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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

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 = {
// "LIVE" will block requests. "DRY_RUN" will log only
mode?: "LIVE" | "DRY_RUN";
// How the client is identified. We recommend setting characteristics on the
// Arcjet client rather than per rule. Global characteristics default to the
// request IP
characteristics?: string[];
// Time window the rate limit applies to
window: string;
// Maximum number of requests allowed in the time window
max: number;
};

Fixed window example

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 = {
// "LIVE" will block requests. "DRY_RUN" will log only
mode?: "LIVE" | "DRY_RUN";
// How the client is identified. We recommend setting characteristics on the
// Arcjet client rather than per rule. Global characteristics default to the
// request IP
characteristics?: string[];
// The time interval in seconds for the rate limit
interval: number;
// Maximum number of requests allowed over the time interval
max: number;
};

Sliding window example

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 = {
// "LIVE" will block requests. "DRY_RUN" will log only
mode?: "LIVE" | "DRY_RUN";
// How the client is identified. We recommend setting characteristics on the
// Arcjet client rather than per rule. Global characteristics default to the
// request IP
characteristics?: string[];
// Number of tokens to add to the bucket at each interval
refillRate: number;
// The interval in seconds to add tokens to the bucket
interval: number;
// The maximum number of tokens the bucket can hold
capacity: number;
};

Token bucket example

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

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.

To identify users with different characteristics e.g. IP address for anonymous users and a user ID for logged in users, you can create a custom fingerprint. See the example in the custom characteristics section.

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).

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.

Decision

Arcjet provides a single protect function that is used to execute your protection rules. This requires a RequestEvent property which is the event 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 the SDK reference for more information.

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:

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.

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.

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 1000ms when NODE_ENV or ARCJET_ENV is development and 500ms 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.

Testing

Arcjet runs the same in any environment, including locally and in CI. You can use the mode set to DRY_RUN to log the results of rule execution without blocking any requests.

We have an example test framework you can use to automatically test your rules. Arcjet can also be triggered based using a sample of your traffic.

See the Testing section of the docs for details.

Discussion