Mini Moka

Mini Moka is a fast, concurrent cache library for Rust. Mini Moka is a light edition of Moka.

Mini Moka provides cache implementations on top of hash maps. They support full concurrency of retrievals and a high expected concurrency for updates. Mini Moka also provides a non-thread-safe cache implementation for single thread applications.

All caches perform a best-effort bounding of a hash map using an entry replacement algorithm to determine which entries to evict when the capacity is exceeded.

Features

• Thread-safe, highly concurrent in-memory cache implementation.
• A cache can be bounded by one of the followings:
  • The maximum number of entries.
  • The total weighted size of entries. (Size aware eviction)
• Maintains near optimal hit ratio by using an entry replacement algorithms inspired by Caffeine:
  • Admission to a cache is controlled by the Least Frequently Used (LFU) policy.
  • Eviction from a cache is controlled by the Least Recently Used (LRU) policy.
  • More details and some benchmark results are available here.
• Supports expiration policies:
  • Time to live
  • Time to idle

Change Log

• CHANGELOG.md

Table of Contents

• Features
• Change Log
• Usage
• Example: Synchronous Cache
• Avoiding to clone the value at get
• Examples (Part 2)
  • Size Aware Eviction
  • Expiration Policies
• Minimum Supported Rust Versions
• Developing Mini Moka
• Credits
• License

Usage

Add this to your Cargo.toml:

[dependencies]
mini_moka = "0.10"

Example: Synchronous Cache

The thread-safe, synchronous caches are defined in the sync module.

Cache entries are manually added using insert method, and are stored in the cache until either evicted or manually invalidated.

Here's an example of reading and updating a cache by using multiple threads:

// Use the synchronous cache.
use mini_moka::sync::Cache;

use std::thread;

fn value(n: usize) -> String {
    format!("value {}", n)
}

fn main() {
    const NUM_THREADS: usize = 16;
    const NUM_KEYS_PER_THREAD: usize = 64;

    // Create a cache that can store up to 10,000 entries.
    let cache = Cache::new(10_000);

    // Spawn threads and read and update the cache simultaneously.
    let threads: Vec<_> = (0..NUM_THREADS)
        .map(|i| {
            // To share the same cache across the threads, clone it.
            // This is a cheap operation.
            let my_cache = cache.clone();
            let start = i * NUM_KEYS_PER_THREAD;
            let end = (i + 1) * NUM_KEYS_PER_THREAD;

            thread::spawn(move || {
                // Insert 64 entries. (NUM_KEYS_PER_THREAD = 64)
                for key in start..end {
                    my_cache.insert(key, value(key));
                    // get() returns Option<String>, a clone of the stored value.
                    assert_eq!(my_cache.get(&key), Some(value(key)));
                }

                // Invalidate every 4 element of the inserted entries.
                for key in (start..end).step_by(4) {
                    my_cache.invalidate(&key);
                }
            })
        })
        .collect();

    // Wait for all threads to complete.
    threads.into_iter().for_each(|t| t.join().expect("Failed"));

    // Verify the result.
    for key in 0..(NUM_THREADS * NUM_KEYS_PER_THREAD) {
        if key % 4 == 0 {
            assert_eq!(cache.get(&key), None);
        } else {
            assert_eq!(cache.get(&key), Some(value(key)));
        }
    }
}

Avoiding to clone the value at get

For the concurrent cache (sync cache), the return type of get method is Option<V> instead of Option<&V>, where V is the value type. Every time get is called for an existing key, it creates a clone of the stored value V and returns it. This is because the Cache allows concurrent updates from threads so a value stored in the cache can be dropped or replaced at any time by any other thread. get cannot return a reference &V as it is impossible to guarantee the value outlives the reference.

If you want to store values that will be expensive to clone, wrap them by std::sync::Arc before storing in a cache. Arc is a thread-safe reference-counted pointer and its clone() method is cheap.

use std::sync::Arc;

let key = ...
let large_value = vec![0u8; 2 * 1024 * 1024]; // 2 MiB

// When insert, wrap the large_value by Arc.
cache.insert(key.clone(), Arc::new(large_value));

// get() will call Arc::clone() on the stored value, which is cheap.
cache.get(&key);

Example: Size Aware Eviction

If different cache entries have different "weights" — e.g. each entry has different memory footprints — you can specify a weigher closure at the cache creation time. The closure should return a weighted size (relative size) of an entry in u32, and the cache will evict entries when the total weighted size exceeds its max_capacity.

use std::convert::TryInto;
use mini_moka::sync::Cache;

fn main() {
    let cache = Cache::builder()
        // A weigher closure takes &K and &V and returns a u32 representing the
        // relative size of the entry. Here, we use the byte length of the value
        // String as the size.
        .weigher(|_key, value: &String| -> u32 {
            value.len().try_into().unwrap_or(u32::MAX)
        })
        // This cache will hold up to 32MiB of values.
        .max_capacity(32 * 1024 * 1024)
        .build();
    cache.insert(0, "zero".to_string());
}

Note that weighted sizes are not used when making eviction selections.

Example: Expiration Policies

Mini Moka supports the following expiration policies:

• Time to live: A cached entry will be expired after the specified duration past from insert.
• Time to idle: A cached entry will be expired after the specified duration past from get or insert.

To set them, use the CacheBuilder.

use mini_moka::sync::Cache;
use std::time::Duration;

fn main() {
    let cache = Cache::builder()
        // Time to live (TTL): 30 minutes
        .time_to_live(Duration::from_secs(30 * 60))
        // Time to idle (TTI):  5 minutes
        .time_to_idle(Duration::from_secs( 5 * 60))
        // Create the cache.
        .build();

    // This entry will expire after 5 minutes (TTI) if there is no get().
    cache.insert(0, "zero");

    // This get() will extend the entry life for another 5 minutes.
    cache.get(&0);

    // Even though we keep calling get(), the entry will expire
    // after 30 minutes (TTL) from the insert().
}

A note on expiration policies

The cache builders will panic if configured with either time_to_live or time to idle longer than 1000 years. This is done to protect against overflow when computing key expiration.

Minimum Supported Rust Versions

Mini Moka's minimum supported Rust versions (MSRV) are the followings:

Feature	MSRV
default features	Rust 1.61.0 (May 19, 2022)

It will keep a rolling MSRV policy of at least 6 months. If only the default features are enabled, MSRV will be updated conservatively. When using other features, MSRV might be updated more frequently, up to the latest stable. In both cases, increasing MSRV is not considered a semver-breaking change.

Developing Mini Moka

Running All Tests

To run all tests including doc tests on the README, use the following command:

$ RUSTFLAGS='--cfg trybuild' cargo test --all-features

Generating the Doc

$ cargo +nightly -Z unstable-options --config 'build.rustdocflags="--cfg docsrs"' \
    doc --no-deps

Credits

Caffeine

Mini Moka's architecture is heavily inspired by the Caffeine library for Java. Thanks go to Ben Manes and all contributors of Caffeine.

License

Mini Moka is distributed under either of

• The MIT license
• The Apache License (Version 2.0)

at your option.

See LICENSE-MIT and LICENSE-APACHE for details.