Rate Limiting – Token and Leaky Bucket Implementation and Explanation or rather rate limiting is a technique used to control the rate of traffic on a network. These are implemented using Congestive-Avoidance Algorithms (CAA), namely Token and Leaky Bucket algorithm.

For example, if the system is experiencing a high load, the rate limit can be reduced to prevent overload. Conversely, if the system is underutilized, the rate limit can be increased to allow for more traffic or requests.

For example, if the response time of the system is slow or the error rate is high, the rate limit can be reduced to prevent further degradation. Conversely, if the response time is fast and the error rate is low, the rate limit can be increased to allow for more traffic or requests.

Both dynamic and adaptive rate limiting can be implemented in various ways, such as through the use of software algorithms or machine learning models. They are useful in scenarios where the traffic or load on a system is highly variable or unpredictable, such as in cloud computing environments or during peak traffic periods. By adjusting the rate limit dynamically, these techniques can help ensure that a system remains performant and responsive while preventing overload or failure.

• Reactive algorithms: An example of a reactive algorithm would be to reduce the rate limit when the system response time exceeds a certain threshold. For instance, a system that provides API services may implement a reactive algorithm that reduces the rate limit when the average response time exceeds 500 milliseconds.
  
• Proactive algorithms: An example of a proactive algorithm would be to increase the rate limit in anticipation of expected traffic. For example, a website that sells tickets for concerts may implement a proactive algorithm that increases the rate limit for the ticket purchase page in anticipation of high traffic during the presale period. This would prevent users from experiencing delays or timeouts when trying to purchase tickets.
  
• Machine learning models: An example of a machine learning model for dynamic and adaptive rate limiting would be a model trained on historical traffic patterns to predict future traffic. For example, an e-commerce website may train a machine learning model on past holiday sales data to predict traffic for the upcoming holiday season. The model can then adjust the rate limit to handle the anticipated traffic.
  
• Third-party solutions: An example of a third-party solution for dynamic and adaptive rate limiting would be a cloud-based service such as Amazon API Gateway. Amazon API Gateway offers built-in rate limiting features that can be configured to adjust the rate limit based on the current state of the system or network. For example, the rate limit can be increased or decreased based on the volume of incoming requests or the response time of the system.

The tokens are generated at a fixed rate and are used to permit the transmission of packets. If there are not enough tokens available, packets are not transmitted until enough tokens have accumulated. Suppose, to implement a token bucket algorithm with a limit of 5 tokens per user, you can create a map to store the number of tokens each user has accumulated, and a channel to regulate the generation of tokens. Here’s an example implementation in Go using redis at 5 request per user per minute with refill logic

package main

import (
    "fmt"
    "strconv"
    "time"

    "github.com/go-redis/redis"
)

type TokenBucket struct {
    redisClient   *redis.Client
    maxTokens     int
    refillTime    time.Duration
}

func NewTokenBucket(redisClient *redis.Client, maxTokens int, refillTime time.Duration) *TokenBucket {
    return &TokenBucket{
        redisClient:   redisClient,
        maxTokens:     maxTokens,
        refillTime:    refillTime,
    }
}

func (t *TokenBucket) Take(user string) bool {
    // Get the current token count and last refill time for the user
    result, err := t.redisClient.MGet(fmt.Sprintf("%s_tokens", user), fmt.Sprintf("%s_last_refill", user)).Result()
    if err != nil {
        panic(err)
    }

    // Parse the token count and last refill time from the Redis result
    tokenCount, _ := strconv.Atoi(result[0].(string))
    lastRefillTime, _ := time.Parse(time.RFC3339Nano, result[1].(string))

    // Calculate the time elapsed since the last refill
    timeSinceRefill := time.Since(lastRefillTime)

    // Calculate the number of tokens to refill based on the elapsed time and refill rate
    refillCount := int(timeSinceRefill / t.refillTime)

    // Refill the token count up to the maximum
    tokenCount = tokenCount + refillCount
    if tokenCount > t.maxTokens {
        tokenCount = t.maxTokens
    }

    // Check if there are enough tokens for the request
    if tokenCount > 0 {
        // Update the token count in Redis and return true
        err := t.redisClient.MSet(fmt.Sprintf("%s_tokens", user), tokenCount, fmt.Sprintf("%s_last_refill", user), time.Now().Format(time.RFC3339Nano)).Err()
        if err != nil {
            panic(err)
        }
        return true
    } else {
        // Return false if there are not enough tokens
        return false
    }
}

In this implementation, we use the Go Redis library to interact with Redis. The Take function first retrieves the current token count and last refill time from Redis using the MGet command. It then calculates the time elapsed since the last refill, and the number of tokens to refill based on the elapsed time and refill rate.

Next, it refills the token count up to the maximum value, and checks if there are enough tokens for the request. If there are, it updates the token count and last refill time in Redis using the MSet command and returns true. Otherwise, it returns false.

To use this implementation, you can create a Redis client and pass it to the NewTokenBucket function along with the maximum token count and refill time. You can then call the Take function with the user ID for each request to check if there are enough tokens. If there are, the function returns true and decrements the token count in Redis. If there are not, the function returns false without modifying the token count.

In the Leaky Bucket algorithm, the rate of packet transmission is controlled by the size of the bucket and the rate at which it leaks. If packets arrive at a faster rate than the bucket can leak them, they are dropped. This approach helps to smooth out traffic and prevents bursts of traffic from overwhelming the network.

The Leaky Bucket algorithm can be used for both incoming and outgoing traffic, and is often used in Quality of Service (QoS) implementations to prioritize certain types of traffic over others. It is a simple algorithm that can be implemented in hardware or software, and is widely used in networking devices such as routers and switches.

Here is an example implementation of the Leaky Bucket algorithm using Redis to restrict users to a maximum of 5 tokens per minute:

import (
	"fmt"
	"strconv"
	"time"

	"github.com/go-redis/redis"
)

func main() {
	// create Redis client
	client := redis.NewClient(&redis.Options{
		Addr:     "localhost:6379",
		Password: "",
		DB:       0,
	})

	// set the bucket capacity and leak rate
	bucketSize := 5
	leakRate := 1 // token per minute

	// start a loop to handle incoming requests
	for {
		// get the current time in Unix seconds
		now := time.Now().Unix()

		// get the current token count for the user
		key := "user:1"
		val, err := client.Get(key).Result()
		if err != nil {
			// handle error
			fmt.Println(err)
			return
		}

		// convert the token count to integer
		tokenCount, err := strconv.Atoi(val)
		if err != nil {
			// handle error
			fmt.Println(err)
			return
		}

		// calculate the number of tokens that should be in the bucket now
		expectedTokens := int(now/60) * leakRate
		if expectedTokens > bucketSize {
			expectedTokens = bucketSize
		}

		// calculate the number of tokens to add to the bucket
		tokensToAdd := expectedTokens - tokenCount
		if tokensToAdd < 0 {
			tokensToAdd = 0
		}

		// add tokens to the bucket
		_, err = client.IncrBy(key, int64(tokensToAdd)).Result()
		if err != nil {
			// handle error
			fmt.Println(err)
			return
		}

		// check if the bucket has enough tokens for the request
		if tokenCount >= bucketSize {
			// reject the request
			fmt.Println("Request rejected: user has exceeded the token limit")
		} else {
			// process the request
			fmt.Println("Request processed successfully")
		}

		// sleep for 1 second before checking the next request
		time.Sleep(time.Second)
	}
}

In this implementation, we use Redis to store the token count for each user. We set the bucket capacity to 5 tokens and the leak rate to 1 token per minute. We then start a loop to handle incoming requests.

At each iteration of the loop, we first get the current time in Unix seconds and the current token count for the user from Redis. We then calculate the expected number of tokens in the bucket based on the current time and leak rate, and add any missing tokens to the bucket.

We then check if the token count exceeds the bucket size. If it does, we reject the request. Otherwise, we process the request and decrement the token count in Redis.

Note that this implementation assumes that requests are evenly distributed over time, and does not account for bursts of traffic. To handle bursts, we can set a larger bucket size or use a more sophisticated algorithm such as the Token Bucket algorithm.

Adaptive QoS (Quality of Service) Algorithm is a machine learning-based approach to implement rate limiting in network traffic. It is designed to dynamically adjust the rate limits based on the current traffic load, and it uses predictive modeling to forecast future traffic patterns.

The algorithm continuously monitors the incoming traffic and adjusts the rate limit dynamically to maintain a balance between service quality and resource utilization. This is achieved by collecting statistical data about the traffic patterns, such as packet loss rate, latency, and throughput, and using this data to predict future traffic patterns. Based on these predictions, the algorithm adjusts the rate limits to ensure that the network can handle the anticipated traffic while maintaining an acceptable level of service quality.

Adaptive QoS Algorithm is a powerful tool for managing network traffic because it can automatically adjust the rate limits in response to changing traffic patterns. This makes it possible to provide optimal service quality even during periods of high traffic load or unexpected spikes in traffic volume.

In the context of rate limiting, sliding window refers to a technique used in both token bucket and leaky bucket algorithms to track the number of tokens or bytes that have been used within a specific time period.

In a token bucket algorithm, a sliding window is used to track the number of tokens that have been consumed by requests within a fixed time interval. For example, if the rate limit is set to 10 requests per second, the bucket will be refilled with 10 tokens every second. The sliding window will then track how many tokens have been used within the last second, and reject any requests that exceed this limit.

Similarly, in a leaky bucket algorithm, a sliding window is used to track the amount of data that has been transmitted within a fixed time interval. The bucket is refilled at a constant rate, and the sliding window tracks how much data has been transmitted within the last second. If the transmission rate exceeds the capacity of the bucket, the algorithm will discard any excess data.

In both cases, the sliding window is used to enforce the rate limit by keeping track of the amount of traffic that has been sent or received within a specific time period. This allows the algorithm to enforce the rate limit in a granular and precise way, while still allowing some flexibility for bursts of traffic that fall within the limits of the sliding window.

Other System Design Posts:
https://designcodesolve.com/technology/race-conditions-in-distributed-or-multithreaded-system/
https://designcodesolve.com/technology/system-design-db-choices/
https://designcodesolve.com/technology/dropbox-system-design/