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Your knowledge of Durable Objects APIs and configuration may be outdated.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