README

Exponential Backoff GoDoc Build 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.

How To

We define two functions, Retry() and RetryNotify(). They receive an Operation to execute, a BackOff algorithm, and an optional Notify error handler.

The operation will be executed, and will be retried on failure with delay as given by the backoff algorithm. The backoff algorithm can also decide when to stop retrying. In addition, the notify error handler will be called after each failed attempt, except for the last time, whose error should be handled by the caller.

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

// 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 Notify func(error, time.Duration)

func Retry(Operation, BackOff) error
func RetryNotify(Operation, BackOff, Notify)

Examples

See more advanced examples in the godoc.

Retry

Simple retry helper that uses the default exponential backoff algorithm:

operation := func() error {
    // An operation that might fail.
    return nil // or return errors.New("some error")
}

err := Retry(operation, NewExponentialBackOff())
if err != nil {
    // Handle error.
    return err
}

// Operation is successful.
return nil
Ticker
operation := func() error {
    // An operation that might fail
    return nil // or return errors.New("some error")
}

b := NewExponentialBackOff()
ticker := NewTicker(b)

var err error

// 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.
for range ticker.C {
    if err = operation(); err != nil {
        log.Println(err, "will retry...")
        continue
    }

    ticker.Stop()
    break
}

if err != nil {
    // Operation has failed.
    return err
}

// Operation is successful.
return nil

Getting Started

# install
$ go get github.com/cenkalti/backoff

# test
$ cd $GOPATH/src/github.com/cenkalti/backoff
$ go get -t ./...
$ go test -v -cover
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Documentation

Overview

    Package backoff implements backoff algorithms for retrying operations.

    Also has a Retry() helper for retrying operations that may fail.

    Index

    Examples

    Constants

    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

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

        Variables

        View Source
        var SystemClock = systemClock{}

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

          Functions

          func Retry

          func Retry(o Operation, b BackOff) error

            Retry the function f until it does not return error or BackOff stops. f is guaranteed to be run at least once. It is the caller's responsibility to reset b after Retry returns.

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

            Example
            Output:
            
            

            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.

              Types

              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.
              	Reset()
              }

                BackOff is a backoff policy for retrying an operation.

                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 stops.
                    	// It never stops if MaxElapsedTime == 0.
                    	MaxElapsedTime 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().

                          func (*ExponentialBackOff) NextBackOff

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

                            NextBackOff calculates the next backoff interval using the formula:

                            Randomized interval = RetryInterval +/- (RandomizationFactor * RetryInterval)
                            

                            func (*ExponentialBackOff) Reset

                            func (b *ExponentialBackOff) Reset()

                              Reset the interval back to the initial retry interval and restarts the timer.

                              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 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.

                                      Example
                                      Output:
                                      
                                      

                                      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.

                                        func (*Ticker) Stop

                                        func (t *Ticker) Stop()

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

                                          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()