Cloudflare Durable Objects

Status: Production Ready ✅ Last Updated: 2025-11-25 Dependencies: cloudflare-worker-base (recommended) Latest Versions: wrangler@4.50.0+, @cloudflare/workers-types@4.20251125.0+ Official Docs: https://developers.cloudflare.com/durable-objects/

Table of Contents

What are Durable Objects? • Quick Start • When to Load References • Class Structure • State API • WebSocket Hibernation • Alarms • RPC vs HTTP • Stubs & Routing • Migrations • Common Patterns • Critical Rules • Known Issues

What are Durable Objects?

Globally unique, stateful objects with single-point coordination, strong consistency (ACID), WebSocket Hibernation (thousands of connections), SQLite storage (1GB), and alarms API.

Use for: Chat rooms, multiplayer games, rate limiting, session management, leader election, stateful workflows

Quick Start (10 Minutes)

Option 1: Scaffold New DO Project

npm create cloudflare@latest my-durable-app -- \
  --template=cloudflare/durable-objects-template --ts --git --deploy false
cd my-durable-app && bun install && npm run dev

Option 2: Add to Existing Worker

1. Install types:

bun add -d @cloudflare/workers-types

2. Create DO class (src/counter.ts):

import { DurableObject } from 'cloudflare:workers';

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

export default Counter;  // CRITICAL

3. Configure (wrangler.jsonc):

{
  "durable_objects": {
    "bindings": [{ "name": "COUNTER", "class_name": "Counter" }]
  },
  "migrations": [
    { "tag": "v1", "new_sqlite_classes": ["Counter"] }
  ]
}

4. Call from Worker (src/index.ts):

import { Counter } from './counter';

interface Env {
  COUNTER: DurableObjectNamespace<Counter>;
}

export { Counter };

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

Deploy:

bunx wrangler deploy

---

When to Load References

Load immediately when user mentions:

• state-api-reference.md → "storage", "sql", "database", "query", "get/put", "KV", "1GB limit"
• websocket-hibernation.md → "websocket", "real-time", "chat", "hibernation", "serializeAttachment"
• alarms-api.md → "alarms", "scheduled tasks", "cron", "periodic", "batch processing"
• rpc-patterns.md → "RPC", "fetch", "HTTP", "methods", "routing"
• stubs-routing.md → "stubs", "idFromName", "newUniqueId", "location hints", "jurisdiction"
• migrations-guide.md → "migrations", "rename", "delete", "transfer", "schema changes"
• common-patterns.md → "patterns", "examples", "rate limiting", "sessions", "leader election"
• top-errors.md → errors, "not working", debugging, "binding not found"

Load proactively when:

• Building new feature → Load relevant pattern from common-patterns.md
• Debugging issue → Load top-errors.md first
• Implementing WebSocket → Load websocket-hibernation.md before coding
• Setting up storage → Load state-api-reference.md for SQL/KV APIs
• Creating first DO → Load stubs-routing.md for ID methods

---

Durable Object Class Structure

All DOs extend DurableObject and MUST be exported:

import { DurableObject } from 'cloudflare:workers';

export class MyDO extends DurableObject {
  constructor(ctx: DurableObjectState, env: Env) {
    super(ctx, env);  // Required first line
    // Keep minimal - heavy work blocks hibernation
    ctx.blockConcurrencyWhile(async () => {
      // Load from storage before handling requests
    });
  }

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

export default MyDO;  // CRITICAL: Must export

this.ctx provides: storage (SQL/KV), id (unique ID), waitUntil(), acceptWebSocket()

---

State API - Persistent Storage

Durable Objects provide two storage options:

SQL API (SQLite backend, recommended):

• Access via ctx.storage.sql
• Up to 1GB storage per instance
• SQL queries with transactions, indexes, cursors
• Atomic operations (deleteAll is all-or-nothing)
• Use new_sqlite_classes in migrations

Key-Value API (available on both backends):

• Access via ctx.storage (get/put/delete/list)
• Simple key-value operations
• Async transactions supported
• 128MB limit on KV backend, 1GB on SQLite

Quick example:

export class Counter 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 counts (key TEXT PRIMARY KEY, value INTEGER)');
  }

  async increment(): Promise<number> {
    this.sql.exec('INSERT OR REPLACE INTO counts (key, value) VALUES (?, ?)', 'count', 1);
    return this.sql.exec('SELECT value FROM counts WHERE key = ?', 'count').one<{value: number}>().value;
  }
}

Load references/state-api-reference.md for complete SQL and KV API documentation, cursor operations, transactions, parameterized queries, storage limits, and migration patterns.

---

WebSocket Hibernation API

Handle thousands of WebSocket connections per DO instance with automatic hibernation when idle (~10s no activity), saving duration costs. Connections stay open at the edge while DO sleeps.

CRITICAL Rules:

• ✅ Use ctx.acceptWebSocket(server) (enables hibernation)
• ✅ Use ws.serializeAttachment(data) to persist metadata across hibernation
• ✅ Restore connections in constructor with ctx.getWebSockets()
• ❌ Don't use ws.accept() (standard API, no hibernation)
• ❌ Don't use setTimeout/setInterval (prevents hibernation)

Handler methods: webSocketMessage(), webSocketClose(), webSocketError()

Quick pattern:

export class ChatRoom extends DurableObject {
  sessions: Map<WebSocket, any>;

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

    this.ctx.acceptWebSocket(server);  // ← Enables hibernation
    server.serializeAttachment({ userId: 'alice' });  // ← Persists across hibernation
    this.sessions.set(server, { userId: 'alice' });

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

  async webSocketMessage(ws: WebSocket, message: string): Promise<void> {
    const session = this.sessions.get(ws);
    // Broadcast to all
    this.sessions.forEach((_, w) => w.send(message));
  }
}

Load references/websocket-hibernation.md for complete handler patterns, hibernation lifecycle, serializeAttachment API, connection management, broadcasting patterns, and hibernation troubleshooting.

---

Alarms API - Scheduled Tasks

Schedule DO to wake up at a future time for batching, cleanup, reminders, or periodic tasks.

Core API:

• await ctx.storage.setAlarm(timestamp) - Schedule alarm
• await ctx.storage.getAlarm() - Get current alarm time (null if not set)
• await ctx.storage.deleteAlarm() - Cancel alarm
• async alarm(info) - Handler called when alarm fires

Key Features:

• ✅ Guaranteed at-least-once execution with automatic retries (up to 6)
• ✅ Survives hibernation and eviction
• ✅ Deleted automatically after successful execution
• ⚠️ Only ONE alarm per DO (setting new one overwrites previous)

Quick pattern:

export class Batcher extends DurableObject {
  async addItem(item: string): Promise<void> {
    await this.ctx.storage.put('items', [...existingItems, item]);

    // Schedule batch processing if not already scheduled
    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> {
    const items = await this.ctx.storage.get('items');
    await this.processBatch(items);  // Send to API, write to DB, etc.
    await this.ctx.storage.put('items', []);  // Clear buffer
  }
}

Load references/alarms-api.md for periodic alarms pattern, retry handling, error scenarios, cleanup jobs, and batching strategies.

---

RPC vs HTTP Fetch

RPC (Recommended): Call DO methods directly like await stub.increment(). Type-safe, simple, auto-serialization. Requires compatibility_date >= 2024-04-03.

HTTP Fetch: Traditional HTTP request/response with async fetch(request) handler. Required for WebSocket upgrades.

Quick comparison:

// RPC Pattern (simpler)
export class Counter extends DurableObject {
  async increment(): Promise<number> {  // ← Direct method
    let value = await this.ctx.storage.get<number>('count') || 0;
    return ++value;
  }
}
const count = await stub.increment();  // ← Direct call

// HTTP Fetch Pattern
export class Counter extends DurableObject {
  async fetch(request: Request): Promise<Response> {  // ← HTTP handler
    const url = new URL(request.url);
    if (url.pathname === '/increment') { /* ... */ }
  }
}
const response = await stub.fetch('/increment', { method: 'POST' });

Use RPC for: New projects, type safety, simple method calls Use HTTP Fetch for: WebSocket upgrades, complex routing, legacy code

Load references/rpc-patterns.md for complete RPC vs Fetch comparison, migration guide, error handling patterns, and method visibility control.

---

Creating Durable Object Stubs and Routing

To interact with a Durable Object from a Worker: get an ID → create a stub → call methods.

Three ID creation methods:

1. idFromName(name) - Named DOs (most common): Deterministic routing to same instance globally
2. newUniqueId() - Random IDs: New unique instance, must store ID for future access
3. idFromString(idString) - Recreate from saved ID string

Getting stubs:

// Method 1: From ID
const id = env.CHAT_ROOM.idFromName('room-123');
const stub = env.CHAT_ROOM.get(id);

// Method 2: Shortcut for named DOs (recommended)
const stub = env.CHAT_ROOM.getByName('room-123');

await stub.myMethod();

Geographic routing with location hints:

• Set locationHint option when creating stub: { locationHint: 'enam' }
• 9 regions: wnam, enam, sam, weur, eeur, apac, oc, afr, me
• Best-effort (not guaranteed), only affects first creation

Data residency with jurisdiction restrictions:

• Use newUniqueId({ jurisdiction: 'eu' }) or { jurisdiction: 'fedramp' }
• Strictly enforced (DO never leaves jurisdiction)
• Cannot combine with location hints
• Required for GDPR/FedRAMP compliance

Load references/stubs-routing.md for complete guide to ID methods, stub management, location hints, jurisdiction restrictions, use cases, best practices, and error handling patterns.

---

Migrations - Managing DO Classes

Migrations are REQUIRED when creating, renaming, deleting, or transferring DO classes between Workers.

Four migration types:

1. Create New DO: Use new_sqlite_classes (recommended, 1GB) or new_classes (legacy KV, 128MB)
2. Rename DO: Use renamed_classes with from/to mapping (data preserved, bindings forward)
3. Delete DO: Use deleted_classes (⚠️ immediate deletion, cannot undo, all storage lost)
4. Transfer DO: Use transferred_classes with from_script (moves instances to new Worker)

Quick example - Create new DO with SQLite:

{
  "durable_objects": {
    "bindings": [{ "name": "COUNTER", "class_name": "Counter" }]
  },
  "migrations": [
    {
      "tag": "v1",                    // Unique identifier (append-only)
      "new_sqlite_classes": ["Counter"]
    }
  ]
}

CRITICAL rules:

• ❌ Migrations are ATOMIC (all instances migrate at once, no gradual rollout)
• ❌ Cannot enable SQLite on existing KV-backed DOs (must create new class)
• ❌ Migration tags must be unique (cannot reuse, append-only)
• ✅ Code changes don't need migrations (only schema changes do)
• ✅ DO class names are unique per account (across all Workers)

Load references/migrations-guide.md for complete migration patterns, rename/delete/transfer procedures, rollback strategies, and migration gotchas.

---

Common Patterns

Four production-ready patterns for Cloudflare Durable Objects:

1. Rate Limiting - Per-user rate limiting with sliding window, KV storage for request tracking
2. Session Management - User sessions with TTL, SQL storage, automatic cleanup via alarms
3. Leader Election - Single leader guarantee using SQL constraints, heartbeat mechanism
4. Multi-DO Coordination - Game coordinator + game rooms pattern, parent-child DO relationships

Quick example - Rate limiter:

export class RateLimiter extends DurableObject {
  async checkLimit(userId: string, limit: number, window: number): Promise<boolean> {
    const requests = await this.ctx.storage.get<number[]>(`rate:${userId}`) || [];
    const validRequests = requests.filter(t => Date.now() - t < window);

    if (validRequests.length >= limit) return false;

    validRequests.push(Date.now());
    await this.ctx.storage.put(`rate:${userId}`, validRequests);
    return true;
  }
}

Load references/common-patterns.md for complete implementations of all 4 patterns with full code examples, SQL schemas, alarm usage, error handling, and best practices.

---

Critical Rules

✅ Always:

• Export DO class: export default MyDO
• Call super(ctx, env) first in constructor
• Use new_sqlite_classes in migrations (1GB vs 128MB KV)
• Use ctx.acceptWebSocket() for hibernation (not ws.accept())
• Persist state to storage (not just memory)
• Use alarms instead of setTimeout/setInterval
• Use parameterized SQL: sql.exec('... WHERE id = ?', id)
• Minimize constructor work, use blockConcurrencyWhile()

❌ Never:

• Create DO without migration (error)
• Forget to export class (binding not found)
• Use setTimeout/setInterval (prevents hibernation)
• Rely only on in-memory state for WebSockets (use serializeAttachment)
• Deploy migrations gradually (migrations are atomic)
• Enable SQLite on existing KV-backed DO (must create new class)
• Assume location hints are guaranteed (best-effort only)

---

Known Issues Prevention

This skill prevents 15+ documented issues. Top 3 most critical:

Issue #1: Class Not Exported

Error: "binding not found" | Why: DO class not exported Fix: export default MyDO;

Issue #2: Missing Migration

Error: "migrations required" | Why: Created DO without migration entry Fix: Add { "tag": "v1", "new_sqlite_classes": ["MyDO"] } to migrations

Issue #3: setTimeout Breaks Hibernation

Error: DO never hibernates, high charges | Why: setTimeout prevents hibernation Fix: Use await ctx.storage.setAlarm(Date.now() + 1000) instead

12 more issues covered: Wrong migration type, constructor overhead, in-memory state lost, outgoing WebSocket no hibernation, global uniqueness confusion, partial deleteAll, binding mismatch, state size exceeded, migration not atomic, location hint ignored, alarm retry failures, fetch blocks hibernation.

Load references/top-errors.md for complete error catalog with all 15+ issues, detailed prevention strategies, debugging steps, and resolution patterns.

---

Configuration & TypeScript

wrangler.jsonc structure:

{
  "durable_objects": {
    "bindings": [
      { "name": "COUNTER", "class_name": "Counter" }  // Binding name must match code
    ]
  },
  "migrations": [
    { "tag": "v1", "new_sqlite_classes": ["Counter"] }  // Required for new DOs
  ]
}

TypeScript types:

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);
  }
}

---

Official Docs: 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/ Questions? Load references/top-errors.md for common problems or check templates/ for working examples