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Your knowledge of Durable Objects APIs and configuration may be outdated.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

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 - 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 casesconst stub = env.MY_DO.getByName("room-123");
// From existing ID stringconst id = env.MY_DO.idFromString(storedIdString);const stub = env.MY_DO.get(id);
// New unique ID - store mapping externallyconst 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);
// Handlerasync alarm(): Promise<void> {  // Process scheduled work  // Optionally reschedule: await this.ctx.storage.setAlarm(...)}
// Cancelawait this.ctx.storage.deleteAlarm();

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);  });});