Add the jittered_expiry_policy example

Cargo.toml

@@ -78,6 +78,7 @@ env_logger = "0.10.0"
 getrandom = "0.2"
 once_cell = "1.7"
 paste = "1.0.9"
+rand = "0.8.5"
 reqwest = { version = "0.11.11", default-features = false, features = ["rustls-tls"] }
 tokio = { version = "1.19", features = ["fs", "io-util", "macros", "rt-multi-thread", "sync", "time" ] }
 
@@ -158,6 +159,10 @@ required-features = ["sync"]
 name = "eviction_listener_sync"
 required-features = ["sync"]
 
+[[example]]
+name = "jittered_expiry_policy_sync"
+required-features = ["sync"]
+
 [[example]]
 name = "reinsert_expired_entries_sync"
 required-features = ["sync"]

examples/README.md

@@ -60,6 +60,17 @@ existence of the entry.
     - Demonstrates when the expired entries will be actually evicted from the cache,
       and why the `run_pending_tasks` method could be important in some cases.
 
+- [jittered_expiry_policy_sync](./jittered_expiry_policy_sync.rs)
+    - Implements a jittered expiry policy for a cache.
+    - The `JitteredExpiry` struct is a custom expiry policy that adds jitter to the
+      base expiry duration.
+        - It implements the `moka::Expiry` trait and calculates the expiry duration
+          after a write or read operation.
+        - The jitter is randomly generated and added to or subtracted from the base
+          expiry duration.
+    - This example uses the `moka::sync::Cache` type, but The same expiry policy can
+      be used with the `moka::future::Cache`.
+
 - [cascading_drop_async](./cascading_drop_async.rs)
     - Controls the lifetime of the objects in a separate `BTreeMap` collection from
       the cache using an eviction listener.

examples/jittered_expiry_policy_sync.rs

@@ -0,0 +1,190 @@
+//! This example demonstrates how to implement a jittered expiry policy for a cache.
+//!
+//! The `JitteredExpiry` struct is a custom expiry policy that adds jitter to the
+//! expiry duration. It implements the `moka::Expiry` trait and calculates the expiry
+//! duration after a write or read operation. The jitter is randomly generated and
+//! added to or subtracted from the base expiry duration.
+//!
+//! This example uses the `moka::sync::Cache` type, which is a synchronous cache. The
+//! same expiry policy can be used with the asynchronous cache, `moka::future::Cache`.
+
+use std::time::{Duration, Instant};
+
+use moka::{sync::Cache, Expiry};
+use rand::{
+    distributions::{Distribution, Uniform},
+    Rng,
+};
+
+/// A `moka::Expiry` implementation that adds jitter to the expiry duration.
+pub struct JitteredExpiry<J> {
+    /// Optional time-to-live duration.
+    time_to_live: Option<Duration>,
+    /// Optional time-to-idle duration.
+    time_to_idle: Option<Duration>,
+    /// The distribution to randomly generate the jitter. The jitter is added to
+    /// or subtracted from the expiry duration.
+    jitter_gen: J,
+}
+
+impl<J> JitteredExpiry<J>
+where
+    J: Distribution<Duration>,
+{
+    pub fn new(
+        time_to_live: Option<Duration>,
+        time_to_idle: Option<Duration>,
+        jitter_gen: J,
+    ) -> Self {
+        Self {
+            time_to_live,
+            time_to_idle,
+            jitter_gen,
+        }
+    }
+
+    /// Calculates the expiry duration after a write operation.
+    pub fn calc_expiry_for_write(&self) -> Option<Duration> {
+        if matches!((self.time_to_live, self.time_to_idle), (None, None)) {
+            return None;
+        }
+
+        let expiry = match (self.time_to_live, self.time_to_idle) {
+            (Some(ttl), None) => ttl,
+            (None, Some(tti)) => tti,
+            (Some(ttl), Some(tti)) => ttl.min(tti),
+            (None, None) => unreachable!(),
+        };
+
+        Some(self.add_jitter(expiry))
+    }
+
+    /// Calculates the expiry duration after a read operation.
+    pub fn calc_expiry_for_read(&self, read_at: Instant, modified_at: Instant) -> Option<Duration> {
+        if matches!((self.time_to_live, self.time_to_idle), (None, None)) {
+            return None;
+        }
+
+        let expiry = match (self.time_to_live, self.time_to_idle) {
+            (Some(ttl), None) => {
+                let elapsed = Self::elapsed_since_write(read_at, modified_at);
+                Self::remaining_to_ttl(ttl, elapsed)
+            }
+            (None, Some(tti)) => tti,
+            (Some(ttl), Some(tti)) => {
+                // Ensure that the expiry duration does not exceed the
+                // time-to-live since last write.
+                let elapsed = Self::elapsed_since_write(read_at, modified_at);
+                let remaining = Self::remaining_to_ttl(ttl, elapsed);
+                tti.min(remaining)
+            }
+            (None, None) => unreachable!(),
+        };
+
+        Some(self.add_jitter(expiry))
+    }
+
+    /// Calculates the elapsed time between `modified_at` and `read_at`.
+    fn elapsed_since_write(read_at: Instant, modified_at: Instant) -> Duration {
+        // NOTE: `duration_since` panics if `read_at` is earlier than `modified_at`.
+        if read_at >= modified_at {
+            read_at.duration_since(modified_at)
+        } else {
+            Duration::default() // zero duration
+        }
+    }
+
+    /// Calculates the remaining time to live based on the `ttl` and `elapsed` time.
+    fn remaining_to_ttl(ttl: Duration, elapsed: Duration) -> Duration {
+        ttl.saturating_sub(elapsed)
+    }
+
+    /// Adds jitter to the given duration.
+    fn add_jitter(&self, duration: Duration) -> Duration {
+        let mut rng = rand::thread_rng();
+        let jitter = self.jitter_gen.sample(&mut rng);
+
+        // Add or subtract the jitter to/from the duration.
+        if rng.gen() {
+            duration.saturating_add(jitter)
+        } else {
+            duration.saturating_sub(jitter)
+        }
+    }
+}
+
+/// The implementation of the `moka::Expiry` trait for `JitteredExpiry`.
+/// https://docs.rs/moka/latest/moka/policy/trait.Expiry.html
+impl<K, V, J> Expiry<K, V> for JitteredExpiry<J>
+where
+    J: Distribution<Duration>,
+{
+    /// 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.
+    fn expire_after_create(&self, _key: &K, _value: &V, _created_at: Instant) -> Option<Duration> {
+        dbg!(self.calc_expiry_for_write())
+    }
+
+    /// 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.
+    fn expire_after_update(
+        &self,
+        _key: &K,
+        _value: &V,
+        _updated_at: Instant,
+        duration_until_expiry: Option<Duration>,
+    ) -> Option<Duration> {
+        dbg!(self.calc_expiry_for_write().or(duration_until_expiry))
+    }
+
+    /// 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.
+    fn expire_after_read(
+        &self,
+        _key: &K,
+        _value: &V,
+        read_at: Instant,
+        duration_until_expiry: Option<Duration>,
+        last_modified_at: Instant,
+    ) -> Option<Duration> {
+        dbg!(self
+            .calc_expiry_for_read(read_at, last_modified_at)
+            .or(duration_until_expiry))
+    }
+}
+
+fn main() {
+    let expiry = JitteredExpiry::new(
+        // TTL 10 minutes
+        Some(Duration::from_secs(10 * 60)),
+        // TTI 3 minutes
+        Some(Duration::from_secs(3 * 60)),
+        // Jitter +/- 30 seconds, 1 second resolution, uniformly distributed
+        Uniform::from(0..30).map(Duration::from_secs),
+    );
+
+    let cache = Cache::builder().expire_after(expiry).build();
+
+    const NUM_KEYS: usize = 10;
+
+    // Insert some key-value pairs.
+    for key in 0..NUM_KEYS {
+        cache.insert(key, format!("value-{key}"));
+    }
+
+    // Get all entries.
+    for key in 0..NUM_KEYS {
+        assert_eq!(cache.get(&key), Some(format!("value-{key}")));
+    }
+
+    // Update all entries.
+    for key in 0..NUM_KEYS {
+        cache.insert(key, format!("new-value-{key}"));
+    }
+}