---

name: cloudflare-durable-objects description: Build stateful, coordinated applications on Cloudflare's edge using Durable Objects.

Durable Objects

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

When to Use

Do NOT Use For

• Stateless request handling (use plain Workers)
• Maximum global distribution needs
• High fan-out independent requests

Core Principles

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

Wrangler Configuration

Copy// wrangler.jsonc
{
  "durable_objects": {
    "bindings": [{ "name": "MY_DO", "class_name": "MyDurableObject" }]
  },
  "migrations": [{ "tag": "v1", "new_sqlite_classes": ["MyDurableObject"] }]
}

Basic Durable Object Pattern

Copyimport { 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

Copy// 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

Copy// 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

Copy// 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

Copyexport 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

Copyimport { 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

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

By Partition Key

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

Parent-Child

Copy// 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

1. Keep DOs small - Model around natural coordination boundaries
2. Use SQL for structured data - SQLite is recommended over KV
3. Initialize schemas in constructor - Use blockConcurrencyWhile() once
4. Persist before response - Don't rely on in-memory state alone
5. Handle DO eviction - State is lost on eviction, storage persists
6. Use alarms for timeouts - Not external cron jobs
7. Test with @cloudflare/vitest-pool-workers - Full DO lifecycle testing

Attribution

Based on cloudflare/skills durable-objects skill.