tokio_retry/strategy/
exponential_backoff.rs

1use std::iter::Iterator;
2use std::time::Duration;
3use std::u64::MAX as U64_MAX;
4
5/// A retry strategy driven by exponential back-off.
6///
7/// The power corresponds to the number of past attempts.
8#[derive(Debug, Clone)]
9pub struct ExponentialBackoff {
10    current: u64,
11    base: u64,
12    factor: u64,
13    max_delay: Option<Duration>,
14}
15
16impl ExponentialBackoff {
17    /// Constructs a new exponential back-off strategy,
18    /// given a base duration in milliseconds.
19    ///
20    /// The resulting duration is calculated by taking the base to the `n`-th power,
21    /// where `n` denotes the number of past attempts.
22    pub fn from_millis(base: u64) -> ExponentialBackoff {
23        ExponentialBackoff {
24            current: base,
25            base: base,
26            factor: 1u64,
27            max_delay: None,
28        }
29    }
30
31    /// A multiplicative factor that will be applied to the retry delay.
32    ///
33    /// For example, using a factor of `1000` will make each delay in units of seconds.
34    ///
35    /// Default factor is `1`.
36    pub fn factor(mut self, factor: u64) -> ExponentialBackoff {
37        self.factor = factor;
38        self
39    }
40
41    /// Apply a maximum delay. No retry delay will be longer than this `Duration`.
42    pub fn max_delay(mut self, duration: Duration) -> ExponentialBackoff {
43        self.max_delay = Some(duration);
44        self
45    }
46}
47
48impl Iterator for ExponentialBackoff {
49    type Item = Duration;
50
51    fn next(&mut self) -> Option<Duration> {
52        // set delay duration by applying factor
53        let duration = if let Some(duration) = self.current.checked_mul(self.factor) {
54            Duration::from_millis(duration)
55        } else {
56            Duration::from_millis(U64_MAX)
57        };
58
59        // check if we reached max delay
60        if let Some(ref max_delay) = self.max_delay {
61            if duration > *max_delay {
62                return Some(*max_delay);
63            }
64        }
65
66        if let Some(next) = self.current.checked_mul(self.base) {
67            self.current = next;
68        } else {
69            self.current = U64_MAX;
70        }
71
72        Some(duration)
73    }
74}
75
76#[test]
77fn returns_some_exponential_base_10() {
78    let mut s = ExponentialBackoff::from_millis(10);
79
80    assert_eq!(s.next(), Some(Duration::from_millis(10)));
81    assert_eq!(s.next(), Some(Duration::from_millis(100)));
82    assert_eq!(s.next(), Some(Duration::from_millis(1000)));
83}
84
85#[test]
86fn returns_some_exponential_base_2() {
87    let mut s = ExponentialBackoff::from_millis(2);
88
89    assert_eq!(s.next(), Some(Duration::from_millis(2)));
90    assert_eq!(s.next(), Some(Duration::from_millis(4)));
91    assert_eq!(s.next(), Some(Duration::from_millis(8)));
92}
93
94#[test]
95fn saturates_at_maximum_value() {
96    let mut s = ExponentialBackoff::from_millis(U64_MAX - 1);
97
98    assert_eq!(s.next(), Some(Duration::from_millis(U64_MAX - 1)));
99    assert_eq!(s.next(), Some(Duration::from_millis(U64_MAX)));
100    assert_eq!(s.next(), Some(Duration::from_millis(U64_MAX)));
101}
102
103#[test]
104fn can_use_factor_to_get_seconds() {
105    let factor = 1000;
106    let mut s = ExponentialBackoff::from_millis(2).factor(factor);
107
108    assert_eq!(s.next(), Some(Duration::from_secs(2)));
109    assert_eq!(s.next(), Some(Duration::from_secs(4)));
110    assert_eq!(s.next(), Some(Duration::from_secs(8)));
111}
112
113#[test]
114fn stops_increasing_at_max_delay() {
115    let mut s = ExponentialBackoff::from_millis(2).max_delay(Duration::from_millis(4));
116
117    assert_eq!(s.next(), Some(Duration::from_millis(2)));
118    assert_eq!(s.next(), Some(Duration::from_millis(4)));
119    assert_eq!(s.next(), Some(Duration::from_millis(4)));
120}
121
122#[test]
123fn returns_max_when_max_less_than_base() {
124    let mut s = ExponentialBackoff::from_millis(20).max_delay(Duration::from_millis(10));
125
126    assert_eq!(s.next(), Some(Duration::from_millis(10)));
127    assert_eq!(s.next(), Some(Duration::from_millis(10)));
128}
tokio_retry/strategy/exponential_backoff.rs

tokio_retry/strategy/
exponential_backoff.rs
