axum-limit 0.1.0

Production-oriented rate limiting for Axum via extractors, with pluggable algorithms, storage backends, and dynamic quotas.
Documentation 
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

use super::{PolicyState, RateLimitPolicy};
use crate::codec::{decode_json, encode_json, CodecError};
use crate::quota::Quota;
use crate::snapshot::RateLimitSnapshot;
use crate::time::saturating_sub_ms;
use serde::{Deserialize, Serialize};

/// Fixed window rate limiting policy.
///
/// Allows up to `quota.max` requests per window of `quota.per`. The counter resets
/// when a new window starts.
#[derive(Debug, Clone, Copy, Default)]
pub struct FixedWindowPolicy;

impl RateLimitPolicy for FixedWindowPolicy {
    type State = FixedWindowState;
    const STATE_ID: &'static str = "fixed_window";
}

/// Fixed window state for a single key.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct FixedWindowState {
    count: usize,
    window_start_ms: u64,
    window_size_ms: u64,
    max: usize,
}

impl FixedWindowState {
    pub(crate) fn new_at(quota: Quota, now_ms: u64) -> Self {
        Self {
            count: 0,
            window_start_ms: now_ms,
            window_size_ms: quota.per_ms,
            max: quota.max.max(1),
        }
    }

    fn maybe_reset_window(&mut self, now_ms: u64) {
        if saturating_sub_ms(now_ms, self.window_start_ms) >= self.window_size_ms {
            self.count = 0;
            self.window_start_ms = now_ms;
        }
    }

    fn reset_at_ms(&self) -> u64 {
        self.window_start_ms.saturating_add(self.window_size_ms)
    }
}

impl PolicyState for FixedWindowState {
    fn try_acquire(&mut self, now_ms: u64) -> RateLimitSnapshot {
        self.maybe_reset_window(now_ms);
        let limit = self.max;

        if self.count < limit {
            self.count += 1;
            RateLimitSnapshot {
                allowed: true,
                limit,
                remaining: limit.saturating_sub(self.count),
                reset_at_ms: self.reset_at_ms(),
            }
        } else {
            RateLimitSnapshot {
                allowed: false,
                limit,
                remaining: 0,
                reset_at_ms: self.reset_at_ms(),
            }
        }
    }

    fn encode(&self) -> Result<Vec<u8>, CodecError> {
        encode_json(self)
    }

    fn decode(bytes: &[u8], _quota: Quota) -> Result<Self, CodecError> {
        decode_json(bytes)
    }

    fn create(quota: Quota, now_ms: u64) -> Self {
        Self::new_at(quota, now_ms)
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    const BASE_MS: u64 = 2_000_000;

    fn at(offset_ms: u64) -> u64 {
        BASE_MS + offset_ms
    }

    fn window(quota: Quota) -> FixedWindowState {
        FixedWindowState::new_at(quota, BASE_MS)
    }

    #[test]
    fn allows_up_to_max_requests_in_window() {
        let mut window = window(Quota::new(3, 1000));

        assert!(window.try_acquire(at(0)).allowed);
        assert!(window.try_acquire(at(100)).allowed);
        assert!(window.try_acquire(at(500)).allowed);
        assert!(!window.try_acquire(at(900)).allowed);
    }

    #[test]
    fn resets_after_window_expires() {
        let mut window = window(Quota::new(2, 1000));

        assert!(window.try_acquire(at(0)).allowed);
        assert!(window.try_acquire(at(100)).allowed);
        assert!(!window.try_acquire(at(200)).allowed);

        assert!(window.try_acquire(at(1000)).allowed);
        assert!(window.try_acquire(at(1100)).allowed);
        assert!(!window.try_acquire(at(1200)).allowed);
    }

    #[test]
    fn round_trips_through_json() {
        let mut state = window(Quota::new(3, 1000));
        assert!(state.try_acquire(at(0)).allowed);
        assert!(state.try_acquire(at(100)).allowed);

        let encoded = state.encode().expect("encode");
        let mut decoded = FixedWindowState::decode(&encoded, Quota::new(3, 1000)).expect("decode");

        assert!(decoded.try_acquire(at(200)).allowed);
        assert!(!decoded.try_acquire(at(300)).allowed);
    }
}