Durable Objects

Build stateful, coordinated applications on Cloudflare's edge using Durable Objects.

When to Use

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

Core Principles

• 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

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 }); }, };

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();

WebSocket Handler

export class ChatRoom extends DurableObject<Env> { async webSocketMessage(ws: WebSocket, message: string) { // Broadcast to all connected clients for (const client of this.ctx.getWebSockets()) { if (client !== ws) { client.send(message); } } }

async webSocketClose(ws: WebSocket) { // Clean up connection state } }

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); }); });

Sharding Strategies

By Entity ID

// Each user gets their own DO const stub = env.USER_DO.getByName(user-${userId});

By Partition Key

// Shard by region or tenant const stub = env.DATA_DO.getByName(${region}-${tenantId});

Parent-Child

// Parent coordinates, children store data const parent = env.COORDINATOR.getByName("main"); const childId = await parent.getShardFor(key); const child = env.DATA_DO.get(childId);

Best Practices

• Keep DOs small - Model around natural coordination boundaries

• Use SQL for structured data - SQLite is recommended over KV

• Initialize schemas in constructor - Use blockConcurrencyWhile() once

• Persist before response - Don't rely on in-memory state alone

• Handle DO eviction - State is lost on eviction, storage persists

• Use alarms for timeouts - Not external cron jobs

• Test with @cloudflare/vitest-pool-workers

• Full DO lifecycle testing

Attribution

Based on cloudflare/skills durable-objects skill.