/
exponential.go
141 lines (118 loc) · 5.23 KB
/
exponential.go
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package backoff
import (
"math/rand"
"time"
)
/*
ExponentialBackOff is an implementation of BackOff that increases the back off
period for each retry attempt using a randomization function that grows exponentially.
NextBackOff() is calculated using the following formula:
randomized_interval =
retry_interval * (random value in range [1 - randomization_factor, 1 + randomization_factor])
In other words NextBackOff() will range between the randomization factor
percentage below and above the retry interval. For example, using 2 seconds as the base retry
interval and 0.5 as the randomization factor, the actual back off period used in the next retry
attempt will be between 1 and 3 seconds.
Note: max_interval caps the retry_interval and not the randomized_interval.
If the time elapsed since an ExponentialBackOff instance is created goes past the
max_elapsed_time then the method NextBackOff() starts returning backoff.Stop.
The elapsed time can be reset by calling Reset().
Example: The default retry_interval is .5 seconds, default randomization_factor is 0.5, default
multiplier is 1.5 and the default max_interval is 1 minute. For 10 tries the sequence will be
(values in seconds) and assuming we go over the max_elapsed_time on the 10th try:
request# retry_interval randomized_interval
1 0.5 [0.25, 0.75]
2 0.75 [0.375, 1.125]
3 1.125 [0.562, 1.687]
4 1.687 [0.8435, 2.53]
5 2.53 [1.265, 3.795]
6 3.795 [1.897, 5.692]
7 5.692 [2.846, 8.538]
8 8.538 [4.269, 12.807]
9 12.807 [6.403, 19.210]
10 19.210 backoff.Stop
Implementation is not thread-safe.
*/
type ExponentialBackOff struct {
InitialInterval time.Duration
RandomizationFactor float64
Multiplier float64
MaxInterval time.Duration
// After MaxElapsedTime the ExponentialBackOff stops.
// It never stops if MaxElapsedTime == 0.
MaxElapsedTime time.Duration
Clock Clock
currentInterval time.Duration
startTime time.Time
}
// Clock is an interface that returns current time for BackOff.
type Clock interface {
Now() time.Time
}
// Default values for ExponentialBackOff.
const (
DefaultInitialInterval = 500 * time.Millisecond
DefaultRandomizationFactor = 0.5
DefaultMultiplier = 1.5
DefaultMaxInterval = 60 * time.Second
DefaultMaxElapsedTime = 15 * time.Minute
)
// NewExponentialBackOff creates an instance of ExponentialBackOff using default values.
func NewExponentialBackOff() *ExponentialBackOff {
return &ExponentialBackOff{
InitialInterval: DefaultInitialInterval,
RandomizationFactor: DefaultRandomizationFactor,
Multiplier: DefaultMultiplier,
MaxInterval: DefaultMaxInterval,
MaxElapsedTime: DefaultMaxElapsedTime,
Clock: SystemClock,
}
}
type systemClock struct{}
func (t systemClock) Now() time.Time {
return time.Now()
}
// SystemClock implements Clock interface that uses time.Now().
var SystemClock = systemClock{}
// Reset the interval back to the initial retry interval and restarts the timer.
func (b *ExponentialBackOff) Reset() {
b.currentInterval = b.InitialInterval
b.startTime = b.Clock.Now()
}
// NextBackOff calculates the next back off interval using the formula:
// randomized_interval = retry_interval +/- (randomization_factor * retry_interval)
func (b *ExponentialBackOff) NextBackOff() time.Duration {
// Make sure we have not gone over the maximum elapsed time.
if b.MaxElapsedTime != 0 && b.GetElapsedTime() > b.MaxElapsedTime {
return Stop
}
defer b.incrementCurrentInterval()
return getRandomValueFromInterval(b.RandomizationFactor, rand.Float64(), b.currentInterval)
}
// GetElapsedTime returns the elapsed time since an ExponentialBackOff instance
// is created and is reset when Reset() is called.
//
// The elapsed time is computed using time.Now().UnixNano().
func (b *ExponentialBackOff) GetElapsedTime() time.Duration {
return b.Clock.Now().Sub(b.startTime)
}
// Increments the current interval by multiplying it with the multiplier.
func (b *ExponentialBackOff) incrementCurrentInterval() {
// Check for overflow, if overflow is detected set the current interval to the max interval.
if float64(b.currentInterval) >= float64(b.MaxInterval)/b.Multiplier {
b.currentInterval = b.MaxInterval
} else {
b.currentInterval = time.Duration(float64(b.currentInterval) * b.Multiplier)
}
}
// Returns a random value from the interval:
// [randomizationFactor * currentInterval, randomizationFactor * currentInterval].
func getRandomValueFromInterval(randomizationFactor, random float64, currentInterval time.Duration) time.Duration {
var delta = randomizationFactor * float64(currentInterval)
var minInterval = float64(currentInterval) - delta
var maxInterval = float64(currentInterval) + delta
// Get a random value from the range [minInterval, maxInterval].
// The formula used below has a +1 because if the minInterval is 1 and the maxInterval is 3 then
// we want a 33% chance for selecting either 1, 2 or 3.
return time.Duration(minInterval + (random * (maxInterval - minInterval + 1)))
}