# [−][src]Crate backoff

`ExponentialBackoff` is a backoff implementation that increases the backoff period for each retry attempt using a randomization function that grows exponentially.

`next_backoff` is calculated using the following formula:

```  randomized interval =
retry_interval * (random value in range [1 - randomization_factor, 1 + randomization_factor])```

In other words `next_backoff` will range between the randomization factor percentage below and above the retry interval.

For example, given the following parameters:

```retry_interval = 2
randomization_factor = 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: `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 `next_backoff` starts returning `None`.

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 `max_elapsed_time` on the 10th try:

Request # `retry_interval` (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 None

## Modules

 backoff default Constants for the exponential backoff policy. exponential

## Structs

 SystemClock `SystemClock` uses the system's clock to get the current time. This Clock should be used for real use-cases.

## Enums

 Error Error is the error value in an operation's result.

## Traits

 Clock Clock returns the current time. Notify Notify is called in `retry_notify` in case of errors. Operation Operation is an operation that can be retried if it fails.

## Type Definitions

 ExponentialBackoff Exponential backoff policy with system's clock.