Moka is a fast, concurrent cache library for Rust. Moka is inspired by the Caffeine library for Java.
Moka provides cache implementations on top of hash maps. They support full concurrency of retrievals and a high expected concurrency for updates. 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.
- Thread-safe, highly concurrent in-memory cache implementations:
- Synchronous caches that can be shared across OS threads.
- An asynchronous (futures aware) cache that can be accessed inside and outside of asynchronous contexts.
- 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
- Supports eviction listener, a callback function that will be called when an entry is removed from the cache.
Moka is powering production services as well as embedded Linux devices like home routers. Here are some highlights:
- crates.io: The official crate registry has been using Moka in its API service to reduce the loads on PostgreSQL. Moka is maintaining cache hit rates of ~85% for the high-traffic download endpoint. (Moka used: Nov 2021 — present)
- aliyundrive-webdav: This WebDAV gateway for a cloud drive may have been deployed in hundreds of home Wi-Fi routers, including inexpensive models with 32-bit MIPS or ARMv5TE-based SoCs. Moka is used to cache the metadata of remote files. (Moka used: Aug 2021 — present)
- Features
- Moka in Production
- Change Log
- Usage
- Examples (Part 1)
- Avoiding to clone the value at
get - Examples (Part 2)
- Hashing Algorithm
- Minimum Supported Rust Versions
- Troubleshooting
- Developing Moka
- Road Map
- About the Name
- Credits
- License
Add this to your Cargo.toml:
[dependencies]
moka = "0.9"To use the asynchronous cache, enable a crate feature called "future".
[dependencies]
moka = { version = "0.9", features = ["future"] }The thread-safe, synchronous caches are defined in the sync module.
Cache entries are manually added using insert or get_with 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 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)));
}
}
}If you want to atomically initialize and insert a value when the key is not present,
you might want to check the document for other insertion methods
get_with and try_get_with.
The asynchronous (futures aware) cache is defined in the future module.
It works with asynchronous runtime such as Tokio,
async-std or actix-rt.
To use the asynchronous cache, enable a crate feature called "future".
Cache entries are manually added using an insert method, and are stored in the cache until either evicted or manually invalidated:
- Inside an async context (
async fnorasyncblock), useinsertorinvalidatemethod for updating the cache andawaitthem. - Outside any async context, use
blockingmethod to access blocking version ofinsertorinvalidatemethods.
Here is a similar program to the previous example, but using asynchronous cache with Tokio runtime:
// Cargo.toml
//
// [dependencies]
// moka = { version = "0.9", features = ["future"] }
// tokio = { version = "1", features = ["rt-multi-thread", "macros" ] }
// futures-util = "0.3"
// Use the asynchronous cache.
use moka::future::Cache;
#[tokio::main]
async fn main() {
const NUM_TASKS: usize = 16;
const NUM_KEYS_PER_TASK: usize = 64;
fn value(n: usize) -> String {
format!("value {}", n)
}
// Create a cache that can store up to 10,000 entries.
let cache = Cache::new(10_000);
// Spawn async tasks and write to and read from the cache.
let tasks: Vec<_> = (0..NUM_TASKS)
.map(|i| {
// To share the same cache across the async tasks, clone it.
// This is a cheap operation.
let my_cache = cache.clone();
let start = i * NUM_KEYS_PER_TASK;
let end = (i + 1) * NUM_KEYS_PER_TASK;
tokio::spawn(async move {
// Insert 64 entries. (NUM_KEYS_PER_TASK = 64)
for key in start..end {
// insert() is an async method, so await it.
my_cache.insert(key, value(key)).await;
// 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) {
// invalidate() is an async method, so await it.
my_cache.invalidate(&key).await;
}
})
})
.collect();
// Wait for all tasks to complete.
futures_util::future::join_all(tasks).await;
// Verify the result.
for key in 0..(NUM_TASKS * NUM_KEYS_PER_TASK) {
if key % 4 == 0 {
assert_eq!(cache.get(&key), None);
} else {
assert_eq!(cache.get(&key), Some(value(key)));
}
}
}If you want to atomically initialize and insert a value when the key is not present,
you might want to check the document for other insertion methods
get_with and try_get_with.
For the concurrent caches (sync and future caches), 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);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 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.
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
getorinsert.
To set them, use the CacheBuilder.
use 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().
}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.
By default, a cache uses a hashing algorithm selected to provide resistance against HashDoS attacks.
The default hashing algorithm is the one used by std::collections::HashMap, which
is currently SipHash 1-3, though this is subject to change at any point in the
future.
While its performance is very competitive for medium sized keys, other hashing algorithms will outperform it for small keys such as integers as well as large keys such as long strings. However those algorithms will typically not protect against attacks such as HashDoS.
The hashing algorithm can be replaced on a per-Cache basis using the
build_with_hasher method of the CacheBuilder. Many alternative algorithms are
available on crates.io, such as the aHash crate.
This crate's minimum supported Rust versions (MSRV) are the followings:
| Feature | Enabled by default? | MSRV |
|---|---|---|
| no feature | Rust 1.51.0 | |
atomic64 |
yes | Rust 1.51.0 |
quanta |
yes | Rust 1.51.0 |
future |
Rust 1.51.0 | |
dash |
Rust 1.51.0 |
If only the default features are enabled, MSRV will be updated conservatively. When
using other features, like future, MSRV might be updated more frequently, up to the
latest stable. In both cases, increasing MSRV is not considered a semver-breaking
change.
Quanta crate up to v0.9.3 has an issue on some specific x86_64-based machines. It will cause intermittent panic due to integer overflow:
- metrics-rs/quanta — Intermittent panic due to overflowing our source calibration denominator. #61 (Fixed by Quanta v0.10.0)
The overflows have been reported by a couple of users who use AMD-based Lenovo laptops or Circle CI.
When this issue occurs, you will get a stacktrace containing the following lines:
... panicked at 'attempt to add with overflow', ...
...
quanta::Calibration::calibrate
at ... /quanta-0.9.3/src/lib.rs:226:13
quanta::Clock::new::{{closure}}
at ... /quanta-0.9.3/src/lib.rs:307:17
...This issue was fixed by Quanta v0.10.0.
You can prevent the issue by upgrading Moka to v0.8.4 or newer, which depends on Quanta v0.10.0 or newer.
On some 32-bit target platforms including the followings, you may encounter compile errors:
armv5te-unknown-linux-musleabimips-unknown-linux-muslmipsel-unknown-linux-musl
error[E0432]: unresolved import `std::sync::atomic::AtomicU64`
--> ... /moka-0.5.3/src/sync.rs:10:30
|
10 | atomic::{AtomicBool, AtomicU64, Ordering},
| ^^^^^^^^^
| |
| no `AtomicU64` in `sync::atomic`Such errors can occur because std::sync::atomic::AtomicU64 is not provided on these
platforms but Moka uses it.
You can resolve the errors by disabling atomic64 feature, which is one of the
default features of Moka. Edit your Cargo.toml to add default-features = false
to the dependency declaration.
[dependencies]
moka = { version = "0.9", default-feautures = false }
# Or
moka = { version = "0.9", default-feautures = false, features = ["future"] }This will make Moka to switch to a fall-back implementation, so it will compile.
Running All Tests
To run all tests including future feature and doc tests on the README, use the
following command:
$ RUSTFLAGS='--cfg skeptic --cfg trybuild' cargo test --all-featuresRunning All Tests without Default Features
$ RUSTFLAGS='--cfg skeptic --cfg trybuild' cargo test \
--no-default-features --features futureGenerating the Doc
$ cargo +nightly -Z unstable-options --config 'build.rustdocflags="--cfg docsrs"' \
doc --no-deps --features 'future, dash'-
asyncoptimized caches. (v0.2.0) - Size-aware eviction. (
v0.7.0via #24) - API stabilization. (Smaller core cache API, shorter names for frequently
used methods) (
v0.8.0via #105)- e.g.
get_or_insert_with(K, F)→get_with(K, F)get_or_try_insert_with(K, F)→try_get_with(K, F)blocking_insert(K, V)→blocking().insert(K, V)time_to_live()→policy().time_to_live()
- Notifications on eviction. (
v0.9.0via #145) - Cache statistics. (Hit rate, etc.)
- Upgrade TinyLFU to Window-TinyLFU. (details)
- The variable (per-entry) expiration, using a hierarchical timer wheel.
Moka is named after the moka pot, a stove-top coffee maker that brews espresso-like coffee using boiling water pressurized by steam.
This name would imply the following facts and hopes:
- Moka is a part of the Java Caffeine cache family.
- It is written in Rust. (Many moka pots are made of aluminum alloy or stainless steel. We know they don't rust though)
- It should be fast. ("Espresso" in Italian means express)
- It should be easy to use, like a moka pot.
Moka's architecture is heavily inspired by the Caffeine library for Java. Thanks go to Ben Manes and all contributors of Caffeine.
The source files of the concurrent hash table under moka::cht module were copied
from the cht crate v0.4.1 and modified by us. We did so for better
integration. cht v0.4.1 and earlier are licensed under the MIT license.
Thanks go to Gregory Meyer.
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.
