nautilus_network/ratelimiter/
mod.rs

1// -------------------------------------------------------------------------------------------------
2//  Copyright (C) 2015-2026 Nautech Systems Pty Ltd. All rights reserved.
3//  https://nautechsystems.io
4//
5//  Licensed under the GNU Lesser General Public License Version 3.0 (the "License");
6//  You may not use this file except in compliance with the License.
7//  You may obtain a copy of the License at https://www.gnu.org/licenses/lgpl-3.0.en.html
8//
9//  Unless required by applicable law or agreed to in writing, software
10//  distributed under the License is distributed on an "AS IS" BASIS,
11//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12//  See the License for the specific language governing permissions and
13//  limitations under the License.
14// -------------------------------------------------------------------------------------------------
15
16//! A rate limiter implementation heavily inspired by [governor](https://github.com/antifuchs/governor).
17//!
18//! The governor does not support different quota for different key. It is an open [issue](https://github.com/antifuchs/governor/issues/193).
19pub mod clock;
20mod gcra;
21mod nanos;
22pub mod quota;
23
24use std::{
25    fmt::Debug,
26    hash::Hash,
27    num::NonZeroU64,
28    sync::atomic::{AtomicU64, Ordering},
29    time::Duration,
30};
31
32use dashmap::DashMap;
33use futures_util::StreamExt;
34
35use self::{
36    clock::{Clock, FakeRelativeClock, MonotonicClock},
37    gcra::{Gcra, NotUntil},
38    nanos::Nanos,
39    quota::Quota,
40};
41
42/// An in-memory representation of a GCRA's rate-limiting state.
43///
44/// Implemented using [`AtomicU64`] operations, this state representation can be used to
45/// construct rate limiting states for other in-memory states: e.g., this crate uses
46/// `InMemoryState` as the states it tracks in the keyed rate limiters it implements.
47///
48/// Internally, the number tracked here is the theoretical arrival time (a GCRA term) in number of
49/// nanoseconds since the rate limiter was created.
50#[derive(Debug, Default)]
51pub struct InMemoryState(AtomicU64);
52
53impl InMemoryState {
54    /// Measures and updates the GCRA's state atomically, retrying on concurrent modifications.
55    ///
56    /// # Errors
57    ///
58    /// Returns an error if the provided closure returns an error.
59    pub(crate) fn measure_and_replace_one<T, F, E>(&self, mut f: F) -> Result<T, E>
60    where
61        F: FnMut(Option<Nanos>) -> Result<(T, Nanos), E>,
62    {
63        let mut prev = self.0.load(Ordering::Acquire);
64        let mut decision = f(NonZeroU64::new(prev).map(|n| n.get().into()));
65        while let Ok((result, new_data)) = decision {
66            // Lock-free CAS loop: retry with current value if another thread modified it,
67            // uses weak variant (faster) since spurious failures are fine in a retry loop.
68            match self.0.compare_exchange_weak(
69                prev,
70                new_data.into(),
71                Ordering::Release,
72                Ordering::Relaxed,
73            ) {
74                Ok(_) => return Ok(result),
75                Err(e) => prev = e, // Retry with value written by another thread
76            }
77            decision = f(NonZeroU64::new(prev).map(|n| n.get().into()));
78        }
79        // This map shouldn't be needed, as we only get here in the error case, but the compiler
80        // can't see it.
81        decision.map(|(result, _)| result)
82    }
83}
84
85/// A concurrent, thread-safe and fairly performant hashmap based on [`DashMap`].
86pub type DashMapStateStore<K> = DashMap<K, InMemoryState>;
87
88/// A way for rate limiters to keep state.
89///
90/// There are two important kinds of state stores: Direct and keyed. The direct kind have only
91/// one state, and are useful for "global" rate limit enforcement (e.g. a process should never
92/// do more than N tasks a day). The keyed kind allows one rate limit per key (e.g. an API
93/// call budget per client API key).
94///
95/// A direct state store is expressed as [`StateStore::Key`] = `NotKeyed`.
96/// Keyed state stores have a
97/// type parameter for the key and set their key to that.
98pub trait StateStore {
99    /// The type of key that the state store can represent.
100    type Key;
101
102    /// Updates a state store's rate limiting state for a given key, using the given closure.
103    ///
104    /// The closure parameter takes the old value (`None` if this is the first measurement) of the
105    /// state store at the key's location, checks if the request an be accommodated and:
106    ///
107    /// - If the request is rate-limited, returns `Err(E)`.
108    /// - If the request can make it through, returns `Ok(T)` (an arbitrary positive return
109    ///   value) and the updated state.
110    ///
111    /// It is `measure_and_replace`'s job then to safely replace the value at the key - it must
112    /// only update the value if the value hasn't changed. The implementations in this
113    /// crate use `AtomicU64` operations for this.
114    ///
115    /// # Errors
116    ///
117    /// Returns `Err(E)` if the closure returns an error or the request is rate-limited.
118    fn measure_and_replace<T, F, E>(&self, key: &Self::Key, f: F) -> Result<T, E>
119    where
120        F: Fn(Option<Nanos>) -> Result<(T, Nanos), E>;
121}
122
123impl<K: Hash + Eq + Clone> StateStore for DashMapStateStore<K> {
124    type Key = K;
125
126    fn measure_and_replace<T, F, E>(&self, key: &Self::Key, f: F) -> Result<T, E>
127    where
128        F: Fn(Option<Nanos>) -> Result<(T, Nanos), E>,
129    {
130        if let Some(v) = self.get(key) {
131            // fast path: measure existing entry
132            return v.measure_and_replace_one(f);
133        }
134        // make an entry and measure that:
135        let entry = self.entry(key.clone()).or_default();
136        (*entry).measure_and_replace_one(f)
137    }
138}
139
140/// A rate limiter that enforces different quotas per key using the GCRA algorithm.
141///
142/// This implementation allows setting different rate limits for different keys,
143/// with an optional default quota for keys that don't have specific quotas.
144pub struct RateLimiter<K, C>
145where
146    C: Clock,
147{
148    default_gcra: Option<Gcra>,
149    state: DashMapStateStore<K>,
150    gcra: DashMap<K, Gcra>,
151    clock: C,
152    start: C::Instant,
153}
154
155impl<K, C> Debug for RateLimiter<K, C>
156where
157    K: Debug,
158    C: Clock,
159{
160    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
161        f.debug_struct(stringify!(RateLimiter)).finish()
162    }
163}
164
165impl<K> RateLimiter<K, MonotonicClock>
166where
167    K: Eq + Hash,
168{
169    /// Creates a new rate limiter with a base quota and keyed quotas.
170    ///
171    /// The base quota applies to all keys that don't have specific quotas.
172    /// Keyed quotas override the base quota for specific keys.
173    #[must_use]
174    pub fn new_with_quota(base_quota: Option<Quota>, keyed_quotas: Vec<(K, Quota)>) -> Self {
175        let clock = MonotonicClock {};
176        let start = MonotonicClock::now(&clock);
177        let gcra: DashMap<_, _> = keyed_quotas
178            .into_iter()
179            .map(|(k, q)| (k, Gcra::new(q)))
180            .collect();
181        Self {
182            default_gcra: base_quota.map(Gcra::new),
183            state: DashMapStateStore::new(),
184            gcra,
185            clock,
186            start,
187        }
188    }
189}
190
191impl<K> RateLimiter<K, FakeRelativeClock>
192where
193    K: Hash + Eq + Clone,
194{
195    /// Advances the fake clock by the specified duration.
196    ///
197    /// This is only available for testing with `FakeRelativeClock`.
198    pub fn advance_clock(&self, by: Duration) {
199        self.clock.advance(by);
200    }
201}
202
203impl<K, C> RateLimiter<K, C>
204where
205    K: Hash + Eq + Clone,
206    C: Clock,
207{
208    /// Adds or updates a quota for a specific key.
209    pub fn add_quota_for_key(&self, key: K, value: Quota) {
210        self.gcra.insert(key, Gcra::new(value));
211    }
212
213    /// Checks if the given key is allowed under the rate limit.
214    ///
215    /// # Errors
216    ///
217    /// Returns `Err(NotUntil)` if the key is rate-limited, indicating when it will be allowed.
218    pub fn check_key(&self, key: &K) -> Result<(), NotUntil<C::Instant>> {
219        match self.gcra.get(key) {
220            Some(quota) => quota.test_and_update(self.start, key, &self.state, self.clock.now()),
221            None => self.default_gcra.as_ref().map_or(Ok(()), |gcra| {
222                gcra.test_and_update(self.start, key, &self.state, self.clock.now())
223            }),
224        }
225    }
226
227    /// Waits until the specified key is ready (not rate-limited).
228    pub async fn until_key_ready(&self, key: &K) {
229        loop {
230            match self.check_key(key) {
231                Ok(()) => {
232                    break;
233                }
234                Err(e) => {
235                    self.clock.sleep(e.wait_time_from(self.clock.now())).await;
236                }
237            }
238        }
239    }
240
241    /// Waits until all specified keys are ready (not rate-limited).
242    ///
243    /// If no keys are provided, this function returns immediately.
244    /// Uses fast paths for 0-2 keys to avoid stream scheduling overhead.
245    pub async fn await_keys_ready(&self, keys: Option<&[K]>) {
246        let Some(keys) = keys else {
247            return;
248        };
249
250        match keys.len() {
251            0 => {}
252            1 => self.until_key_ready(&keys[0]).await,
253            2 => {
254                tokio::join!(
255                    self.until_key_ready(&keys[0]),
256                    self.until_key_ready(&keys[1]),
257                );
258            }
259            _ => {
260                let tasks = keys.iter().map(|key| self.until_key_ready(key));
261                futures::stream::iter(tasks)
262                    .for_each_concurrent(None, |key_future| async move {
263                        key_future.await;
264                    })
265                    .await;
266            }
267        }
268    }
269}
270
271#[cfg(test)]
272mod tests {
273    use std::{
274        num::NonZeroU32,
275        sync::atomic::{AtomicU32, Ordering},
276        time::Duration,
277    };
278
279    use dashmap::DashMap;
280    use rstest::rstest;
281
282    use super::{
283        DashMapStateStore, RateLimiter,
284        clock::{Clock, FakeRelativeClock},
285        gcra::{Gcra, StateSnapshot},
286        nanos::Nanos,
287        quota::Quota,
288    };
289
290    fn initialize_mock_rate_limiter() -> RateLimiter<String, FakeRelativeClock> {
291        let clock = FakeRelativeClock::default();
292        let start = clock.now();
293        let gcra = DashMap::new();
294        let base_quota = Quota::per_second(NonZeroU32::new(2).unwrap()).unwrap();
295        RateLimiter {
296            default_gcra: Some(Gcra::new(base_quota)),
297            state: DashMapStateStore::new(),
298            gcra,
299            clock,
300            start,
301        }
302    }
303
304    #[rstest]
305    fn test_default_quota() {
306        let mock_limiter = initialize_mock_rate_limiter();
307
308        // Check base quota is not exceeded
309        assert!(mock_limiter.check_key(&"default".to_string()).is_ok());
310        assert!(mock_limiter.check_key(&"default".to_string()).is_ok());
311
312        // Check base quota is exceeded
313        assert!(mock_limiter.check_key(&"default".to_string()).is_err());
314
315        // Increment clock and check base quota is reset
316        mock_limiter.advance_clock(Duration::from_secs(1));
317        assert!(mock_limiter.check_key(&"default".to_string()).is_ok());
318    }
319
320    #[rstest]
321    fn test_custom_key_quota() {
322        let mock_limiter = initialize_mock_rate_limiter();
323
324        // Add new key quota pair
325        mock_limiter.add_quota_for_key(
326            "custom".to_string(),
327            Quota::per_second(NonZeroU32::new(1).unwrap()).unwrap(),
328        );
329
330        // Check custom quota
331        assert!(mock_limiter.check_key(&"custom".to_string()).is_ok());
332        assert!(mock_limiter.check_key(&"custom".to_string()).is_err());
333
334        // Check that default quota still applies to other keys
335        assert!(mock_limiter.check_key(&"default".to_string()).is_ok());
336        assert!(mock_limiter.check_key(&"default".to_string()).is_ok());
337        assert!(mock_limiter.check_key(&"default".to_string()).is_err());
338    }
339
340    #[rstest]
341    fn test_multiple_keys() {
342        let mock_limiter = initialize_mock_rate_limiter();
343
344        mock_limiter.add_quota_for_key(
345            "key1".to_string(),
346            Quota::per_second(NonZeroU32::new(1).unwrap()).unwrap(),
347        );
348        mock_limiter.add_quota_for_key(
349            "key2".to_string(),
350            Quota::per_second(NonZeroU32::new(3).unwrap()).unwrap(),
351        );
352
353        // Test key1
354        assert!(mock_limiter.check_key(&"key1".to_string()).is_ok());
355        assert!(mock_limiter.check_key(&"key1".to_string()).is_err());
356
357        // Test key2
358        assert!(mock_limiter.check_key(&"key2".to_string()).is_ok());
359        assert!(mock_limiter.check_key(&"key2".to_string()).is_ok());
360        assert!(mock_limiter.check_key(&"key2".to_string()).is_ok());
361        assert!(mock_limiter.check_key(&"key2".to_string()).is_err());
362    }
363
364    #[rstest]
365    fn test_quota_reset() {
366        let mock_limiter = initialize_mock_rate_limiter();
367
368        // Exhaust quota
369        assert!(mock_limiter.check_key(&"reset".to_string()).is_ok());
370        assert!(mock_limiter.check_key(&"reset".to_string()).is_ok());
371        assert!(mock_limiter.check_key(&"reset".to_string()).is_err());
372
373        // Advance clock by less than a second
374        mock_limiter.advance_clock(Duration::from_millis(499));
375        assert!(mock_limiter.check_key(&"reset".to_string()).is_err());
376
377        // Advance clock to reset
378        mock_limiter.advance_clock(Duration::from_millis(501));
379        assert!(mock_limiter.check_key(&"reset".to_string()).is_ok());
380    }
381
382    #[rstest]
383    fn test_different_quotas() {
384        let mock_limiter = initialize_mock_rate_limiter();
385
386        mock_limiter.add_quota_for_key(
387            "per_second".to_string(),
388            Quota::per_second(NonZeroU32::new(2).unwrap()).unwrap(),
389        );
390        mock_limiter.add_quota_for_key(
391            "per_minute".to_string(),
392            Quota::per_minute(NonZeroU32::new(3).unwrap()),
393        );
394
395        // Test per_second quota
396        assert!(mock_limiter.check_key(&"per_second".to_string()).is_ok());
397        assert!(mock_limiter.check_key(&"per_second".to_string()).is_ok());
398        assert!(mock_limiter.check_key(&"per_second".to_string()).is_err());
399
400        // Test per_minute quota
401        assert!(mock_limiter.check_key(&"per_minute".to_string()).is_ok());
402        assert!(mock_limiter.check_key(&"per_minute".to_string()).is_ok());
403        assert!(mock_limiter.check_key(&"per_minute".to_string()).is_ok());
404        assert!(mock_limiter.check_key(&"per_minute".to_string()).is_err());
405
406        // Advance clock and check reset
407        mock_limiter.advance_clock(Duration::from_secs(1));
408        assert!(mock_limiter.check_key(&"per_second".to_string()).is_ok());
409        assert!(mock_limiter.check_key(&"per_minute".to_string()).is_err());
410    }
411
412    #[tokio::test]
413    async fn test_await_keys_ready() {
414        let mock_limiter = initialize_mock_rate_limiter();
415
416        // Check base quota is not exceeded
417        assert!(mock_limiter.check_key(&"default".to_string()).is_ok());
418        assert!(mock_limiter.check_key(&"default".to_string()).is_ok());
419
420        // Check base quota is exceeded
421        assert!(mock_limiter.check_key(&"default".to_string()).is_err());
422
423        // Wait keys to be ready and check base quota is reset
424        mock_limiter.advance_clock(Duration::from_secs(1));
425        let keys = ["default".to_string()];
426        mock_limiter.await_keys_ready(Some(keys.as_slice())).await;
427        assert!(mock_limiter.check_key(&"default".to_string()).is_ok());
428    }
429
430    #[rstest]
431    fn test_remaining_burst_capacity_zero_t() {
432        let snapshot = StateSnapshot::new(
433            Nanos::from(0u64),
434            Nanos::from(1_000_000u64),
435            Nanos::from(0u64),
436            Nanos::from(0u64),
437        );
438        assert_eq!(snapshot.remaining_burst_capacity(), 0);
439    }
440
441    #[rstest]
442    fn test_per_second_returns_none_on_zero_replenish_interval() {
443        assert!(Quota::per_second(NonZeroU32::new(u32::MAX).unwrap()).is_none());
444    }
445
446    #[rstest]
447    fn test_per_minute_accepts_max_burst() {
448        let quota = Quota::per_minute(NonZeroU32::new(u32::MAX).unwrap());
449        assert!(quota.replenish_interval().as_nanos() > 0);
450    }
451
452    #[rstest]
453    fn test_per_hour_accepts_max_burst() {
454        let quota = Quota::per_hour(NonZeroU32::new(u32::MAX).unwrap());
455        assert!(quota.replenish_interval().as_nanos() > 0);
456    }
457
458    mod property_tests {
459        use proptest::prelude::*;
460        use rstest::rstest;
461
462        use crate::ratelimiter::{gcra::StateSnapshot, nanos::Nanos};
463
464        // Upper bound: ~1 hour in nanoseconds (realistic GCRA range)
465        const MAX_NANOS: u64 = 3_600_000_000_000;
466
467        proptest! {
468            #![proptest_config(ProptestConfig {
469                failure_persistence: Some(Box::new(
470                    proptest::test_runner::FileFailurePersistence::WithSource("ratelimiter")
471                )),
472                ..ProptestConfig::default()
473            })]
474
475            #[rstest]
476            fn remaining_burst_capacity_never_panics(
477                t in 0u64..=MAX_NANOS,
478                tau in 0u64..=MAX_NANOS,
479                time_of_measurement in 0u64..=MAX_NANOS,
480                tat in 0u64..=MAX_NANOS,
481            ) {
482                let snapshot = StateSnapshot::new(
483                    Nanos::from(t),
484                    Nanos::from(tau),
485                    Nanos::from(time_of_measurement),
486                    Nanos::from(tat),
487                );
488
489                let _ = snapshot.remaining_burst_capacity();
490            }
491        }
492    }
493
494    #[rstest]
495    fn test_gcra_boundary_exact_replenishment() {
496        // Test GCRA boundary condition where t0 equals earliest_time exactly.
497        // This exercises the saturating_sub edge case deterministically without sleeps.
498        let mock_limiter = initialize_mock_rate_limiter();
499        let key = "boundary_test".to_string();
500
501        assert!(mock_limiter.check_key(&key).is_ok());
502        assert!(mock_limiter.check_key(&key).is_ok());
503        assert!(mock_limiter.check_key(&key).is_err());
504
505        // Advance clock by exactly one replenish interval (500ms for 2 req/sec)
506        let quota = Quota::per_second(NonZeroU32::new(2).unwrap()).unwrap();
507        let replenish_interval = quota.replenish_interval();
508        mock_limiter.advance_clock(replenish_interval);
509
510        assert!(
511            mock_limiter.check_key(&key).is_ok(),
512            "Request at exact replenish boundary should be allowed"
513        );
514        assert!(
515            mock_limiter.check_key(&key).is_err(),
516            "Immediate follow-up should be rate-limited"
517        );
518    }
519
520    #[rstest]
521    fn test_per_second_boundary_exact_limit() {
522        // 1_000_000_000ns / 1_000_000_000 = 1ns per replenish, the exact boundary
523        let quota = Quota::per_second(NonZeroU32::new(1_000_000_000).unwrap()).unwrap();
524        assert_eq!(quota.replenish_interval().as_nanos(), 1);
525    }
526
527    #[rstest]
528    fn test_per_second_returns_none_above_one_billion() {
529        // 1_000_000_000ns / 1_000_000_001 rounds to 0ns
530        assert!(Quota::per_second(NonZeroU32::new(1_000_000_001).unwrap()).is_none());
531    }
532
533    #[rstest]
534    fn test_burst_size_replenished_in_truncation() {
535        // 100_000_000_000ns * u32::MAX overflows u64, `as u64` silently truncates
536        let quota = Quota::with_period(Duration::from_secs(100))
537            .unwrap()
538            .allow_burst(NonZeroU32::new(u32::MAX).unwrap());
539
540        let replenished_in = quota.burst_size_replenished_in();
541        let full: u128 = 100_000_000_000u128 * u128::from(u32::MAX);
542        let truncated = full as u64;
543
544        assert_eq!(replenished_in, Duration::from_nanos(truncated));
545        assert_ne!(
546            full,
547            u128::from(truncated),
548            "Truncation should have occurred"
549        );
550    }
551
552    #[rstest]
553    #[should_panic(expected = "t cannot be zero")]
554    fn test_from_gcra_parameters_panics_on_zero_t() {
555        let _ = Quota::from_gcra_parameters(Nanos::from(0u64), Nanos::from(100u64));
556    }
557
558    #[rstest]
559    #[should_panic(expected = "tau/t results in zero burst capacity")]
560    fn test_from_gcra_parameters_panics_on_zero_division() {
561        // tau=1, t=2 → integer division yields 0
562        let _ = Quota::from_gcra_parameters(Nanos::from(2u64), Nanos::from(1u64));
563    }
564
565    #[rstest]
566    #[should_panic(expected = "tau/t exceeds u32::MAX")]
567    fn test_from_gcra_parameters_panics_on_overflow() {
568        let _ = Quota::from_gcra_parameters(Nanos::from(1u64), Nanos::from(u64::MAX));
569    }
570
571    #[rstest]
572    fn test_concurrent_check_key_respects_burst() {
573        let rate = 10u32;
574        let clock = FakeRelativeClock::default();
575        let start = clock.now();
576        let limiter = RateLimiter {
577            default_gcra: Some(Gcra::new(
578                Quota::per_second(NonZeroU32::new(rate).unwrap()).unwrap(),
579            )),
580            state: DashMapStateStore::new(),
581            gcra: DashMap::new(),
582            clock,
583            start,
584        };
585
586        let accepted = AtomicU32::new(0);
587        let num_threads = 50;
588
589        // Clock is frozen: no replenishment occurs
590        std::thread::scope(|s| {
591            for _ in 0..num_threads {
592                s.spawn(|| {
593                    if limiter.check_key(&"hot_key".to_string()).is_ok() {
594                        accepted.fetch_add(1, Ordering::Relaxed);
595                    }
596                });
597            }
598        });
599
600        let total = accepted.load(Ordering::Relaxed);
601        assert!(total >= 1, "At least one request should be accepted");
602        assert!(
603            total <= rate,
604            "Accepted {total} but burst capacity is {rate}"
605        );
606    }
607}
nautilus_network/ratelimiter/mod.rs

nautilus_network/ratelimiter/
mod.rs
