# Moka
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> **note**
> `v0.12.0` had major breaking changes on the API and internal behavior. Please read
> the [MIGRATION-GUIDE.md][migration-guide-v012] for the details.
* * *
Moka is a fast, concurrent cache library for Rust. Moka is inspired by the
[Caffeine][caffeine-git] 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.
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.
[gh-actions-badge]: https://github.com/moka-rs/moka/workflows/CI/badge.svg
[release-badge]: https://img.shields.io/crates/v/moka.svg
[docs-badge]: https://docs.rs/moka/badge.svg
[deps-rs-badge]: https://deps.rs/repo/github/moka-rs/moka/status.svg
[codecov-badge]: https://codecov.io/gh/moka-rs/moka/graph/badge.svg?token=7GYZNS7O67
[license-badge]: https://img.shields.io/crates/l/moka.svg
[fossa-badge]: https://app.fossa.com/api/projects/git%2Bgithub.com%2Fmoka-rs%2Fmoka.svg?type=shield
[gh-actions]: https://github.com/moka-rs/moka/actions?query=workflow%3ACI
[crate]: https://crates.io/crates/moka
[docs]: https://docs.rs/moka
[deps-rs]: https://deps.rs/repo/github/moka-rs/moka
[codecov]: https://codecov.io/gh/moka-rs/moka
[fossa]: https://app.fossa.com/projects/git%2Bgithub.com%2Fmoka-rs%2Fmoka?ref=badge_shield
[caffeine-git]: https://github.com/ben-manes/caffeine
## Features
Moka provides a rich and flexible feature set while maintaining high hit ratio and a
high level of concurrency for concurrent access.
- Thread-safe, highly concurrent in-memory cache implementations:
- Synchronous caches that can be shared across OS threads.
- An asynchronous (futures aware) cache.
- 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][tiny-lfu].
- Supports expiration policies:
- Time to live.
- Time to idle.
- Per-entry variable expiration.
- Supports eviction listener, a callback function that will be called when an entry
is removed from the cache.
### Choosing the right cache for your use case
No cache implementation is perfect for every use cases. Moka is a complex software
and can be overkill for your use case. Sometimes simpler caches like
[Mini Moka][mini-moka-crate] or [Quick Cache][quick-cache] might be a better fit.
The following table shows the trade-offs between the different cache implementations:
| Thread-safe, sync cache | ✅ | ✅ | ✅ |
| Thread-safe, async cache | ✅ | ❌ | ✅ |
| Non-concurrent cache | ❌ | ✅ | ✅ |
| Bounded by the maximum number of entries | ✅ | ✅ | ✅ |
| Bounded by the total weighted size of entries | ✅ | ✅ | ✅ |
| Near optimal hit ratio | ✅ TinyLFU | ✅ TinyLFU | ✅ CLOCK-Pro |
| Per-key, atomic insertion. (e.g. `get_with` method) | ✅ | ❌ | ✅ |
| Cache-level expiration policies (time-to-live and time-to-idle) | ✅ | ✅ | ❌ |
| Per-entry variable expiration | ✅ | ❌ | ❌ |
| Eviction listener | ✅ | ❌ | ❌ |
| Lock-free, concurrent iterator | ✅ | ❌ | ❌ |
| Lock-per-shard, concurrent iterator | ❌ | ✅ | ❌ |
| Small overhead compared to a concurrent hash table | ❌ | ❌ | ✅ |
| Does not use background threads | ❌ → ✅ Removed from v0.12 | ✅ | ✅ |
| Small dependency tree | ❌ | ✅ | ✅ |
[tiny-lfu]: https://github.com/moka-rs/moka/wiki#admission-and-eviction-policies
[quick-cache]: https://crates.io/crates/quick_cache
[mini-moka-crate]: https://crates.io/crates/mini-moka
## Moka in Production
Moka is powering production services as well as embedded Linux devices like home
routers. Here are some highlights:
- [crates.io](https://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%][gh-discussions-51] for the high-traffic download endpoint.
(Moka used: Nov 2021 — present)
- [aliyundrive-webdav][aliyundrive-webdav-git]: 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)
[gh-discussions-51]: https://github.com/moka-rs/moka/discussions/51
[aliyundrive-webdav-git]: https://github.com/messense/aliyundrive-webdav
## Recent Changes
> **Note**
> `v0.12.0` had major breaking changes on the API and internal behavior. Please read
> the [MIGRATION-GUIDE.md][migration-guide-v012] for the details.
- [MIGRATION-GUIDE.md][migration-guide-v012]
- [CHANGELOG.md](https://github.com/moka-rs/moka/blob/main/CHANGELOG.md)
[migration-guide-v012]: https://github.com/moka-rs/moka/blob/main/MIGRATION-GUIDE.md
## Table of Contents
- [Features](#features)
- [Choosing the right cache for your use case](#choosing-the-right-cache-for-your-use-case)
- [Moka in Production](#moka-in-production)
- [Change Log](#change-log)
- [Supported Platforms](#supported-platforms)
- [Usage](#usage)
- Examples (Part 1)
- [Synchronous Cache](#example-synchronous-cache)
- [Asynchronous Cache](#example-asynchronous-cache)
- [Avoiding to clone the value at `get`](#avoiding-to-clone-the-value-at-get)
- Example (Part 2)
- [Size Aware Eviction](#example-size-aware-eviction)
- [Expiration Policies](#expiration-policies)
- [Minimum Supported Rust Versions](#minimum-supported-rust-versions)
- Troubleshooting
- [Compile Errors on Some 32-bit Platforms](#compile-errors-on-some-32-bit-platforms)
- [Developing Moka](#developing-moka)
- [Road Map](#road-map)
- [About the Name](#about-the-name)
- [Credits](#credits)
- [License](#license)
## Supported Platforms
Moka should work on most 64-bit and 32-bit platforms if Rust `std` library is
available with threading support. However, WebAssembly (Wasm) and WASI targets are
not supported.
The following platforms are tested on CI:
- Linux 64-bit (x86_64, arm aarch64)
- Linux 32-bit (i646, armv7, armv5, mips)
- If you get compile errors on 32-bit platforms, see
[troubleshooting](#compile-errors-on-some-32-bit-platforms).
The following platforms are not tested on CI but should work:
- macOS (arm64)
- Windows (x86_64 msvc and gnu)
- iOS (arm64)
The following platforms are _not_ supported:
- WebAssembly (Wasm) and WASI targets are not supported.
(See [this project task][gh-proj-49877487])
- `nostd` environment (platforms without `std` library) are not supported.
- 16-bit platforms are not supported.
[gh-proj-49877487]: https://github.com/orgs/moka-rs/projects/1?pane=issue&itemId=49877487
## Usage
To add Moka to your dependencies, run `cargo add` as the followings:
```console
# To use the synchronous cache:
cargo add moka --features sync
# To use the asynchronous cache:
cargo add moka --features future
```
If you want to use the cache under an async runtime such as `tokio` or `async-std`, you should specify the `future` feature. Otherwise, specify the `sync` feature.
## Example: Synchronous Cache
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:
```rust
// 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)));
}
}
}
```
You can try the synchronous example by cloning the repository and running the
following cargo instruction:
```console
$ cargo run --example sync_example
```
If you want to atomically initialize and insert a value when the key is not present,
you might want to check [the document][doc-sync-cache] for other insertion methods
`get_with` and `try_get_with`.
[doc-sync-cache]: https://docs.rs/moka/*/moka/sync/struct.Cache.html#method.get_with
## Example: Asynchronous Cache
The asynchronous (futures aware) cache is defined in the `future` module.
It works with asynchronous runtime such as [Tokio][tokio-crate],
[async-std][async-std-crate] or [actix-rt][actix-rt-crate].
To use the asynchronous cache, [enable a crate feature called "future"](#usage).
[tokio-crate]: https://crates.io/crates/tokio
[async-std-crate]: https://crates.io/crates/async-std
[actix-rt-crate]: https://crates.io/crates/actix-rt
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 fn` or `async` block), use `insert` or `invalidate`
method for updating the cache and `await` them.
- Outside any async context, use `blocking` method to access blocking version of
`insert` or `invalidate` methods.
Here is a similar program to the previous example, but using asynchronous cache with
[Tokio][tokio-crate] runtime:
```rust,ignore
// Cargo.toml
//
// [dependencies]
// moka = { version = "0.12", 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).await, 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).await, None);
} else {
assert_eq!(cache.get(&key).await, Some(value(key)));
}
}
}
```
You can try the asynchronous example by cloning the repository and running the
following cargo instruction:
```console
$ cargo run --example async_example --features future
```
If you want to atomically initialize and insert a value when the key is not present,
you might want to check [the document][doc-future-cache] for other insertion methods
`get_with` and `try_get_with`.
[doc-future-cache]: https://docs.rs/moka/*/moka/future/struct.Cache.html#method.get_with
## Avoiding to clone the value at `get`
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`][rustdoc-std-arc] is a thread-safe
reference-counted pointer and its `clone()` method is cheap.
[rustdoc-std-arc]: https://doc.rust-lang.org/stable/std/sync/struct.Arc.html
```rust,ignore
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`.
```rust
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.
You can try the size aware eviction example by cloning the repository and running the
following cargo instruction:
```console
$ cargo run --example size_aware_eviction
```
## Expiration Policies
Moka supports the following expiration policies:
- **Cache-level expiration policies:**
- Cache-level policies are applied to all entries in the cache.
- **Time to live (TTL)**: A cached entry will be expired after the specified
duration past from `insert`.
- **Time to idle (TTI)**: A cached entry will be expired after the specified
duration past from `get` or `insert`.
- **Per-entry expiration policy:**
- The per-entry expiration lets you sets a different expiration time for each
entry.
For details and examples of above policies, see the "Example: Time-based Expiration"
section ([`sync::Cache`][doc-sync-cache-expiration],
[`future::Cache`][doc-future-cache-expiration]) of the document.
[doc-sync-cache-expiration]: https://docs.rs/moka/latest/moka/sync/struct.Cache.html#example-time-based-expirations
[doc-future-cache-expiration]: https://docs.rs/moka/latest/moka/future/struct.Cache.html#example-time-based-expirations
## Minimum Supported Rust Versions
Moka's minimum supported Rust versions (MSRV) are the followings:
| default features | Rust 1.65.0 (Nov 3, 2022) |
| `future` | Rust 1.65.0 (Nov 3, 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, 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.
## Troubleshooting
### Compile Errors on Some 32-bit Platforms
On some 32-bit target platforms including the followings, you may encounter compile
errors:
- `armv5te-unknown-linux-musleabi`
- `mips-unknown-linux-musl`
- `mipsel-unknown-linux-musl`
```console
error[E0432]: unresolved import `std::sync::atomic::AtomicU64`
--> ... /moka-0.5.3/src/sync.rs:10:30
|
| |
| 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.
```toml:Cargo.toml
[dependencies]
moka = { version = "0.12", default-features = false, features = ["sync"] }
# Or
moka = { version = "0.12", default-features = false, features = ["future"] }
```
This will make Moka to switch to a fall-back implementation, so it will compile.
## Developing Moka
**Running All Tests**
To run all tests including `future` feature and doc tests on the README, use the
following command:
```console
$ RUSTFLAGS='--cfg trybuild' cargo test --all-features
```
**Running All Tests without Default Features**
```console
$ RUSTFLAGS='--cfg trybuild' cargo test \
--no-default-features --features 'future, sync'
```
**Generating the Doc**
```console
$ cargo +nightly -Z unstable-options --config 'build.rustdocflags="--cfg docsrs"' \
doc --no-deps --features 'future, sync'
```
## Roadmap
See the [project roadmap][gh-proj-1] for the updated and detailed plans.
But here are some highlights:
[gh-proj-1]: https://github.com/orgs/moka-rs/projects/1/views/1
- [x] Size-aware eviction. (`v0.7.0` via [#24][gh-pull-024])
- [x] API stabilization. (Smaller core API, shorter names for frequently used
methods) (`v0.8.0` via [#105][gh-pull-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)`
- `time_to_live()` → `policy().time_to_live()`
- [x] Notifications on eviction. (`v0.9.0` via [#145][gh-pull-145])
- [x] Variable (per-entry) expiration, using hierarchical timer wheels.
(`v0.11.0` via [#248][gh-pull-248])
- [x] Remove background threads. (`v0.12.0` via [#294][gh-pull-294] and
[#316][gh-pull-316])
- [x] Add upsert and compute methods. (`v0.12.3` via [#370][gh-pull-370])
- [ ] Cache statistics (Hit rate, etc.). ([details][cache-stats])
- [ ] Upgrade TinyLFU to Window-TinyLFU. ([details][tiny-lfu])
- [ ] Restore cache from a snapshot. ([details][restore])
[gh-pull-024]: https://github.com/moka-rs/moka/pull/24
[gh-pull-105]: https://github.com/moka-rs/moka/pull/105
[gh-pull-145]: https://github.com/moka-rs/moka/pull/145
[gh-pull-248]: https://github.com/moka-rs/moka/pull/248
[gh-pull-294]: https://github.com/moka-rs/moka/pull/294
[gh-pull-316]: https://github.com/moka-rs/moka/pull/316
[gh-pull-370]: https://github.com/moka-rs/moka/pull/370
[cache-stats]: https://github.com/moka-rs/moka/issues/234
[restore]: https://github.com/moka-rs/moka/issues/314
## About the Name
Moka is named after the [moka pot][moka-pot-wikipedia], 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-pot-wikipedia]: https://en.wikipedia.org/wiki/Moka_pot
## Credits
### Caffeine
Moka's architecture is heavily inspired by the [Caffeine][caffeine-git] library for
Java. Thanks go to Ben Manes and all contributors of Caffeine.
### cht
The source files of the concurrent hash table under `moka::cht` module were copied
from the [cht crate v0.4.1][cht-v041] 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.
[cht-v041]: https://github.com/Gregory-Meyer/cht/tree/v0.4.1
## License
Moka is distributed under either of
- The MIT license
- The Apache License (Version 2.0)
at your option.
See [LICENSE-MIT](LICENSE-MIT) and [LICENSE-APACHE](LICENSE-APACHE) for details.