/durable-objects

name: durable-objects
description: Build, debug, or review Cloudflare Durable Objects code for persistent state and coordination.
license: Apache-2.0

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Your knowledge of Durable Objects APIs and configuration may be outdated.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

// 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** - 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 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);

Storage Operations

// SQL (synchronous, recommended)
this.ctx.storage.sql.exec("INSERT INTO t (c) VALUES (?)", value);
const rows = this.ctx.storage.sql.exec<Row>("SELECT * FROM t").toArray();

// KV (async)
await this.ctx.storage.put("key", value);
const val = await this.ctx.storage.get<Type>("key");

Alarms

// Schedule (replaces existing)
await this.ctx.storage.setAlarm(Date.now() + 60_000);

// Handler
async alarm(): Promise<void> {
  // Process scheduled work
  // Optionally reschedule: await this.ctx.storage.setAlarm(...)
}

// Cancel
await this.ctx.storage.deleteAlarm();

Testing

Read the [testing reference](./references/testing.md) before configuring a suite or writing Durable Object tests. It routes to current setup, APIs, and examples and identifies the behavior to cover.