1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149

/*
 * Copyright (c) Dell Inc., or its subsidiaries. All Rights Reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 */

use std::iter::Iterator;
use std::time::Duration;
use std::u64::MAX as U64_MAX;

/// The retry policy that can retry something with
/// backoff policy.
pub trait BackoffSchedule: Iterator<Item = Duration> {}

/// Any implementation which implements the Iterator trait would also implement BackoffSchedule.
impl<T> BackoffSchedule for T where T: Iterator<Item = Duration> {}

/// The retry policy that can retry something with
/// exp backoff policy.
#[derive(Debug, Clone, Copy, Default, PartialEq)]
pub struct RetryWithBackoff {
    current: u64,
    base: u64,
    max_delay: Option<Duration>,
}

impl RetryWithBackoff {
    /// Constructs a new exponential back-off strategy,
    /// using default setting.
    pub fn default() -> RetryWithBackoff {
        let delay = Some(Duration::from_millis(10000));
        RetryWithBackoff {
            current: 1,
            base: 10,
            max_delay: delay,
        }
    }

    /// Constructs a new exponential back-off strategy,
    /// given a base duration in milliseconds.
    pub fn from_millis(base: u64) -> RetryWithBackoff {
        RetryWithBackoff {
            current: base,
            base,
            max_delay: None,
        }
    }

    /// Apply a maximum delay. No retry delay will be longer than this `Duration`.
    pub fn max_delay(mut self, duration: Duration) -> RetryWithBackoff {
        self.max_delay = Some(duration);
        self
    }

    /// Apply a the max number of tries.
    pub fn max_tries(self, tries: i32) -> std::iter::Take<RetryWithBackoff> {
        self.take(tries as usize)
    }
}

impl Iterator for RetryWithBackoff {
    type Item = Duration;

    fn next(&mut self) -> Option<Duration> {
        // set delay duration by applying factor
        let duration = Duration::from_millis(self.current);

        // check if we reached max delay
        if let Some(ref max_delay) = self.max_delay {
            if duration > *max_delay {
                return Some(*max_delay);
            }
        }

        if let Some(next) = self.current.checked_mul(self.base) {
            self.current = next;
        } else {
            self.current = U64_MAX;
        }

        Some(duration)
    }
}

#[test]
fn test_uses_default_setting() {
    let mut s = RetryWithBackoff::default();

    assert_eq!(s.next(), Some(Duration::from_millis(1)));
    assert_eq!(s.next(), Some(Duration::from_millis(10)));
    assert_eq!(s.next(), Some(Duration::from_millis(100)));
    assert_eq!(s.next(), Some(Duration::from_millis(1000)));
}

#[test]
fn test_returns_some_exponential_base_10() {
    let mut s = RetryWithBackoff::from_millis(10);

    assert_eq!(s.next(), Some(Duration::from_millis(10)));
    assert_eq!(s.next(), Some(Duration::from_millis(100)));
    assert_eq!(s.next(), Some(Duration::from_millis(1000)));
}

#[test]
fn test_returns_with_finite_retries() {
    let mut s = RetryWithBackoff::from_millis(10).max_tries(5);
    assert_eq!(s.next(), Some(Duration::from_millis(10)));
    assert_eq!(s.next(), Some(Duration::from_millis(100)));
    assert_eq!(s.next(), Some(Duration::from_millis(1000)));
    assert_eq!(s.next(), Some(Duration::from_millis(10000)));
    assert_eq!(s.next(), Some(Duration::from_millis(100000)));
    assert_eq!(s.next(), None);
}
#[test]
fn test_returns_some_exponential_base_2() {
    let mut s = RetryWithBackoff::from_millis(2);

    assert_eq!(s.next(), Some(Duration::from_millis(2)));
    assert_eq!(s.next(), Some(Duration::from_millis(4)));
    assert_eq!(s.next(), Some(Duration::from_millis(8)));
}

#[test]
fn test_saturates_at_maximum_value() {
    let mut s = RetryWithBackoff::from_millis(U64_MAX - 1);
    assert_eq!(s.next(), Some(Duration::from_millis(U64_MAX - 1)));
    assert_eq!(s.next(), Some(Duration::from_millis(U64_MAX)));
    assert_eq!(s.next(), Some(Duration::from_millis(U64_MAX)));
}

#[test]
fn stops_increasing_at_max_delay() {
    let mut s = RetryWithBackoff::from_millis(2).max_delay(Duration::from_millis(4));

    assert_eq!(s.next(), Some(Duration::from_millis(2)));
    assert_eq!(s.next(), Some(Duration::from_millis(4)));
    assert_eq!(s.next(), Some(Duration::from_millis(4)));
}

#[test]
fn returns_max_when_max_less_than_base() {
    let mut s = RetryWithBackoff::from_millis(20).max_delay(Duration::from_millis(10));
    assert_eq!(s.next(), Some(Duration::from_millis(10)));
    assert_eq!(s.next(), Some(Duration::from_millis(10)));
}