rust 47 lines · 8 steps

A token bucket rate limiter in Rust

A token bucket refills tokens over time and only allows work when enough tokens are on hand.

Explained by highlit
Intermediate rate-limiting token-bucket lazy-refill state-machine
1use std::time::{Duration, Instant};
2 
3pub struct TokenBucket {
4 capacity: f64,
5 tokens: f64,
6 refill_per_sec: f64,
7 last_refill: Instant,
8}
9 
10impl TokenBucket {
11 pub fn new(capacity: f64, refill_per_sec: f64) -> Self {
12 Self {
13 capacity,
14 tokens: capacity,
15 refill_per_sec,
16 last_refill: Instant::now(),
17 }
18 }
19 
20 fn refill(&mut self) {
21 let now = Instant::now();
22 let elapsed = now.duration_since(self.last_refill).as_secs_f64();
23 if elapsed > 0.0 {
24 self.tokens = (self.tokens + elapsed * self.refill_per_sec).min(self.capacity);
25 self.last_refill = now;
26 }
27 }
28 
29 pub fn try_acquire(&mut self, cost: f64) -> bool {
30 self.refill();
31 if self.tokens >= cost {
32 self.tokens -= cost;
33 true
34 } else {
35 false
36 }
37 }
38 
39 pub fn time_until_available(&mut self, cost: f64) -> Duration {
40 self.refill();
41 if self.tokens >= cost {
42 return Duration::ZERO;
43 }
44 let deficit = cost - self.tokens;
45 Duration::from_secs_f64(deficit / self.refill_per_sec)
46 }
47}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Lazy refill on each access avoids background timers by computing tokens from elapsed time.
  2. 2Clamping to capacity keeps a bucket from accumulating credit while idle.
  3. 3Storing the last refill instant lets you derive both availability and wait time from one clock read.

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