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Use the Alarms API

Use the Durable Objects Alarms API to batch requests to a Durable Object.

This example implements an alarm() handler that allows batching of requests to a single Durable Object.

When a request is received and no alarm is set, it sets an alarm for 10 seconds in the future. The alarm() handler processes all requests received within that 10-second window.

If no new requests are received, no further alarms will be set until the next request arrives.

import { DurableObject } from 'cloudflare:workers';


// Worker
export default {
  async fetch(request, env) {
    let id = env.BATCHER.idFromName("foo");
    return await env.BATCHER.get(id).fetch(request);
  },
};


const SECONDS = 10;


// Durable Object
export class Batcher extends DurableObject {
  constructor(state, env) {
    this.state = state;
    this.storage = state.storage;
    this.state.blockConcurrencyWhile(async () => {
      let vals = await this.storage.list({ reverse: true, limit: 1 });
      this.count = vals.size == 0 ? 0 : parseInt(vals.keys().next().value);
    });
  }


  async fetch(request) {
    this.count++;


    // If there is no alarm currently set, set one for 10 seconds from now
    // Any further POSTs in the next 10 seconds will be part of this batch.
    let currentAlarm = await this.storage.getAlarm();
    if (currentAlarm == null) {
      this.storage.setAlarm(Date.now() + (1000 * SECONDS));
    }


    // Add the request to the batch.
    await this.storage.put(this.count, await request.text());
    return new Response(JSON.stringify({ queued: this.count }), {
      headers: {
        "content-type": "application/json;charset=UTF-8",
      },
    });
  }


  async alarm() {
    let vals = await this.storage.list();
    await fetch("http://example.com/some-upstream-service", {
      method: "POST",
      body: Array.from(vals.values()),
    });
    await this.storage.deleteAll();
    this.count = 0;
  }
}

The alarm() handler will be called once every 10 seconds. If an unexpected error terminates the Durable Object, the alarm() handler will be re-instantiated on another machine. Following a short delay, the alarm() handler will run from the beginning on the other machine.

Finally, configure your Wrangler file to include a Durable Object binding and migration based on the namespace and class name chosen previously.

{
  "name": "durable-object-alarm",
  "durable_objects": {
    "bindings": [
      {
        "name": "BATCHER",
        "class_name": "Batcher"
      }
    ]
  },
  "migrations": [
    {
      "tag": "v1",
      "new_classes": [
        "Batcher"
      ]
    }
  ]
}