Durable Objects

Build stateful, coordinated applications on Cloudflare's edge using Durable Objects.

Retrieval-First Development

Prefer retrieval from official docs over pre-training for Durable Objects tasks.

Fetch the relevant doc page when implementing features.

When to Use

• Creating new Durable Object classes for stateful coordination
• Implementing RPC methods, alarms, or WebSocket handlers
• Reviewing existing DO code for best practices
• Configuring wrangler.jsonc/toml for DO bindings and migrations
• Writing tests with @cloudflare/vitest-pool-workers
• Designing sharding strategies and parent-child relationships

Reference Documentation

• ./references/rules.md - Core rules, storage, concurrency, RPC, alarms
• ./references/testing.md - Vitest setup, unit/integration tests, alarm testing
• ./references/workers.md - Workers handlers, types, wrangler config, observability

Search: blockConcurrencyWhile, idFromName, getByName, setAlarm, sql.exec

Core Principles

Use Durable Objects For

Do NOT Use For

• Stateless request handling (use plain Workers)
• Maximum global distribution needs
• High fan-out independent requests

Quick Reference

Wrangler Configuration

// wrangler.jsonc
{
  "durable_objects": {
    "bindings": [{ "name": "MY_DO", "class_name": "MyDurableObject" }],
  },
  "migrations": [{ "tag": "v1", "new_sqlite_classes": ["MyDurableObject"] }],
}

Basic Durable Object Pattern

import { DurableObject } from 'cloudflare:workers'

export interface Env {
  MY_DO: DurableObjectNamespace<MyDurableObject>
}

export class MyDurableObject extends DurableObject<Env> {
  constructor(ctx: DurableObjectState, env: Env) {
    super(ctx, env)
    ctx.blockConcurrencyWhile(async () => {
      this.ctx.storage.sql.exec(`
        CREATE TABLE IF NOT EXISTS items (
          id INTEGER PRIMARY KEY AUTOINCREMENT,
          data TEXT NOT NULL
        )
      `)
    })
  }

  async addItem(data: string): Promise<number> {
    const result = this.ctx.storage.sql.exec<{ id: number }>(
      'INSERT INTO items (data) VALUES (?) RETURNING id',
      data
    )
    return result.one().id
  }
}

export default {
  async fetch(request: Request, env: Env): Promise<Response> {
    const stub = env.MY_DO.getByName('my-instance')
    const id = await stub.addItem('hello')
    return Response.json({ id })
  },
}

Critical Rules

1. Model around coordination atoms - One DO per chat room/game/user, not one global DO
2. Use `getByName()` for deterministic routing - Same input = same DO instance
3. Use SQLite storage - Configure new_sqlite_classes in migrations
4. Initialize in constructor - Use blockConcurrencyWhile() for schema setup only
5. Use RPC methods - Use RPC for DO-to-DO communication (compatibility date >= 2024-04-03). The outer Worker can still use the standard fetch() handler to route to the DO stub.
6. Persist first, cache second - Always write to storage before updating in-memory state
7. One alarm per DO - setAlarm() replaces any existing alarm

Anti-Patterns (NEVER)

• Single global DO handling all requests (bottleneck)
• Using blockConcurrencyWhile() on every request (kills throughput)
• Storing critical state only in memory (lost on eviction/crash)
• Using await between related storage writes (breaks atomicity)
• Holding blockConcurrencyWhile() across fetch() or external I/O

Stub Creation

// Deterministic - preferred for most cases
const stub = env.MY_DO.getByName('room-123')

// From existing ID string
const id = env.MY_DO.idFromString(storedIdString)
const stub = env.MY_DO.get(id)

// New unique ID - store mapping externally
const id = env.MY_DO.newUniqueId()
const stub = env.MY_DO.get(id)

Advanced Features

See references/advanced_features.md for detailed examples of Storage Operations (SQL and KV) and Alarms scheduling.

Security

• RPC Access Control: RPC methods are public to any caller with a stub. Implement authorization inside the method body (e.g., check this.env.AUTH_TOKEN).
• WebSockets: Validate WebSocket origin in onConnect() to prevent cross-site hijacking.

Testing Quick Start

import { env } from 'cloudflare:test'
import { describe, it, expect } from 'vitest'

describe('MyDO', () => {
  it('should work', async () => {
    const stub = env.MY_DO.getByName('test')
    const result = await stub.addItem('test')
    expect(result).toBe(1)
  })
})