Exponential Backoff GoDoc Build Status Coverage Status

This is a Go port of the exponential backoff algorithm from Google's HTTP Client Library for Java.

Exponential backoff is an algorithm that uses feedback to multiplicatively decrease the rate of some process, in order to gradually find an acceptable rate. The retries exponentially increase and stop increasing when a certain threshold is met.


Import path is Please note the version part at the end.

Use to view the documentation.


  • I would like to keep this library as small as possible.
  • Please don't send a PR without opening an issue and discussing it first.
  • If proposed change is not a common use case, I will probably not accept it.
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    Package backoff implements backoff algorithms for retrying operations.

    Use Retry function for retrying operations that may fail. If Retry does not meet your needs, copy/paste the function into your project and modify as you wish.

    There is also Ticker type similar to time.Ticker. You can use it if you need to work with channels.

    See Examples section below for usage examples.




    View Source
    const (
    	DefaultInitialInterval     = 500 * time.Millisecond
    	DefaultRandomizationFactor = 0.5
    	DefaultMultiplier          = 1.5
    	DefaultMaxInterval         = 60 * time.Second
    	DefaultMaxElapsedTime      = 15 * time.Minute

      Default values for ExponentialBackOff.

      View Source
      const Stop time.Duration = -1

        Stop indicates that no more retries should be made for use in NextBackOff().


        View Source
        var SystemClock = systemClock{}

          SystemClock implements Clock interface that uses time.Now().


          func Permanent

          func Permanent(err error) error

            Permanent wraps the given err in a *PermanentError.

            func Retry

            func Retry(o Operation, b BackOff) error

              Retry the operation o until it does not return error or BackOff stops. o is guaranteed to be run at least once.

              If o returns a *PermanentError, the operation is not retried, and the wrapped error is returned.

              Retry sleeps the goroutine for the duration returned by BackOff after a failed operation returns.


              func RetryNotify

              func RetryNotify(operation Operation, b BackOff, notify Notify) error

                RetryNotify calls notify function with the error and wait duration for each failed attempt before sleep.

                func RetryNotifyWithTimer

                func RetryNotifyWithTimer(operation Operation, b BackOff, notify Notify, t Timer) error

                  RetryNotifyWithTimer calls notify function with the error and wait duration using the given Timer for each failed attempt before sleep. A default timer that uses system timer is used when nil is passed.


                  type BackOff

                  type BackOff interface {
                  	// NextBackOff returns the duration to wait before retrying the operation,
                  	// or backoff. Stop to indicate that no more retries should be made.
                  	// Example usage:
                  	// 	duration := backoff.NextBackOff();
                  	// 	if (duration == backoff.Stop) {
                  	// 		// Do not retry operation.
                  	// 	} else {
                  	// 		// Sleep for duration and retry operation.
                  	// 	}
                  	NextBackOff() time.Duration
                  	// Reset to initial state.

                    BackOff is a backoff policy for retrying an operation.

                    func WithMaxRetries

                    func WithMaxRetries(b BackOff, max uint64) BackOff

                      WithMaxRetries creates a wrapper around another BackOff, which will return Stop if NextBackOff() has been called too many times since the last time Reset() was called

                      Note: Implementation is not thread-safe.

                      type BackOffContext

                      type BackOffContext interface {
                      	Context() context.Context

                        BackOffContext is a backoff policy that stops retrying after the context is canceled.

                        func WithContext

                        func WithContext(b BackOff, ctx context.Context) BackOffContext

                          WithContext returns a BackOffContext with context ctx

                          ctx must not be nil

                          type Clock

                          type Clock interface {
                          	Now() time.Time

                            Clock is an interface that returns current time for BackOff.

                            type ConstantBackOff

                            type ConstantBackOff struct {
                            	Interval time.Duration

                              ConstantBackOff is a backoff policy that always returns the same backoff delay. This is in contrast to an exponential backoff policy, which returns a delay that grows longer as you call NextBackOff() over and over again.

                              func NewConstantBackOff

                              func NewConstantBackOff(d time.Duration) *ConstantBackOff

                              func (*ConstantBackOff) NextBackOff

                              func (b *ConstantBackOff) NextBackOff() time.Duration

                              func (*ConstantBackOff) Reset

                              func (b *ConstantBackOff) Reset()

                              type ExponentialBackOff

                              type ExponentialBackOff struct {
                              	InitialInterval     time.Duration
                              	RandomizationFactor float64
                              	Multiplier          float64
                              	MaxInterval         time.Duration
                              	// After MaxElapsedTime the ExponentialBackOff returns Stop.
                              	// It never stops if MaxElapsedTime == 0.
                              	MaxElapsedTime time.Duration
                              	Stop           time.Duration
                              	Clock          Clock
                              	// contains filtered or unexported fields

                                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.

                                func NewExponentialBackOff

                                func NewExponentialBackOff() *ExponentialBackOff

                                  NewExponentialBackOff creates an instance of ExponentialBackOff using default values.

                                  func (*ExponentialBackOff) GetElapsedTime

                                  func (b *ExponentialBackOff) GetElapsedTime() time.Duration

                                    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 (*ExponentialBackOff) NextBackOff

                                    func (b *ExponentialBackOff) NextBackOff() time.Duration

                                      NextBackOff calculates the next backoff interval using the formula:

                                      Randomized interval = RetryInterval * (1 ± RandomizationFactor)

                                      func (*ExponentialBackOff) Reset

                                      func (b *ExponentialBackOff) Reset()

                                        Reset the interval back to the initial retry interval and restarts the timer. Reset must be called before using b.

                                        type Notify

                                        type Notify func(error, time.Duration)

                                          Notify is a notify-on-error function. It receives an operation error and backoff delay if the operation failed (with an error).

                                          NOTE that if the backoff policy stated to stop retrying, the notify function isn't called.

                                          type Operation

                                          type Operation func() error

                                            An Operation is executing by Retry() or RetryNotify(). The operation will be retried using a backoff policy if it returns an error.

                                            type PermanentError

                                            type PermanentError struct {
                                            	Err error

                                              PermanentError signals that the operation should not be retried.

                                              func (*PermanentError) Error

                                              func (e *PermanentError) Error() string

                                              func (*PermanentError) Is

                                              func (e *PermanentError) Is(target error) bool

                                              func (*PermanentError) Unwrap

                                              func (e *PermanentError) Unwrap() error

                                              type StopBackOff

                                              type StopBackOff struct{}

                                                StopBackOff is a fixed backoff policy that always returns backoff.Stop for NextBackOff(), meaning that the operation should never be retried.

                                                func (*StopBackOff) NextBackOff

                                                func (b *StopBackOff) NextBackOff() time.Duration

                                                func (*StopBackOff) Reset

                                                func (b *StopBackOff) Reset()

                                                type Ticker

                                                type Ticker struct {
                                                	C <-chan time.Time
                                                	// contains filtered or unexported fields

                                                  Ticker holds a channel that delivers `ticks' of a clock at times reported by a BackOff.

                                                  Ticks will continue to arrive when the previous operation is still running, so operations that take a while to fail could run in quick succession.


                                                  func NewTicker

                                                  func NewTicker(b BackOff) *Ticker

                                                    NewTicker returns a new Ticker containing a channel that will send the time at times specified by the BackOff argument. Ticker is guaranteed to tick at least once. The channel is closed when Stop method is called or BackOff stops. It is not safe to manipulate the provided backoff policy (notably calling NextBackOff or Reset) while the ticker is running.

                                                    func NewTickerWithTimer

                                                    func NewTickerWithTimer(b BackOff, timer Timer) *Ticker

                                                      NewTickerWithTimer returns a new Ticker with a custom timer. A default timer that uses system timer is used when nil is passed.

                                                      func (*Ticker) Stop

                                                      func (t *Ticker) Stop()

                                                        Stop turns off a ticker. After Stop, no more ticks will be sent.

                                                        type Timer

                                                        type Timer interface {
                                                        	Start(duration time.Duration)
                                                        	C() <-chan time.Time

                                                        type ZeroBackOff

                                                        type ZeroBackOff struct{}

                                                          ZeroBackOff is a fixed backoff policy whose backoff time is always zero, meaning that the operation is retried immediately without waiting, indefinitely.

                                                          func (*ZeroBackOff) NextBackOff

                                                          func (b *ZeroBackOff) NextBackOff() time.Duration

                                                          func (*ZeroBackOff) Reset

                                                          func (b *ZeroBackOff) Reset()