---
title: Build a rate limiter · Cloudflare Durable Objects docs
description: Build a rate limiter using Durable Objects and Workers.
source_url:
  html: https://kate-fixes-httprbi.preview.developers.cloudflare.com/durable-objects/examples/build-a-rate-limiter/index.md
  md: https://kate-fixes-httprbi.preview.developers.cloudflare.com/durable-objects/examples/build-a-rate-limiter/
---

This example shows how to build a rate limiter using Durable Objects and Workers that can be used to protect upstream resources, including third-party APIs that your application relies on and/or services that may be costly for you to invoke.

This example also discusses some decisions that need to be made when designing a system, such as a rate limiter, with Durable Objects.

The Worker creates a `RateLimiter` Durable Object on a per IP basis to protect upstream resources. IP based rate limiting can be effective without negatively impacting latency because any given IP will remain within a small geographic area colocated with the `RateLimiter` Durable Object. Furthermore, throughput is also improved because each IP gets its own Durable Object.

It might seem simpler to implement a global rate limiter, `const id = env.RATE_LIMITER.idFromName("global");`, which can provide better guarantees on the request rate to the upstream resource. However:

* This would require all requests globally to make a sub-request to a single Durable Object.
* Implementing a global rate limiter would add additional latency for requests not colocated with the Durable Object, and global throughput would be capped to the throughput of a single Durable Object.
* A single Durable Object that all requests rely on is typically considered an anti-pattern. Durable Objects work best when they are scoped to a user, room, service and/or the specific subset of your application that requires global co-ordination.

Note

If you do not need unique or custom rate-limiting capabilities, refer to [Rate limiting rules](https://developers.cloudflare.com/waf/rate-limiting-rules/) that are part of Cloudflare's Web Application Firewall (WAF) product.

The Durable Object uses a token bucket algorithm to implement rate limiting. The naive idea is that each request requires a token to complete, and the tokens are replenished according to the reciprocal of the desired number of requests per second. As an example, a 1000 requests per second rate limit will have a token replenished every millisecond (as specified by milliseconds\_per\_request) up to a given capacity limit.

This example uses Durable Object's [Alarms API](https://developers.cloudflare.com/durable-objects/api/alarms) to schedule the Durable Object to be woken up at a time in the future.

* When the alarm's scheduled time comes, the `alarm()` handler method is called, and in this case, the alarm will add a token to the "Bucket".
* The implementation is made more efficient by adding tokens in bulk (as specified by milliseconds\_for\_updates) and preventing the alarm handler from being invoked every millisecond. More frequent invocations of Durable Objects will lead to higher invocation and duration charges.

The first implementation of a rate limiter is below:

* JavaScript

  ```js
  import { DurableObject } from "cloudflare:workers";


  // Worker
  export default {
    async fetch(request, env, _ctx) {
      // Determine the IP address of the client
      const ip = request.headers.get("CF-Connecting-IP");
      if (ip === null) {
        return new Response("Could not determine client IP", { status: 400 });
      }


      // Obtain an identifier for a Durable Object based on the client's IP address
      const id = env.RATE_LIMITER.idFromName(ip);


      try {
        const stub = env.RATE_LIMITER.get(id);
        const milliseconds_to_next_request =
          await stub.getMillisecondsToNextRequest();
        if (milliseconds_to_next_request > 0) {
          // Alternatively one could sleep for the necessary length of time
          return new Response("Rate limit exceeded", { status: 429 });
        }
      } catch (error) {
        return new Response("Could not connect to rate limiter", { status: 502 });
      }


      // TODO: Implement me
      return new Response("Call some upstream resource...");
    },
  };


  // Durable Object
  export class RateLimiter extends DurableObject {
    static milliseconds_per_request = 1;
    static milliseconds_for_updates = 5000;
    static capacity = 10000;


    constructor(ctx, env) {
      super(ctx, env);
      this.tokens = RateLimiter.capacity;
    }


    async getMillisecondsToNextRequest() {
      this.checkAndSetAlarm();


      let milliseconds_to_next_request = RateLimiter.milliseconds_per_request;
      if (this.tokens > 0) {
        this.tokens -= 1;
        milliseconds_to_next_request = 0;
      }


      return milliseconds_to_next_request;
    }


    async checkAndSetAlarm() {
      let currentAlarm = await this.ctx.storage.getAlarm();
      if (currentAlarm == null) {
        this.ctx.storage.setAlarm(
          Date.now() +
            RateLimiter.milliseconds_for_updates *
              RateLimiter.milliseconds_per_request,
        );
      }
    }


    async alarm() {
      if (this.tokens < RateLimiter.capacity) {
        this.tokens = Math.min(
          RateLimiter.capacity,
          this.tokens + RateLimiter.milliseconds_for_updates,
        );
        this.checkAndSetAlarm();
      }
    }
  }
  ```

* TypeScript

  ```ts
  import { DurableObject } from "cloudflare:workers";


  export interface Env {
    RATE_LIMITER: DurableObjectNamespace<RateLimiter>;
  }


  // Worker
  export default {
    async fetch(request, env, _ctx): Promise<Response> {
      // Determine the IP address of the client
      const ip = request.headers.get("CF-Connecting-IP");
      if (ip === null) {
        return new Response("Could not determine client IP", { status: 400 });
      }


      // Obtain an identifier for a Durable Object based on the client's IP address
      const id = env.RATE_LIMITER.idFromName(ip);


      try {
        const stub = env.RATE_LIMITER.get(id);
        const milliseconds_to_next_request =
          await stub.getMillisecondsToNextRequest();
        if (milliseconds_to_next_request > 0) {
          // Alternatively one could sleep for the necessary length of time
          return new Response("Rate limit exceeded", { status: 429 });
        }
      } catch (error) {
        return new Response("Could not connect to rate limiter", { status: 502 });
      }


      // TODO: Implement me
      return new Response("Call some upstream resource...");
    },
  } satisfies ExportedHandler<Env>;


  // Durable Object
  export class RateLimiter extends DurableObject {
    static readonly milliseconds_per_request = 1;
    static readonly milliseconds_for_updates = 5000;
    static readonly capacity = 10000;


    tokens: number;


    constructor(ctx: DurableObjectState, env: Env) {
      super(ctx, env);
      this.tokens = RateLimiter.capacity;
    }


    async getMillisecondsToNextRequest(): Promise<number> {
      this.checkAndSetAlarm();


      let milliseconds_to_next_request = RateLimiter.milliseconds_per_request;
      if (this.tokens > 0) {
        this.tokens -= 1;
        milliseconds_to_next_request = 0;
      }


      return milliseconds_to_next_request;
    }


    private async checkAndSetAlarm() {
      let currentAlarm = await this.ctx.storage.getAlarm();
      if (currentAlarm == null) {
        this.ctx.storage.setAlarm(
          Date.now() +
            RateLimiter.milliseconds_for_updates *
              RateLimiter.milliseconds_per_request,
        );
      }
    }


    async alarm() {
      if (this.tokens < RateLimiter.capacity) {
        this.tokens = Math.min(
          RateLimiter.capacity,
          this.tokens + RateLimiter.milliseconds_for_updates,
        );
        this.checkAndSetAlarm();
      }
    }
  }
  ```

While the token bucket algorithm is popular for implementing rate limiting and uses Durable Object features, there is a simpler approach:

* JavaScript

  ```js
  import { DurableObject } from "cloudflare:workers";


  // Durable Object
  export class RateLimiter extends DurableObject {
    static milliseconds_per_request = 1;
    static milliseconds_for_grace_period = 5000;


    constructor(ctx, env) {
      super(ctx, env);
      this.nextAllowedTime = 0;
    }


    async getMillisecondsToNextRequest() {
      const now = Date.now();


      this.nextAllowedTime = Math.max(now, this.nextAllowedTime);
      this.nextAllowedTime += RateLimiter.milliseconds_per_request;


      const value = Math.max(
        0,
        this.nextAllowedTime - now - RateLimiter.milliseconds_for_grace_period,
      );
      return value;
    }
  }
  ```

* TypeScript

  ```ts
  import { DurableObject } from "cloudflare:workers";


  // Durable Object
  export class RateLimiter extends DurableObject {
    static milliseconds_per_request = 1;
    static milliseconds_for_grace_period = 5000;


    nextAllowedTime: number;


    constructor(ctx: DurableObjectState, env: Env) {
      super(ctx, env);
      this.nextAllowedTime = 0;
    }


    async getMillisecondsToNextRequest(): Promise<number> {
      const now = Date.now();


      this.nextAllowedTime = Math.max(now, this.nextAllowedTime);
      this.nextAllowedTime += RateLimiter.milliseconds_per_request;


      const value = Math.max(
        0,
        this.nextAllowedTime - now - RateLimiter.milliseconds_for_grace_period,
      );
      return value;
    }
  }
  ```

Finally, configure your Wrangler file to include a Durable Object [binding](https://developers.cloudflare.com/durable-objects/get-started/#4-configure-durable-object-bindings) and [migration](https://developers.cloudflare.com/durable-objects/reference/durable-objects-migrations/) based on the namespace and class name chosen previously.

* wrangler.jsonc

  ```jsonc
  {
    "name": "my-counter",
    "main": "src/index.ts",
    "durable_objects": {
      "bindings": [
        {
          "name": "RATE_LIMITER",
          "class_name": "RateLimiter"
        }
      ]
    },
    "migrations": [
      {
        "tag": "v1",
        "new_sqlite_classes": [
          "RateLimiter"
        ]
      }
    ]
  }
  ```

* wrangler.toml

  ```toml
  name = "my-counter"
  main = "src/index.ts"


  [[durable_objects.bindings]]
  name = "RATE_LIMITER"
  class_name = "RateLimiter"


  [[migrations]]
  tag = "v1"
  new_sqlite_classes = ["RateLimiter"]
  ```

### Related resources

* Learn more about Durable Object's [Alarms API](https://developers.cloudflare.com/durable-objects/api/alarms) and how to configure alarms.
* [Understand how to troubleshoot](https://developers.cloudflare.com/durable-objects/observability/troubleshooting/) common errors related with Durable Objects.
* Review how [Durable Objects are priced](https://developers.cloudflare.com/durable-objects/platform/pricing/), including pricing examples.
