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//! Distributed rate limiting via the [`Cache`](crate::cache::Cache) trait.
//!
//! Pair this with `cache::RedisCache` for safe enforcement across many
//! processes / replicas — a single shared counter per `(window, key)`
//! pair, incremented atomically by Redis' `INCRBY`. Pair with the
//! built-in `InMemoryCache` for testing.
//!
//! ## Algorithm: fixed-window counter
//!
//! For each `(key, window)` pair, the bucket id is the unix-seconds
//! window-start (`(now / window_secs) * window_secs`). Each request
//! increments that counter. The counter expires when its window does.
//! Simple, fast, no per-request locks, and works across replicas because
//! Redis owns the shared state.
//!
//! Trade vs. token-bucket: bursts can hit `2 * capacity` in a single
//! second straddling a window edge. For most APIs this is fine — if
//! you need leaky-bucket smoothness, stay with the in-process
//! [`crate::rate_limit::RateLimitLayer`] (single replica) or build a
//! sliding-window counter on top of the same [`Cache`] trait.
//!
//! ## Quick start
//!
//! ```ignore
//! use rustango::rate_limit::KeyBy;
//! use rustango::rate_limit_cache::{CacheRateLimitLayer, CacheRateLimitRouterExt};
//! use rustango::cache::RedisCache;
//! use std::sync::Arc;
//! use std::time::Duration;
//!
//! let cache: rustango::cache::BoxedCache =
//! Arc::new(RedisCache::connect("redis://localhost").await?);
//!
//! let app = axum::Router::new()
//! .route("/api/login", axum::routing::post(login))
//! .cache_rate_limit(
//! CacheRateLimitLayer::new(cache, 5, Duration::from_secs(60))
//! .key_by(KeyBy::Ip)
//! .key_prefix("login"),
//! );
//! ```
use std::net::SocketAddr;
use std::sync::Arc;
use std::time::{Duration, SystemTime, UNIX_EPOCH};
use axum::body::Body;
use axum::extract::{ConnectInfo, Request};
use axum::http::{header, HeaderValue, Response, StatusCode};
use axum::middleware::Next;
use axum::Router;
use crate::cache::BoxedCache;
use crate::rate_limit::KeyBy;
/// Fixed-window counter rate limiter backed by a [`Cache`](crate::cache::Cache).
///
/// Cheap to clone (everything is `Arc`-wrapped or `Copy`).
#[derive(Clone)]
pub struct CacheRateLimitLayer {
cache: BoxedCache,
capacity: u32,
window: Duration,
key_by: KeyBy,
/// Prefix for cache keys. Distinguishes multiple limiters that share
/// the same cache (e.g. `"login"` vs `"signup"`).
key_prefix: Arc<String>,
}
impl CacheRateLimitLayer {
/// New limiter: `capacity` requests per `window`, keyed by IP.
#[must_use]
pub fn new(cache: BoxedCache, capacity: u32, window: Duration) -> Self {
Self {
cache,
capacity,
window,
key_by: KeyBy::Ip,
key_prefix: Arc::new("rl".to_owned()),
}
}
#[must_use]
pub fn key_by(mut self, key_by: KeyBy) -> Self {
self.key_by = key_by;
self
}
#[must_use]
pub fn key_prefix(mut self, prefix: impl Into<String>) -> Self {
self.key_prefix = Arc::new(prefix.into());
self
}
fn extract_key(&self, req: &Request<Body>) -> String {
match &self.key_by {
KeyBy::Ip => req
.extensions()
.get::<ConnectInfo<SocketAddr>>()
.map(|ci| ci.ip().to_string())
.unwrap_or_else(|| "<no-ip>".to_owned()),
KeyBy::Header(name) => req
.headers()
.get(*name)
.and_then(|v| v.to_str().ok())
.map(str::to_owned)
.unwrap_or_else(|| "<no-header>".to_owned()),
KeyBy::Global => "<global>".to_owned(),
}
}
fn window_secs(&self) -> u64 {
self.window.as_secs().max(1)
}
/// Take one slot from the bucket for `key`. Returns
/// `Ok((current_count, reset_at_unix_secs))` on success,
/// `Err(retry_after_secs)` when over capacity.
///
/// # Errors
/// Returns `Err(retry_after_secs)` when the limit has been hit. Any
/// underlying cache error is treated as "fail open" — the request is
/// allowed and `Ok((0, 0))` is returned. This avoids hard outages
/// when Redis is briefly unreachable; flip to fail-closed by reading
/// `cache.incr(...)` directly if your threat model requires it.
pub async fn take(&self, key: &str) -> Result<(u32, u64), u64> {
let window_secs = self.window_secs();
let now = SystemTime::now()
.duration_since(UNIX_EPOCH)
.map_or(0, |d| d.as_secs());
let window_start = (now / window_secs) * window_secs;
let cache_key = format!("{}:{}:{window_start}", self.key_prefix.as_str(), key,);
let count = match self
.cache
.incr(&cache_key, 1, Some(Duration::from_secs(window_secs)))
.await
{
Ok(n) => n,
Err(_e) => {
// Fail-open: cache outage shouldn't deny all traffic.
tracing::warn!(cache_key, "rate-limit cache incr failed; allowing request");
return Ok((0, 0));
}
};
let reset_at = window_start + window_secs;
if count > i64::from(self.capacity) {
let retry = reset_at.saturating_sub(now).max(1);
Err(retry)
} else {
// count fits in u32 because capacity is u32 and we check above.
Ok((u32::try_from(count).unwrap_or(u32::MAX), reset_at))
}
}
}
/// Extension trait — apply a cache-backed rate-limit layer to a router.
pub trait CacheRateLimitRouterExt {
#[must_use]
fn cache_rate_limit(self, layer: CacheRateLimitLayer) -> Self;
}
impl<S: Clone + Send + Sync + 'static> CacheRateLimitRouterExt for Router<S> {
fn cache_rate_limit(self, layer: CacheRateLimitLayer) -> Self {
let cfg = Arc::new(layer);
self.layer(axum::middleware::from_fn(
move |req: Request<Body>, next: Next| {
let cfg = cfg.clone();
async move { handle(cfg, req, next).await }
},
))
}
}
async fn handle(cfg: Arc<CacheRateLimitLayer>, req: Request<Body>, next: Next) -> Response<Body> {
let key = cfg.extract_key(&req);
match cfg.take(&key).await {
Ok((count, reset_at)) => {
let mut response = next.run(req).await;
let remaining = i64::from(cfg.capacity).saturating_sub(i64::from(count));
let _ = response.headers_mut().insert(
"x-ratelimit-limit",
HeaderValue::from_str(&cfg.capacity.to_string())
.unwrap_or(HeaderValue::from_static("0")),
);
let _ = response.headers_mut().insert(
"x-ratelimit-remaining",
HeaderValue::from_str(&remaining.max(0).to_string())
.unwrap_or(HeaderValue::from_static("0")),
);
if reset_at > 0 {
let _ = response.headers_mut().insert(
"x-ratelimit-reset",
HeaderValue::from_str(&reset_at.to_string())
.unwrap_or(HeaderValue::from_static("0")),
);
}
response
}
Err(retry_secs) => Response::builder()
.status(StatusCode::TOO_MANY_REQUESTS)
.header(header::RETRY_AFTER, retry_secs.to_string())
.header("x-ratelimit-limit", cfg.capacity.to_string())
.header("x-ratelimit-remaining", "0")
.body(Body::from(format!(
r#"{{"error":"rate limit exceeded","retry_after":{retry_secs}}}"#
)))
.unwrap_or_else(|_| Response::new(Body::empty())),
}
}
#[cfg(test)]
mod tests {
use super::*;
use crate::cache::InMemoryCache;
fn layer(capacity: u32, window_secs: u64) -> CacheRateLimitLayer {
let cache: BoxedCache = Arc::new(InMemoryCache::new());
CacheRateLimitLayer::new(cache, capacity, Duration::from_secs(window_secs))
.key_prefix("test")
}
#[tokio::test]
async fn first_n_under_capacity_succeed() {
let l = layer(3, 60);
for _ in 0..3 {
assert!(l.take("alice").await.is_ok());
}
}
#[tokio::test]
async fn n_plus_one_returns_retry_after() {
let l = layer(2, 60);
assert!(l.take("alice").await.is_ok());
assert!(l.take("alice").await.is_ok());
let err = l.take("alice").await.unwrap_err();
assert!(err >= 1, "retry_after must be at least 1 sec, got {err}");
}
#[tokio::test]
async fn separate_keys_have_independent_counters() {
let l = layer(1, 60);
assert!(l.take("alice").await.is_ok());
assert!(l.take("alice").await.is_err());
// Different key — fresh bucket
assert!(l.take("bob").await.is_ok());
}
#[tokio::test]
async fn separate_prefixes_have_independent_counters() {
let cache: BoxedCache = Arc::new(InMemoryCache::new());
let a =
CacheRateLimitLayer::new(cache.clone(), 1, Duration::from_secs(60)).key_prefix("login");
let b = CacheRateLimitLayer::new(cache, 1, Duration::from_secs(60)).key_prefix("signup");
assert!(a.take("alice").await.is_ok());
assert!(a.take("alice").await.is_err());
// b uses a different prefix — same key gets its own counter
assert!(b.take("alice").await.is_ok());
}
#[tokio::test]
async fn count_returned_increases_per_call() {
let l = layer(5, 60);
let (c1, _) = l.take("k").await.unwrap();
let (c2, _) = l.take("k").await.unwrap();
let (c3, _) = l.take("k").await.unwrap();
assert_eq!((c1, c2, c3), (1, 2, 3));
}
#[tokio::test]
async fn reset_at_advances_with_window() {
let l = layer(1, 60);
let (_, reset_at) = l.take("k").await.unwrap();
let now = SystemTime::now()
.duration_since(UNIX_EPOCH)
.unwrap()
.as_secs();
// reset_at must be in the future and within one window-length.
assert!(reset_at > now);
assert!(reset_at <= now + 60);
}
#[tokio::test]
async fn fail_open_on_cache_error_is_documented_via_take_succeeding() {
// The InMemoryCache never errors, so we verify the success path
// here. Real fail-open behavior is exercised by the
// CacheRateLimitLayer::take() doc comment + the fact that we
// return Ok((0,0)) on Err.
let l = layer(2, 60);
assert!(l.take("k").await.is_ok());
}
}