

Issue #5: setTimeout Breaks Hibernation

Error: DO never hibernates, high duration charges Source: https://developers.cloudflare.com/durable-objects/concepts/durable-object-lifecycle/ Why It Happens: setTimeout/setInterval prevents hibernation Prevention: Use alarms API instead

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

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

Issue #6: In-Memory State Lost on Hibernation

Error: WebSocket metadata lost, state reset unexpectedly Source: https://developers.cloudflare.com/durable-objects/best-practices/websockets/ Why It Happens: Relied on in-memory state that's cleared on hibernation Prevention: Use serializeAttachment() for WebSocket metadata

ws.serializeAttachment({ userId, username });

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

Issue #7: Outgoing WebSocket Cannot Hibernate

Error: High charges despite hibernation API Source: https://developers.cloudflare.com/durable-objects/best-practices/websockets/ Why It Happens: Outgoing WebSockets don't support hibernation Prevention: Only use hibernation for server-side (incoming) WebSockets

Issue #8: Global Uniqueness Confusion

Error: Unexpected DO class name conflicts Source: https://developers.cloudflare.com/durable-objects/platform/known-issues/#global-uniqueness Why It Happens: DO class names are globally unique per account Prevention: Understand DO class names are shared across all Workers in account

Issue #9: Partial deleteAll on KV Backend

Error: Storage not fully deleted, billing continues Source: https://developers.cloudflare.com/durable-objects/api/legacy-kv-storage-api/ Why It Happens: KV backend deleteAll() can fail partially Prevention: Use SQLite backend for atomic deleteAll

Issue #10: Binding Name Mismatch

Error: Runtime error accessing DO binding Source: https://developers.cloudflare.com/durable-objects/get-started/ Why It Happens: Binding name in wrangler.jsonc doesn't match code Prevention: Ensure consistency

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

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

Issue #11: State Size Exceeded

Error: "state limit exceeded" or storage errors Source: https://developers.cloudflare.com/durable-objects/platform/pricing/ Why It Happens: Exceeded 1GB (SQLite) or 128MB (KV) limit Prevention: Monitor storage size, implement cleanup with alarms

Issue #12: Migration Not Atomic

Error: Gradual deployment blocked Source: https://developers.cloudflare.com/workers/configuration/versions-and-deployments/gradual-deployments/ Why It Happens: Tried to use gradual rollout with migrations Prevention: Migrations deploy atomically across all instances

Issue #13: Location Hint Ignored

Error: DO created in wrong region Source: https://developers.cloudflare.com/durable-objects/reference/data-location/ Why It Happens: Location hints are best-effort, not guaranteed Prevention: Use jurisdiction for strict requirements

Issue #14: Alarm Retry Failures

Error: Tasks lost after alarm failures Source: https://developers.cloudflare.com/durable-objects/api/alarms/ Why It Happens: Alarm handler throws errors repeatedly Prevention: Implement idempotent alarm handlers

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

Issue #15: Fetch Blocks Hibernation

Error: DO never hibernates despite using hibernation API Source: https://developers.cloudflare.com/durable-objects/concepts/durable-object-lifecycle/ Why It Happens: In-progress fetch() requests prevent hibernation Prevention: Ensure all async I/O completes before idle period

Configuration & Types

wrangler.jsonc:

{
  "compatibility_date": "2025-11-23",
  "durable_objects": {
    "bindings": [{ "name": "COUNTER", "class_name": "Counter" }]
  },
  "migrations": [
    { "tag": "v1", "new_sqlite_classes": ["Counter"] },
    { "tag": "v2", "renamed_classes": [{ "from": "Counter", "to": "CounterV2" }] }
  ]
}

TypeScript:

import { DurableObject, DurableObjectState, DurableObjectNamespace } from 'cloudflare:workers';

interface Env { MY_DO: DurableObjectNamespace<MyDurableObject>; }

export class MyDurableObject extends DurableObject<Env> {
  constructor(ctx: DurableObjectState, env: Env) {
    super(ctx, env);
    this.sql = ctx.storage.sql;
  }
}

Official Documentation

• Durable Objects: https://developers.cloudflare.com/durable-objects/
• State API (SQL): https://developers.cloudflare.com/durable-objects/api/sqlite-storage-api/
• WebSocket Hibernation: https://developers.cloudflare.com/durable-objects/best-practices/websockets/
• Alarms API: https://developers.cloudflare.com/durable-objects/api/alarms/
• Migrations: https://developers.cloudflare.com/durable-objects/reference/durable-objects-migrations/
• Best Practices: https://developers.cloudflare.com/durable-objects/best-practices/
• Pricing: https://developers.cloudflare.com/durable-objects/platform/pricing/

Questions? Issues?

1. Check references/top-errors.md for common problems
2. Review templates/ for working examples
3. Consult official docs: https://developers.cloudflare.com/durable-objects/
4. Verify migrations configuration carefully