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Bun rate limiting reference

Arcjet rate limiting allows you to define rules which limit the number of requests a client can make over a period of time.

Configuration options

Each rate limit is configured on an exact path with a set of client characteristics and algorithm specific options.

Fixed window rate limit options

Tracks the number of requests made by a client over a fixed time window. Options are explained in the Configuration documentation. See the fixed window algorithm description for more details about how the algorithm works.

// Options for fixed window rate limit
// See https://docs.arcjet.com/rate-limiting/configuration
type FixedWindowRateLimitOptions = {
mode?: "LIVE" | "DRY_RUN"; // "LIVE" will block requests. "DRY_RUN" will log only
match?: string; // request path the rate limit applies to
characteristics?: string[]; // how the client is identified. Defaults to the global characteristics if unset
window: string; // time window the rate limit applies to
max: number; // maximum number of requests allowed in the time window
};

Fixed window example

import arcjet, { fixedWindow } from "@arcjet/bun";
import { env } from "bun";
const aj = arcjet({
key: env.ARCJET_KEY!, // Get your site key from https://app.arcjet.com
characteristics: ["ip.src"], // track requests by IP address
rules: [
fixedWindow({
mode: "LIVE", // will block requests. Use "DRY_RUN" to log only
match: "/api/hello", // match all requests to /api/hello
window: "60s", // 60 second fixed window
max: 100, // allow a maximum of 100 requests
}),
],
});

Sliding window rate limit options

Tracks the number of requests made by a client over a sliding window so that the window moves with time. Options are explained in the Configuration documentation. See the sliding window algorithm description for more details about how the algorithm works.

// Options for sliding window rate limit
// See https://docs.arcjet.com/rate-limiting/configuration
type SlidingWindowRateLimitOptions = {
mode?: "LIVE" | "DRY_RUN"; // "LIVE" will block requests. "DRY_RUN" will log only
match?: string; // request path the rate limit applies to
characteristics?: string[]; // how the client is identified. Defaults to the global characteristics if unset
interval: number; // the time interval in seconds for the rate limit
max: number; // maximum number of requests allowed over the time interval
};

Sliding window example

import arcjet, { slidingWindow } from "@arcjet/bun";
import { env } from "bun";
const aj = arcjet({
key: env.ARCJET_KEY!, // Get your site key from https://app.arcjet.com
characteristics: ["ip.src"], // track requests by IP address
rules: [
slidingWindow({
mode: "LIVE", // will block requests. Use "DRY_RUN" to log only
match: "/api/hello", // match all requests to /api/hello
interval: 60, // 60 second sliding window
max: 100, // allow a maximum of 100 requests
}),
],
});

Token bucket rate limit options

Based on a bucket filled with a specific number of tokens. Each request withdraws a token from the bucket and the bucket is refilled at a fixed rate. Once the bucket is empty, the client is blocked until the bucket refills. Options are explained in the Configuration documentation. See the token bucket algorithm description for more details about how the algorithm works.

// Options for token bucket rate limit
// See https://docs.arcjet.com/rate-limiting/configuration
type TokenBucketRateLimitOptions = {
mode?: "LIVE" | "DRY_RUN"; // "LIVE" will block requests. "DRY_RUN" will log only
match?: string; // request path the rate limit applies to
characteristics?: string[]; // how the client is identified. Defaults to the global characteristics if unset
refillRate: number; // number of tokens to add to the bucket at each interval
interval: number; // the interval in seconds to add tokens to the bucket
capacity: number; // the maximum number of tokens the bucket can hold
};

Token bucket example

See the token bucket example for how to specify the number of tokens to request.

import arcjet, { tokenBucket } from "@arcjet/bun";
import { env } from "bun";
const aj = arcjet({
key: env.ARCJET_KEY!, // Get your site key from https://app.arcjet.com
characteristics: ["ip.src"], // track requests by IP address
rules: [
tokenBucket({
mode: "LIVE", // will block requests. Use "DRY_RUN" to log only
match: "/api/hello", // match all requests to /api/hello
refillRate: 10, // refill 10 tokens per interval
interval: 60, // 60 second interval
capacity: 100, // bucket maximum capacity of 100 tokens
}),
],
});

Identifying users

Rate limit rules use characteristics to identify the client and apply the limit across requests. The default is to use the client’s IP address. However, you can specify other characteristics such as a user ID or other metadata from your application.

In this example we define a rate limit rule that applies to a specific user ID. The custom characteristic is userId with the value passed as a prop on the protect function. You can use any string for the characteristic name and any string, number or boolean for the value.

import arcjet, { fixedWindow } from "@arcjet/bun";
import { env } from "bun";
const aj = arcjet({
key: env.ARCJET_KEY!, // Get your site key from https://app.arcjet.com
// Define a custom userId characteristic.
// See https://docs.arcjet.com/rate-limiting/configuration#characteristics
characteristics: ["userId"],
rules: [
fixedWindow({
mode: "LIVE",
window: "1h",
max: 60,
}),
],
});
export default {
port: 3000,
fetch: aj.handler(async (req) => {
// Pass userId as a string to identify the user. This could also be a number
// or boolean value
const decision = await aj.protect(req, { userId: "user123" });
console.log("Arcjet decision", decision);
if (decision.isDenied()) {
return new Response("Too many requests", { status: 429 });
}
return new Response("Hello world");
}),
};

Rules

The arcjet client is configured with one or more rules which take one or many of the above options.

Example - single rate limit

Set a single rate limit rule on the /api/hello API route that applies a 60 request limit per hour per IP address (the default if no characteristics are specified).

import arcjet, { slidingWindow } from "@arcjet/bun";
import { env } from "bun";
const aj = arcjet({
key: env.ARCJET_KEY!, // Get your site key from https://app.arcjet.com
characteristics: ["ip.src"], // track requests by IP address
rules: [
slidingWindow({
mode: "LIVE", // will block requests. Use "DRY_RUN" to log only
match: "/api/hello", // match all requests to /api/hello
interval: 60, // 60 second sliding window
max: 100, // allow a maximum of 100 requests
}),
],
});

Example - dry run mode for new rules

Rate limits can be combined in the arcjet client which allows you to test new configurations in dry run mode first before enabling them in live mode. You can inspect the results of each rule by logging them or using the Arcjet Dashboard.

import arcjet, { fixedWindow } from "@arcjet/bun";
import { env } from "bun";
const aj = arcjet({
key: env.ARCJET_KEY!, // Get your site key from https://app.arcjet.com
characteristics: ["ip.src"],
rules: [
fixedWindow(
// This rule is live
{
mode: "LIVE",
match: "/api/hello",
window: "1h",
max: 60,
},
// This rule is in dry run mode, so will log but not block
{
mode: "DRY_RUN",
match: "/api/hello",
// Setting the characteristics in the rule itself overrides the global
// setting
characteristics: ['http.request.headers["x-api-key"]'],
window: "1h",
// max could also be a dynamic value applied after looking up a limit
// elsewhere e.g. in a database for the authenticated user
max: 600,
},
),
],
});

Decision

Arcjet provides a single protect function that is used to execute your protection rules. This requires a request argument which is the request context as passed to the request handler.

This function returns a Promise that resolves to an ArcjetDecision object. This contains the following properties:

  • id (string) - The unique ID for the request. This can be used to look up the request in the Arcjet dashboard. It is prefixed with req_ for decisions involving the Arcjet cloud API. For decisions taken locally, the prefix is lreq_.
  • conclusion (ArcjetConclusion) - The final conclusion based on evaluating each of the configured rules. If you wish to accept Arcjet’s recommended action based on the configured rules then you can use this property.
  • reason (ArcjetReason) - An object containing more detailed information about the conclusion.
  • results (ArcjetRuleResult[]) - An array of ArcjetRuleResult objects containing the results of each rule that was executed.
  • ip (ArcjetIpDetails) - An object containing Arcjet’s analysis of the client IP address. See IP analysis in the SDK reference for more information.

See the SDK reference for more details about the rule results.

You check if a deny conclusion has been returned by a rate limit rule by using decision.isDenied() and decision.reason.isRateLimit().

You can iterate through the results and check whether a rate limit was applied:

for (const result of decision.results) {
console.log("Rule Result", result);
}

This example will log the full result as well as each rate limit rule:

import arcjet, { detectBot, fixedWindow } from "@arcjet/bun";
import { env } from "bun";
const aj = arcjet({
key: env.ARCJET_KEY!, // Get your site key from https://app.arcjet.com
characteristics: ["ip.src"],
rules: [
fixedWindow({
mode: "LIVE",
window: "1h",
max: 60,
}),
detectBot({
mode: "LIVE",
block: ["AUTOMATED", "LIKELY_AUTOMATED"],
}),
],
});
export default {
port: 3000,
fetch: aj.handler(async (req) => {
const decision = await aj.protect(req);
for (const result of decision.results) {
console.log("Rule Result", result);
if (result.reason.isRateLimit()) {
console.log("Rate limit rule", result);
}
if (result.reason.isBot()) {
console.log("Bot protection rule", result);
}
}
if (decision.isDenied()) {
if (decision.reason.isRateLimit()) {
return new Response("Too many requests", { status: 429 });
} else {
return new Response("Forbidden", { status: 403 });
}
}
return new Response("Hello world");
}),
};

Token bucket request

When using a token bucket rule, an additional requested prop should be passed to the protect function. This is the number of tokens the client is requesting to withdraw from the bucket.

import arcjet, { tokenBucket } from "@arcjet/bun";
import { env } from "bun";
const aj = arcjet({
key: env.ARCJET_KEY!, // Get your site key from https://app.arcjet.com
characteristics: ["ip.src"],
rules: [
tokenBucket({
mode: "LIVE",
refillRate: 40_000,
interval: "1d",
capacity: 40_000,
}),
],
});
export default {
port: 3000,
fetch: aj.handler(async (req) => {
const decision = await aj.protect(req, { requested: 50 });
console.log("Arcjet decision", decision);
if (decision.isDenied()) {
return new Response("Too many requests", { status: 429 });
}
return new Response("Hello world");
}),
};

Rate limit headers

With a rate limit rule enabled, you can access additional metadata in every Arcjet decision result:

  • max (number): The configured maximum number of requests applied to this request.
  • remaining (number): The number of requests remaining before max is reached within the window.
  • window (number): The total amount of seconds in which requests are counted.
  • reset (number): The remaining amount of seconds in the window.

These can be used to return RateLimit HTTP headers (draft RFC) to offer the client more detail.

We provide the @arcjet/decorate package for decorating your responses with appropriate RateLimit headers based on a decision.

import arcjet, { fixedWindow, type ArcjetDecision } from "@arcjet/bun";
import { setRateLimitHeaders } from "@arcjet/decorate";
import { env } from "bun";
const aj = arcjet({
key: env.ARCJET_KEY!, // Get your site key from https://app.arcjet.com
// Tracking by ip.src is the default if not specified
//characteristics: ["ip.src"],
rules: [
fixedWindow({
mode: "LIVE",
window: "1h",
max: 60,
}),
],
});
const ResponseWithRateLimit = (
body: string,
init: ResponseInit,
decision: ArcjetDecision,
) => {
const res = new Response(body, init);
setRateLimitHeaders(res, decision);
return res;
};
export default {
port: 3000,
fetch: aj.handler(async (req) => {
const decision = await aj.protect(req);
console.log("Arcjet decision", decision);
if (decision.isDenied()) {
return ResponseWithRateLimit(
"Too many requests",
{ status: 429 },
decision,
);
}
return ResponseWithRateLimit("Hello world", { status: 200 }, decision);
}),
};

Error handling

Arcjet is designed to fail open so that a service issue or misconfiguration does not block all requests. The SDK will also time out and fail open after 500ms when ARCJET_ENV is "production" and 1000ms otherwise. However, in most cases, the response time will be less than 20-30ms.

If there is an error condition, Arcjet will return an ERROR conclusion.

import arcjet, { tokenBucket } from "@arcjet/bun";
import { env } from "bun";
const aj = arcjet({
key: env.ARCJET_KEY!, // Get your site key from https://app.arcjet.com
characteristics: ["userId"], // track requests by a custom user ID
rules: [
// Create a token bucket rate limit. Other algorithms are supported.
tokenBucket({
mode: "LIVE", // will block requests. Use "DRY_RUN" to log only
refillRate: 5, // refill 5 tokens per interval
interval: 10, // refill every 10 seconds
capacity: 10, // bucket maximum capacity of 10 tokens
}),
],
});
export default {
port: 3000,
fetch: aj.handler(async (req) => {
const userId = "user123"; // Replace with your authenticated user ID
const decision = await aj.protect(req, { userId, requested: 5 }); // Deduct 5 tokens from the bucket
console.log("Arcjet decision", decision);
if (decision.isErrored()) {
// Fail open by logging the error and continuing
console.warn("Arcjet error", decision.reason.message);
// You could also fail closed here for very sensitive routes
//return new Response("Service unavailable", { status: 503 });
}
if (decision.isDenied()) {
return new Response("Too many requests", { status: 429 });
}
return new Response("Hello world");
}),
};

Examples

Rate limit by IP address

The example below shows how to configure a rate limit on a single API route. It applies a limit of 60 requests per hour per IP address. If the limit is exceeded, the client is blocked for 10 minutes before being able to make any further requests.

Applying a rate limit by IP address is the default if no characteristics are specified.

import arcjet, { fixedWindow } from "@arcjet/bun";
import { env } from "bun";
const aj = arcjet({
key: env.ARCJET_KEY!, // Get your site key from https://app.arcjet.com
// Tracking by ip.src is the default if not specified
//characteristics: ["ip.src"],
rules: [
fixedWindow({
mode: "LIVE",
window: "1h",
max: 60,
}),
],
});
export default {
port: 3000,
fetch: aj.handler(async (req) => {
const decision = await aj.protect(req);
console.log("Arcjet decision", decision);
if (decision.isDenied()) {
return new Response("Too many requests", { status: 429 });
}
return new Response("Hello world");
}),
};

Rate limit by IP address with custom response

The example below is the same as the one above. However this example also shows a customized response rather than the default

import arcjet, { fixedWindow } from "@arcjet/bun";
import { env } from "bun";
const aj = arcjet({
key: env.ARCJET_KEY!, // Get your site key from https://app.arcjet.com
// Tracking by ip.src is the default if not specified
//characteristics: ["ip.src"],
rules: [
fixedWindow({
mode: "LIVE",
window: "1h",
max: 60,
}),
],
});
export default {
port: 3000,
fetch: aj.handler(async (req) => {
const decision = await aj.protect(req);
console.log("Arcjet decision", decision);
if (decision.isDenied()) {
if (decision.reason.isRateLimit()) {
return new Response("Too many requests", { status: 429 });
} else {
return new Response("Forbidden", { status: 403 });
}
}
return new Response("Hello world");
}),
};

Rate limit by API key header

APIs are commonly protected by keys. You may wish to apply a rate limit based on the key, regardless of which IPs the requests come from. To achieve this, you can specify the characteristics Arcjet will use to track the limit.

The example below shows how to configure a rate limit on a single API route. It applies a limit of 60 requests per hour per API key, where the key is provided in a custom header called x-api-key. If the limit is exceeded, the client is blocked for 10 minutes before being able to make any further requests.

import arcjet, { fixedWindow } from "@arcjet/bun";
import { env } from "bun";
const aj = arcjet({
key: env.ARCJET_KEY!, // Get your site key from https://app.arcjet.com
characteristics: ['http.request.headers["x-api-key"]'],
rules: [
fixedWindow({
mode: "LIVE",
window: "1h",
max: 60,
}),
],
});