Ratelimiters

Ratelimiters are mechanisms used in software systems to control the rate at which a user or a system can access a resource or perform an action. This is essential for preventing abuse, ensuring fair usage, and maintaining the stability and performance of a service. By setting limits on requests per unit of time, ratelimiters help protect against denial-of-service attacks, brute-force attempts, and excessive consumption of resources.

There are several common algorithms used for implementing ratelimiters. The token bucket algorithm is one popular method, where a bucket holds a certain number of tokens. Tokens are added to the bucket at a fixed rate, and each request consumes one token. If the bucket is empty, the request is denied or queued. The leaky bucket algorithm is similar, but it focuses on the rate of output. Requests are added to a queue (the bucket), and they are processed at a constant rate, regardless of the arrival rate of the requests. If the bucket overflows, new requests are dropped.

Another approach is the fixed window counter. This method counts the number of requests within a fixed time window. For example, a limit might be 100 requests per minute. Once the minute is up, the counter resets. A more refined version is the sliding window log, which keeps a timestamped record of each request. The actual limit is calculated by counting requests within the last fixed time period, offering a smoother distribution of requests than the fixed window.

Ratelimiters are crucial for various applications, including web APIs, databases, and network services. They ensure that no single client can overwhelm the system and that all users have a reasonable experience. Implementing effective ratelimiting requires careful consideration of the acceptable rate, the scope of the limit (e.g., per user, per IP, per API key), and the desired behavior when the limit is reached.