Durable
Objects.

A DO is a class with a fetch() method. You access its transactional storage via this.ctx.storage. The binding in wrangler.toml lets Workers create and address DO instances.

# wrangler.toml
[[durable_objects.bindings]]
name = "COUNTER"
class_name = "Counter"

[[migrations]]
tag = "v1"
new_classes = ["Counter"]

// Counter Durable Object
export class Counter {
  constructor(ctx, env) {
    this.ctx = ctx;
  }

  async fetch(req) {
    let val = (await this.ctx.storage.get("count")) || 0;
    val++;
    await this.ctx.storage.put("count", val);
    return new Response(val.toString());
  }
}

Workers address DOs by name or unique ID. The idFromName() method maps a string to a deterministic ID — so "room:lobby" always routes to the same instance.

export default {
  async fetch(req, env) {
    const id = env.COUNTER.idFromName("global-counter");
    const stub = env.COUNTER.get(id);
    return stub.fetch(req);
  }
};

DOs can accept WebSocket connections and hibernate when idle. The webSocketMessage and webSocketClose handlers wake the object only when there's activity.

export class ChatRoom {
  async fetch(req) {
    const [client, server] = Object.values(new WebSocketPair());
    this.ctx.acceptWebSocket(server);
    return new Response(null, { status: 101, webSocket: client });
  }

  async webSocketMessage(ws, msg) {
    // Broadcast to all connected clients
    for (const conn of this.ctx.getWebSockets()) {
      conn.send(msg);
    }
  }
}