---
name: durable-objects
description: Create and review Cloudflare Durable Objects. Use when building stateful coordination (chat rooms, multiplayer games, booking systems), implementing RPC methods, SQLite storage, alarms, WebSockets, or reviewing DO code for best practices. Covers Workers integration, wrangler config, and testing with Vitest.
---

# 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.**

| Resource       | URL                                                               |
| -------------- | ----------------------------------------------------------------- |
| Docs           | https://developers.cloudflare.com/durable-objects/                |
| API Reference  | https://developers.cloudflare.com/durable-objects/api/            |
| Best Practices | https://developers.cloudflare.com/durable-objects/best-practices/ |
| Examples       | https://developers.cloudflare.com/durable-objects/examples/       |

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

| Need                      | Example                                           |
| ------------------------- | ------------------------------------------------- |
| Coordination              | Chat rooms, multiplayer games, collaborative docs |
| Strong consistency        | Inventory, booking systems, turn-based games      |
| Per-entity storage        | Multi-tenant SaaS, per-user data                  |
| Persistent connections    | WebSockets, real-time notifications               |
| Scheduled work per entity | Subscription renewals, game timeouts              |

### Do NOT Use For

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

## Quick Reference

### Wrangler Configuration

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

### Basic Durable Object Pattern

```typescript
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

```typescript
// 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](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

```typescript
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)
  })
})
```
