durable-objects

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---
name: durable-objects
description: Build, debug, or review Cloudflare Durable Objects code for persistent state and coordination.
---

# Durable Objects

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

## Retrieval Sources

Your knowledge of Durable Objects APIs and configuration may be outdated. **Prefer retrieval over pre-training** for any Durable Objects task.

| 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’s Vitest integration
- Designing sharding strategies and parent-child relationships

## Reference Documentation

- `./references/rules.md` - Core rules, storage, concurrency, RPC, alarms
- [Testing reference](./references/testing.md) - Current Vitest documentation, migration choices, and test selection
- `./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** - Not fetch() handler (compatibility date >= 2024-04-03)
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 glo