Exponential Backoff

A method to periodically retry failed requests by multiplicatively increasing the delay of requests (delaying the retry interval).

ex. The first retry is after 1 second, the second is after 2 seconds, the third is after 4 seconds, the fourth is after 8 seconds...

In retry design, it is necessary to consider not only backoff but also retry limits and timeouts (connection timeout and request timeout).

Jitter

A method to prevent requests that failed simultaneously from retrying at the same time by adding a random value to the retry interval of exponential backoff.

With simple exponential intervals, the retry intervals become the same, so jitter is introduced to add temporal variation.

Implementing Exponential Backoff and Jitter

Here's a simple implementation.

package main

import (
	"fmt"
	"log"
	"math"
	"math/rand"
	"time"
)

// Retryer is a retryer.
type Retryer struct {
	MaxRetryCount int
	RetryCount    int
	Jitter        *Jitter
}

func NewRetryer(mrc int) *Retryer {
	return &Retryer{
		MaxRetryCount: mrc,
		RetryCount:    0,
		Jitter: &Jitter{
			base:  10,
			cap:   100,
			sleep: 10,
		},
	}
}

func (r *Retryer) Retry(ja string, f func() error) {
	for i := r.RetryCount; i < r.MaxRetryCount; i++ {
		var d time.Duration
		switch ja {
		case jitterAlgoFull:
			d = r.Jitter.FullJitter(r.RetryCount)
		case jitterAlgoEqual:
			d = r.Jitter.EqualJitter(r.RetryCount)
		case jitterAlgoDecorrelated:
			d = r.Jitter.DecorrelatedJitter()
		}
		time.Sleep(d)
		err := f()
		log.Printf("retry %d times\n", i)
		if err != nil {
			log.Println(err)
			// Continue retrying due to error
			continue
		}
	}

	log.Println("retry done")
	return
}

const jitterAlgoFull = "full"
const jitterAlgoEqual = "equal"
const jitterAlgoDecorrelated = "decorrelated"

// Jitter is a retryer with jitter.
type Jitter struct {
	base  int
	cap   int
	sleep int /// for decorrelated jitter
}

// FullJitter is a full jitter algo.
// sleep = random_between(0 min(cap, base * 2 ** attempt))
// see: https://aws.typepad.com/sajp/2015/03/backoff.html
func (j *Jitter) FullJitter(retryCount int) time.Duration {
	sleep := rand.Intn(min(j.cap, (j.base * int(math.Pow(2, float64(retryCount))))))
	return time.Duration(sleep) * time.Second
}

// EqualJitter is a full equal algo.
// temp = min(cap, base * 2 ** attempt)
// sleep = temp / 2 + random_between(0, temp / 2)
// see: https://aws.typepad.com/sajp/2015/03/backoff.html
func (j *Jitter) EqualJitter(retryCount int) time.Duration {
	temp := rand.Intn(min(j.cap, (j.base * int(math.Pow(2, float64(retryCount))))))
	sleep := (int(math.Ceil(float64(temp/2))) + rand.Intn(int(math.Ceil(float64(temp/2)))))
	return time.Duration(sleep) * time.Second
}

// DecorrelatedJitter is a decorrelated jitter algo.
// sleep = min(cap, random_between(base, sleep * 3))
// see: https://aws.typepad.com/sajp/2015/03/backoff.html
func (j *Jitter) DecorrelatedJitter() time.Duration {
	randomBetween := func(min, max int) int {
		return rand.Intn(max-min) + min
	}
	sleep := min(j.cap, randomBetween(j.base, j.sleep*3))
	j.sleep = sleep
	return time.Duration(sleep) * time.Second
}

func init() {
	rand.New(rand.NewSource(time.Now().UnixNano()))
}

func main() {
	r := NewRetryer(5)
	r.Retry(jitterAlgoFull, func() error {
		return fmt.Errorf("retry error")
	})
	r.Retry(jitterAlgoEqual, func() error {
		return fmt.Errorf("retry error")
	})
	r.Retry(jitterAlgoDecorrelated, func() error {
		return fmt.Errorf("retry error")
	})
}

I referred to the following article for the jitter algorithm, but I'm not entirely confident that it's correctly implemented. It's a bit fuzzy, so there might be some logical oversights. cf. aws.amazon.com - Exponential Backoff And Jitter