---
title: ratelimit.check_rate
summary: null
url: >-
  https://www.fastly.com/documentation/reference/vcl/functions/rate-limiting/ratelimit-check-rate
---


Increment an entry in a rate counter and check if the client has exceeded
some average number of requests per second (RPS).

The lowest rate limit supported by this feature to demonstrate effective
behavior is 10 RPS. Using a limit below 10 RPS may result in unpredictable
accuracy and detection time.

## Parameters

`entry` - The entry to keep track of. Typically `client.ip` and any associated
metadata. An entry can be, at maximum, 256 bytes long.

`rc` - The ratecounter that tracks requests. When a user makes a request, a
ratecounter tracks that users average rate of requests over time.

`delta` - The integer value to increment the entry in the rate counter by.
Typically, a value of 1 is used. The value must be between 0 and 100000.

`window` - The last N seconds of data over which to compute the requests-per-second.
Must be 1, 10, or 60.

`limit` - The RPS limit. If the user goes above this limit during the given
window, then they are penalized and their entry is put into the penalty box
`pb`. Must be between 10 and 70000000.

`pb` - The penalty box. Keeps track of penalized users. If a user is penalized,
their entry is put into this penalty box.

`ttl` - The penalty box TTL. Specifies how long a user remains penalized. Must
be between 1m and 1h, and has minute granularity (rounded to the nearest
minute).

## Return value

Upon completion, this function returns `true` if the user is penalized (i.e.,
should be rate limited) or `false` if not.

In the event an error occurs, the default return value is `false`; therefore,
errors (via `fastly.error`) must be handled specifically.

## Errors

If the given `entry` is longer than 256 bytes, then `false` is returned and
`fastly.error` is set to `EINVAL`.

## Example

The following example will:

check if the user has exceeded 100 requests per second during the last 10 seconds and, if so, penalize them for 10 minutes.

```vcl
penaltybox pbox { }
ratecounter rc { }

sub vcl_recv {
  if (ratelimit.check_rate(client.ip, rc, 1, 10, 100, pbox, 10m)) {
    error 429 "Too many requests";
  }
}
```