> ## Documentation Index
> Fetch the complete documentation index at: https://upstash-vector.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Ratelimiting Algorithms

We provide different algorithms to use out of the box. Each has pros and cons.

## Fixed Window

This algorithm divides time into fixed durations/windows. For example each
window is 10 seconds long. When a new request comes in, the current time is used
to determine the window and a counter is increased. If the counter is larger
than the set limit, the request is rejected.

#### Pros

* Very cheap in terms of data size and computation
* Newer requests are not starved due to a high burst in the past

#### Cons

* Can cause high bursts at the window boundaries to leak through
* Causes request stampedes if many users are trying to access your server,
  whenever a new window begins

#### Usage

Create a new ratelimiter, that allows 10 requests per 10 seconds.

```ts
const ratelimit = new Ratelimit({
  redis: Redis.fromEnv(),
  limiter: Ratelimit.fixedWindow(10, "10 s"),
  analytics: true,
});
```

## Sliding Window

Builds on top of fixed window but instead of a fixed window, we use a rolling
window. Take this example: We have a rate limit of 10 requests per 1 minute. We
divide time into 1 minute slices, just like in the fixed window algorithm.
Window 1 will be from 00:00:00 to 00:01:00 (HH:MM:SS). Let's assume it is
currently 00:01:15 and we have received 4 requests in the first window and 5
requests so far in the current window. The approximation to determine if the
request should pass works like this:

```ts
limit = 10

// 4 request from the old window, weighted + requests in current window
rate = 4 * ((60 - 15) / 60) + 5 = 8

return rate < limit // True means we should allow the request
```

#### Pros

* Solves the issue near boundary from fixed window.

#### Cons

* More expensive in terms of storage and computation
* Is only an approximation, because it assumes a uniform request flow in the
  previous window, but this is fine in most cases

#### Usage

Create a new ratelimiter, that allows 10 requests per 10 seconds.

```ts
const ratelimit = new Ratelimit({
  redis: Redis.fromEnv(),
  limiter: Ratelimit.slidingWindow(10, "10 s"),
  analytics: true,
});
```

## Token Bucket

*Not yet supported for `MultiRegionRatelimit`*

Consider a bucket filled with `{maxTokens}` tokens that refills constantly at
`{refillRate}` per `{interval}`. Every request will remove one token from the
bucket and if there is no token to take, the request is rejected.

#### Pros

* Bursts of requests are smoothed out and you can process them at a constant
  rate.
* Allows to set a higher initial burst limit by setting `maxTokens` higher than
  `refillRate`

#### Cons

* Expensive in terms of computation

#### Usage

Create a new bucket, that refills 5 tokens every 10 seconds and has a maximum
size of 10.

```ts
const ratelimit = new Ratelimit({
  redis: Redis.fromEnv(),
  limiter: Ratelimit.tokenBucket(5, "10 s", 10),
  analytics: true,
});
```