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.
Installation
Details
Usage
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name: durable-objects description: 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.
Durable Objects
Build stateful, coordinated applications on Cloudflare's edge using Durable Objects.
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
// 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_classesin 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
awaitbetween related storage writes (breaks atomicity) - Holding
blockConcurrencyWhile()acrossfetch()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);
});
});
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