moka/policy.rs
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use std::{
fmt,
sync::Arc,
time::{Duration, Instant},
};
#[derive(Clone, Debug)]
/// The policy of a cache.
pub struct Policy {
max_capacity: Option<u64>,
num_segments: usize,
time_to_live: Option<Duration>,
time_to_idle: Option<Duration>,
}
impl Policy {
pub(crate) fn new(
max_capacity: Option<u64>,
num_segments: usize,
time_to_live: Option<Duration>,
time_to_idle: Option<Duration>,
) -> Self {
Self {
max_capacity,
num_segments,
time_to_live,
time_to_idle,
}
}
/// Returns the `max_capacity` of the cache.
pub fn max_capacity(&self) -> Option<u64> {
self.max_capacity
}
#[cfg(feature = "sync")]
pub(crate) fn set_max_capacity(&mut self, capacity: Option<u64>) {
self.max_capacity = capacity;
}
/// Returns the number of internal segments of the cache.
pub fn num_segments(&self) -> usize {
self.num_segments
}
#[cfg(feature = "sync")]
pub(crate) fn set_num_segments(&mut self, num: usize) {
self.num_segments = num;
}
/// Returns the `time_to_live` of the cache.
pub fn time_to_live(&self) -> Option<Duration> {
self.time_to_live
}
/// Returns the `time_to_idle` of the cache.
pub fn time_to_idle(&self) -> Option<Duration> {
self.time_to_idle
}
}
/// The eviction (and admission) policy of a cache.
///
/// When the cache is full, the eviction/admission policy is used to determine which
/// items should be admitted to the cache and which cached items should be evicted.
/// The choice of a policy will directly affect the performance (hit rate) of the
/// cache.
///
/// The following policies are available:
///
/// - **TinyLFU** (default):
/// - Suitable for most workloads.
/// - TinyLFU combines the LRU eviction policy and an admission policy based on the
/// historical popularity of keys.
/// - Note that it tracks not only the keys currently in the cache, but all hit and
/// missed keys. The data structure used to _estimate_ the popularity of keys is
/// a modified Count-Min Sketch, which has a very low memory footprint (thus the
/// name "tiny").
/// - **LRU**:
/// - Suitable for some workloads with strong recency bias, such as streaming data
/// processing.
///
/// LFU stands for Least Frequently Used. LRU stands for Least Recently Used.
///
/// Use associate function [`EvictionPolicy::tiny_lfu`](#method.tiny_lfu) or
/// [`EvictionPolicy::lru`](#method.lru) to obtain an instance of `EvictionPolicy`.
#[derive(Clone, Default)]
pub struct EvictionPolicy {
pub(crate) config: EvictionPolicyConfig,
}
impl EvictionPolicy {
/// Returns the TinyLFU policy, which is suitable for most workloads.
///
/// TinyLFU is a combination of the LRU eviction policy and the admission policy
/// based on the historical popularity of keys.
///
/// Note that it tracks not only the keys currently in the cache, but all hit and
/// missed keys. The data structure used to _estimate_ the popularity of keys is
/// a modified Count-Min Sketch, which has a very low memory footprint (thus the
/// name "tiny").
pub fn tiny_lfu() -> Self {
Self {
config: EvictionPolicyConfig::TinyLfu,
}
}
/// Returns the LRU policy.
///
/// Suitable for some workloads with strong recency bias, such as streaming data
/// processing.
pub fn lru() -> Self {
Self {
config: EvictionPolicyConfig::Lru,
}
}
}
impl fmt::Debug for EvictionPolicy {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self.config {
EvictionPolicyConfig::TinyLfu => write!(f, "EvictionPolicy::TinyLfu"),
EvictionPolicyConfig::Lru => write!(f, "EvictionPolicy::Lru"),
}
}
}
#[derive(Clone, Debug, PartialEq, Eq)]
pub(crate) enum EvictionPolicyConfig {
TinyLfu,
Lru,
}
impl Default for EvictionPolicyConfig {
fn default() -> Self {
Self::TinyLfu
}
}
/// Calculates when cache entries expire. A single expiration time is retained on
/// each entry so that the lifetime of an entry may be extended or reduced by
/// subsequent evaluations.
///
/// `Expiry` trait provides three methods. They specify the expiration time of an
/// entry by returning a `Some(duration)` until the entry expires:
///
/// - [`expire_after_create`](#method.expire_after_create) — Returns the
/// duration (or none) after the entry's creation.
/// - [`expire_after_read`](#method.expire_after_read) — Returns the duration
/// (or none) after its last read.
/// - [`expire_after_update`](#method.expire_after_update) — Returns the
/// duration (or none) after its last update.
///
/// The default implementations are provided that return `None` (no expiration) or
/// `current_duration: Option<Instant>` (not modify the current expiration time).
/// Override some of them as you need.
///
pub trait Expiry<K, V> {
/// Specifies that the entry should be automatically removed from the cache once
/// the duration has elapsed after the entry's creation. This method is called
/// for cache write methods such as `insert` and `get_with` but only when the key
/// was not present in the cache.
///
/// # Parameters
///
/// - `key` — A reference to the key of the entry.
/// - `value` — A reference to the value of the entry.
/// - `created_at` — The time when this entry was inserted.
///
/// # Return value
///
/// The returned `Option<Duration>` is used to set the expiration time of the
/// entry.
///
/// - Returning `Some(duration)` — The expiration time is set to
/// `created_at + duration`.
/// - Returning `None` — The expiration time is cleared (no expiration).
/// - This is the value that the default implementation returns.
///
/// # Notes on `time_to_live` and `time_to_idle` policies
///
/// When the cache is configured with `time_to_live` and/or `time_to_idle`
/// policies, the entry will be evicted after the earliest of the expiration time
/// returned by this expiry, the `time_to_live` and `time_to_idle` policies.
#[allow(unused_variables)]
fn expire_after_create(&self, key: &K, value: &V, created_at: Instant) -> Option<Duration> {
None
}
/// Specifies that the entry should be automatically removed from the cache once
/// the duration has elapsed after its last read. This method is called for cache
/// read methods such as `get` and `get_with` but only when the key is present in
/// the cache.
///
/// # Parameters
///
/// - `key` — A reference to the key of the entry.
/// - `value` — A reference to the value of the entry.
/// - `read_at` — The time when this entry was read.
/// - `duration_until_expiry` — The remaining duration until the entry
/// expires. (Calculated by `expiration_time - read_at`)
/// - `last_modified_at` — The time when this entry was created or updated.
///
/// # Return value
///
/// The returned `Option<Duration>` is used to set the expiration time of the
/// entry.
///
/// - Returning `Some(duration)` — The expiration time is set to
/// `read_at + duration`.
/// - Returning `None` — The expiration time is cleared (no expiration).
/// - Returning `duration_until_expiry` will not modify the expiration time.
/// - This is the value that the default implementation returns.
///
/// # Notes on `time_to_live` and `time_to_idle` policies
///
/// When the cache is configured with `time_to_live` and/or `time_to_idle`
/// policies, then:
///
/// - The entry will be evicted after the earliest of the expiration time
/// returned by this expiry, the `time_to_live` and `time_to_idle` policies.
/// - The `duration_until_expiry` takes in account the `time_to_live` and
/// `time_to_idle` policies.
#[allow(unused_variables)]
fn expire_after_read(
&self,
key: &K,
value: &V,
read_at: Instant,
duration_until_expiry: Option<Duration>,
last_modified_at: Instant,
) -> Option<Duration> {
duration_until_expiry
}
/// Specifies that the entry should be automatically removed from the cache once
/// the duration has elapsed after the replacement of its value. This method is
/// called for cache write methods such as `insert` but only when the key is
/// already present in the cache.
///
/// # Parameters
///
/// - `key` — A reference to the key of the entry.
/// - `value` — A reference to the value of the entry.
/// - `updated_at` — The time when this entry was updated.
/// - `duration_until_expiry` — The remaining duration until the entry
/// expires. (Calculated by `expiration_time - updated_at`)
///
/// # Return value
///
/// The returned `Option<Duration>` is used to set the expiration time of the
/// entry.
///
/// - Returning `Some(duration)` — The expiration time is set to
/// `updated_at + duration`.
/// - Returning `None` — The expiration time is cleared (no expiration).
/// - Returning `duration_until_expiry` will not modify the expiration time.
/// - This is the value that the default implementation returns.
///
/// # Notes on `time_to_live` and `time_to_idle` policies
///
/// When the cache is configured with `time_to_live` and/or `time_to_idle`
/// policies, then:
///
/// - The entry will be evicted after the earliest of the expiration time
/// returned by this expiry, the `time_to_live` and `time_to_idle` policies.
/// - The `duration_until_expiry` takes in account the `time_to_live` and
/// `time_to_idle` policies.
#[allow(unused_variables)]
fn expire_after_update(
&self,
key: &K,
value: &V,
updated_at: Instant,
duration_until_expiry: Option<Duration>,
) -> Option<Duration> {
duration_until_expiry
}
}
pub(crate) struct ExpirationPolicy<K, V> {
time_to_live: Option<Duration>,
time_to_idle: Option<Duration>,
expiry: Option<Arc<dyn Expiry<K, V> + Send + Sync + 'static>>,
}
impl<K, V> Default for ExpirationPolicy<K, V> {
fn default() -> Self {
Self {
time_to_live: None,
time_to_idle: None,
expiry: None,
}
}
}
impl<K, V> Clone for ExpirationPolicy<K, V> {
fn clone(&self) -> Self {
Self {
time_to_live: self.time_to_live,
time_to_idle: self.time_to_idle,
expiry: self.expiry.clone(),
}
}
}
impl<K, V> ExpirationPolicy<K, V> {
#[cfg(test)]
pub(crate) fn new(
time_to_live: Option<Duration>,
time_to_idle: Option<Duration>,
expiry: Option<Arc<dyn Expiry<K, V> + Send + Sync + 'static>>,
) -> Self {
Self {
time_to_live,
time_to_idle,
expiry,
}
}
/// Returns the `time_to_live` of the cache.
pub(crate) fn time_to_live(&self) -> Option<Duration> {
self.time_to_live
}
pub(crate) fn set_time_to_live(&mut self, duration: Duration) {
self.time_to_live = Some(duration);
}
/// Returns the `time_to_idle` of the cache.
pub(crate) fn time_to_idle(&self) -> Option<Duration> {
self.time_to_idle
}
pub(crate) fn set_time_to_idle(&mut self, duration: Duration) {
self.time_to_idle = Some(duration);
}
pub(crate) fn expiry(&self) -> Option<Arc<dyn Expiry<K, V> + Send + Sync + 'static>> {
self.expiry.clone()
}
pub(crate) fn set_expiry(&mut self, expiry: Arc<dyn Expiry<K, V> + Send + Sync + 'static>) {
self.expiry = Some(expiry);
}
}
#[cfg(test)]
pub(crate) mod test_utils {
use std::sync::atomic::{AtomicU8, Ordering};
#[derive(Default)]
pub(crate) struct ExpiryCallCounters {
expected_creations: AtomicU8,
expected_reads: AtomicU8,
expected_updates: AtomicU8,
actual_creations: AtomicU8,
actual_reads: AtomicU8,
actual_updates: AtomicU8,
}
impl ExpiryCallCounters {
pub(crate) fn incl_expected_creations(&self) {
self.expected_creations.fetch_add(1, Ordering::Relaxed);
}
pub(crate) fn incl_expected_reads(&self) {
self.expected_reads.fetch_add(1, Ordering::Relaxed);
}
pub(crate) fn incl_expected_updates(&self) {
self.expected_updates.fetch_add(1, Ordering::Relaxed);
}
pub(crate) fn incl_actual_creations(&self) {
self.actual_creations.fetch_add(1, Ordering::Relaxed);
}
pub(crate) fn incl_actual_reads(&self) {
self.actual_reads.fetch_add(1, Ordering::Relaxed);
}
pub(crate) fn incl_actual_updates(&self) {
self.actual_updates.fetch_add(1, Ordering::Relaxed);
}
pub(crate) fn verify(&self) {
assert_eq!(
self.expected_creations.load(Ordering::Relaxed),
self.actual_creations.load(Ordering::Relaxed),
"expected_creations != actual_creations"
);
assert_eq!(
self.expected_reads.load(Ordering::Relaxed),
self.actual_reads.load(Ordering::Relaxed),
"expected_reads != actual_reads"
);
assert_eq!(
self.expected_updates.load(Ordering::Relaxed),
self.actual_updates.load(Ordering::Relaxed),
"expected_updates != actual_updates"
);
}
}
}