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 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255
// Copyright 2023 The Matrix.org Foundation C.I.C.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//! A TTL cache which can be used to time out repeated operations that might
//! experience intermittent failures.
use std::{
borrow::Borrow,
collections::HashMap,
hash::Hash,
sync::{Arc, RwLock},
time::Duration,
};
use ruma::time::Instant;
const MAX_DELAY: u64 = 15 * 60;
const MULTIPLIER: u64 = 15;
/// A TTL cache where items get inactive instead of discarded.
///
/// The items need to be explicitly removed from the cache. This allows us to
/// implement exponential backoff based TTL.
#[derive(Clone, Debug)]
pub struct FailuresCache<T: Eq + Hash> {
inner: Arc<InnerCache<T>>,
}
#[derive(Debug)]
struct InnerCache<T: Eq + Hash> {
max_delay: Duration,
backoff_multiplier: u64,
items: RwLock<HashMap<T, FailuresItem>>,
}
impl<T: Eq + Hash> Default for InnerCache<T> {
fn default() -> Self {
Self {
max_delay: Duration::from_secs(MAX_DELAY),
backoff_multiplier: MULTIPLIER,
items: Default::default(),
}
}
}
#[derive(Debug, Clone, Copy)]
struct FailuresItem {
insertion_time: Instant,
duration: Duration,
/// Number of times that this item has failed after it was first added to
/// the cache. (In other words, one less than the total number of
/// failures.)
failure_count: u8,
}
impl FailuresItem {
/// Has the item expired.
fn expired(&self) -> bool {
self.insertion_time.elapsed() >= self.duration
}
/// Force the expiry of this item.
///
/// This doesn't reset the failure count, but does mark the item as ready
/// for immediate retry.
fn expire(&mut self) {
self.duration = Duration::from_secs(0);
}
}
impl<T> FailuresCache<T>
where
T: Eq + Hash,
{
pub fn new() -> Self {
Self { inner: Default::default() }
}
pub fn with_settings(max_delay: Duration, multiplier: u8) -> Self {
Self {
inner: InnerCache {
max_delay,
backoff_multiplier: multiplier.into(),
items: Default::default(),
}
.into(),
}
}
/// Is the given key non-expired and part of the cache.
pub fn contains<Q>(&self, key: &Q) -> bool
where
T: Borrow<Q>,
Q: Hash + Eq + ?Sized,
{
let lock = self.inner.items.read().unwrap();
let contains = if let Some(item) = lock.get(key) { !item.expired() } else { false };
contains
}
/// Get the failure count for a given key.
///
/// # Returns
///
/// * `None` if this key is not in the failure cache. (It has never failed,
/// or it has been [`FailuresCache::remove()`]d since the last failure.)
///
/// * `Some(u8)`: the number of times it has failed since it was first
/// added to the failure cache. (In other words, one less than the total
/// number of failures.)
pub fn failure_count<Q>(&self, key: &Q) -> Option<u8>
where
T: Borrow<Q>,
Q: Hash + Eq + ?Sized,
{
let lock = self.inner.items.read().unwrap();
lock.get(key).map(|i| i.failure_count)
}
/// This will calculate a duration that determines how long an item is
/// considered to be valid while being in the cache.
///
/// The returned duration will follow this sequence if the default
/// multiplier and `max_delay` values are used, values are in minutes:
/// [0.25, 0.5, 1.0, 2.0, 4.0, 8.0, 15.0]
fn calculate_delay(&self, failure_count: u8) -> Duration {
let exponential_backoff = 2u64.saturating_pow(failure_count.into());
let delay = exponential_backoff.saturating_mul(self.inner.backoff_multiplier);
Duration::from_secs(delay).clamp(Duration::from_secs(1), self.inner.max_delay)
}
/// Add a single item to the cache.
pub fn insert(&self, item: T) {
self.extend([item]);
}
/// Extend the cache with the given iterator of items.
///
/// Items that are already part of the cache, whether they are expired or
/// not, will have their TTL extended using an exponential backoff
/// algorithm.
pub fn extend(&self, iterator: impl IntoIterator<Item = T>) {
let mut lock = self.inner.items.write().unwrap();
let now = Instant::now();
for key in iterator {
let failure_count = if let Some(value) = lock.get(&key) {
value.failure_count.saturating_add(1)
} else {
0
};
let delay = self.calculate_delay(failure_count);
let item = FailuresItem { insertion_time: now, duration: delay, failure_count };
lock.insert(key, item);
}
}
/// Remove the items contained in the iterator from the cache.
pub fn remove<'a, I, Q>(&'a self, iterator: I)
where
I: Iterator<Item = &'a Q>,
T: Borrow<Q>,
Q: Hash + Eq + 'a + ?Sized,
{
let mut lock = self.inner.items.write().unwrap();
for item in iterator {
lock.remove(item);
}
}
/// Force the expiry of the given item, if it is present in the cache.
///
/// This doesn't reset the failure count, but does mark the item as ready
/// for immediate retry.
#[doc(hidden)]
pub fn expire(&self, item: &T) {
let mut lock = self.inner.items.write().unwrap();
lock.get_mut(item).map(FailuresItem::expire);
}
}
impl<T: Eq + Hash> Default for FailuresCache<T> {
fn default() -> Self {
Self::new()
}
}
#[cfg(test)]
mod tests {
use std::time::Duration;
use proptest::prelude::*;
use super::FailuresCache;
#[test]
fn failures_cache() {
let cache = FailuresCache::new();
assert!(!cache.contains(&1));
cache.extend([1u8].iter());
assert!(cache.contains(&1));
cache.inner.items.write().unwrap().get_mut(&1).unwrap().duration = Duration::from_secs(0);
assert!(!cache.contains(&1));
cache.remove([1u8].iter());
assert!(cache.inner.items.read().unwrap().get(&1).is_none())
}
#[test]
fn failures_cache_timeout() {
let cache: FailuresCache<u8> = FailuresCache::new();
assert_eq!(cache.calculate_delay(0).as_secs(), 15);
assert_eq!(cache.calculate_delay(1).as_secs(), 30);
assert_eq!(cache.calculate_delay(2).as_secs(), 60);
assert_eq!(cache.calculate_delay(3).as_secs(), 120);
assert_eq!(cache.calculate_delay(4).as_secs(), 240);
assert_eq!(cache.calculate_delay(5).as_secs(), 480);
assert_eq!(cache.calculate_delay(6).as_secs(), 900);
assert_eq!(cache.calculate_delay(7).as_secs(), 900);
}
proptest! {
#[test]
fn failures_cache_proptest_timeout(count in 0..10u8) {
let cache: FailuresCache<u8> = FailuresCache::new();
let delay = cache.calculate_delay(count).as_secs();
assert!(delay <= 900);
assert!(delay >= 15);
}
}
}