--- /dev/null
+package backoff
+
+import (
+ "math/rand"
+ "time"
+)
+
+/*
+ExponentialBackOff is a backoff implementation that increases the backoff
+period for each retry attempt using a randomization function that grows exponentially.
+
+NextBackOff() is calculated using the following formula:
+
+ randomized interval =
+ RetryInterval * (random value in range [1 - RandomizationFactor, 1 + RandomizationFactor])
+
+In other words NextBackOff() will range between the randomization factor
+percentage below and above the retry interval.
+
+For example, given the following parameters:
+
+ RetryInterval = 2
+ RandomizationFactor = 0.5
+ Multiplier = 2
+
+the actual backoff period used in the next retry attempt will range between 1 and 3 seconds,
+multiplied by the exponential, that is, between 2 and 6 seconds.
+
+Note: MaxInterval caps the RetryInterval and not the randomized interval.
+
+If the time elapsed since an ExponentialBackOff instance is created goes past the
+MaxElapsedTime, then the method NextBackOff() starts returning backoff.Stop.
+
+The elapsed time can be reset by calling Reset().
+
+Example: Given the following default arguments, for 10 tries the sequence will be,
+and assuming we go over the MaxElapsedTime on the 10th try:
+
+ Request # RetryInterval (seconds) Randomized Interval (seconds)
+
+ 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
+
+Note: 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 {
+ b := &ExponentialBackOff{
+ InitialInterval: DefaultInitialInterval,
+ RandomizationFactor: DefaultRandomizationFactor,
+ Multiplier: DefaultMultiplier,
+ MaxInterval: DefaultMaxInterval,
+ MaxElapsedTime: DefaultMaxElapsedTime,
+ Clock: SystemClock,
+ }
+ b.Reset()
+ return b
+}
+
+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 backoff interval using the formula:
+// Randomized interval = RetryInterval +/- (RandomizationFactor * RetryInterval)
+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(). It is
+// safe to call even while the backoff policy is used by a running
+// ticker.
+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 following 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)))
+}
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