Cloudflare Durable Objects

# 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Your knowledge of Durable Objects APIs and configuration may be outdated.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); }); }); ```