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 ## Retrieval Sources Your knowledge of Durable Objects APIs and configuration may be outdated.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’s Vitest integration - Designing sharding strategies and parent-child relationships ## Reference Documentation - `./references/rules.md` - Core rules, storage, concurrency, RPC, alarms - [Testing reference](./references/testing.md) - Current Vitest documentation, migration choices, and test selection - `./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 Read the [testing reference](./references/testing.md) before configuring a suite or writing Durable Object tests. It routes to current setup, APIs, and examples and identifies the behavior to cover.