durable-objects

$2a

• 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

// 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

• Model around coordination atoms - One DO per chat room/game/user, not one global DO

• **Use getByName() for deterministic routing** - Same input = same DO instance

• Use SQLite storage - Configure new_sqlite_classes in migrations

• Initialize in constructor - Use blockConcurrencyWhile() for schema setup only

• Use RPC methods - Not fetch() handler (compatibility date >= 2024-04-03)

• Persist first, cache second - Always write to storage before updating in-memory state

• 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 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);

  });

});