Agent skill

durable-objects

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. Biases towards retrieval from Cloudflare docs over pre-trained knowledge.

Stars 766
Forks 85

Install this agent skill to your Project

npx add-skill https://github.com/cloudflare/skills/tree/main/skills/durable-objects

SKILL.md

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Your knowledge of Durable Objects APIs and configuration may be outdated.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

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 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

typescript
// 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

typescript
// 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

typescript
// 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

typescript
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);
  });
});

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