durable-objects

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. Prefer current Cloudflare docs over memory for runtime, storage, RPC, and migration details.

Routing

• Use agents-sdk when the code uses Cloudflare Agents SDK classes such as Agent, AIChatAgent, McpAgent, or Code Mode.
• Use this skill for raw Durable Objects, storage, RPC methods, alarms, WebSockets, sharding, Workers integration, and DO tests.

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

Retrieval Rules

Check current Cloudflare Durable Objects documentation before using uncertain syntax, compatibility-date behavior, SQLite storage details, alarms, WebSockets, or RPC features. Read existing wrangler.jsonc/wrangler.toml and compatibility date before edits.

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

// 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
8. Keep atomic state changes together - Avoid await between logically related storage writes; use synchronous SQL operations or explicit transactions where appropriate

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
• Using an Agent SDK abstraction when a raw Durable Object is required, or vice versa, without explaining the tradeoff

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 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);
  });
});