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<img src="./logo.png" alt="Cachet Logo" width="96">
# Cachet
[](https://crates.io/crates/cachet)
[](https://docs.rs/cachet)
[](https://crates.io/crates/cachet)
[](https://github.com/microsoft/oxidizer/actions/workflows/main.yml)
[](https://codecov.io/gh/microsoft/oxidizer)
[](../../LICENSE)
<a href="../.."><img src="../../logo.svg" alt="This crate was developed as part of the Oxidizer project" width="20"></a>
</div>
A composable, multi-tier caching library with stampede protection, background
refresh, and structured telemetry.
## Why Multi-Tier Caching?
A single cache is a single point of failure and a capacity ceiling. Multi-tier
caching layers fast, small caches in front of slower, larger ones:
* **L1 (primary)** - an in-process memory cache: microsecond latency, bounded
capacity, evicts under pressure.
* **L2 (fallback)** - a remote or larger cache: millisecond latency, much larger
capacity, survives process restarts.
On a miss in L1, `cachet` transparently queries L2 and optionally *promotes* the
value back into L1 so the next request is fast again. The result is lower average
latency, reduced load on the backing store, and resilience when either tier is
temporarily unavailable.
## Why Background Refresh?
TTL-based expiration causes a *synchronous* miss every time an entry ages out:
the next caller blocks while the value is recomputed. Background refresh
(time-to-refresh, TTR) decouples freshness from latency:
* While an entry is still within its TTR, all callers receive the cached value
immediately (a “refresh hit”).
* Once the TTR elapses, the *next* caller still receives the stale value, but a
background task is spawned to pull a fresh value from the fallback tier.
* Subsequent callers continue to hit the cache while the refresh happens, so
latency never spikes.
Use [`TimeToRefresh`][__link0] together with a fallback tier to enable this pattern.
## Cache Stampede Protection
A **cache stampede** (also called a thundering herd) occurs when many concurrent
requests all miss the cache at the same time - for example, after a cold start or
after a popular entry expires. Every request independently computes the value,
spiking load on the backing store.
`cachet` avoids this with *request coalescing* via the [`uniflight`][__link1] crate: when
stampede protection is enabled, all concurrent requests for the same key are merged
so that only one computes the value. The rest wait and share the result, including
any error. Enable it with [`CacheBuilder::stampede_protection`][__link2].
## Flexibility
`cachet` is designed to adapt to your infrastructure rather than the other way
around:
* **Any storage backend** - implement [`CacheTier`][__link3] to plug in Redis, Memcached,
a database, or any other store.
* **Service middleware** - with the `service` feature, any
`Service<CacheOperation>` becomes a `CacheTier`, so you can compose retry,
timeout, and circuit-breaker middleware around your storage using standard Tower
or `layered` patterns.
* **Dynamic dispatch** - the builder type-erases the storage tier into a
[`DynamicCache<K, V>`][__link4], so all builders produce the same `Cache<K, V>`
output type regardless of the underlying storage or tier composition.
* **Configurable insert policy** - choose whether, and under what conditions,
values are inserted into a tier ([`InsertPolicy`][__link5]).
* **Clock injection** - all time-based logic (TTL, TTR, timestamps) goes through
a [`tick::Clock`][__link6], making caches fully controllable in tests without sleeping.
## Why Use This Instead of Moka/Other Caches?
Moka (and similar crates) are excellent single-tier in-process caches. `cachet`
builds on top of them and adds:
|In-process memory cache|✅|✅ (via `cachet_memory`)|
|Multi-tier / fallback|❌|✅|
|Stampede protection|❌|✅|
|Background refresh|❌|✅|
|Service middleware integration|❌|✅|
|Structured telemetry|❌|✅|
|Pluggable storage backends|❌|✅|
|Clock injection for testing|❌|✅|
If you only need a single in-process cache with no telemetry requirements,
`moka` directly may be simpler. If you need any of the above, `cachet` is the
right choice.
## Major Types
|[`Cache`][__link7]|The user-facing cache. Wraps any `CacheTier` with `get`, `insert`, `invalidate`, `clear`, `get_or_insert`, `try_get_or_insert`, and `optionally_get_or_insert`.|
|[`CacheBuilder`][__link8]|Builder for `Cache`. Configure storage, TTL, name, telemetry, fallback, insert policy, stampede protection, and background refresh.|
|[`CacheEntry<V>`][__link9]|A value together with an optional cached-at timestamp and TTL. Returned by all `get` operations.|
|[`CacheTier`][__link10]|The core trait for storage backends. Implement this to add your own storage.|
|[`InsertPolicy`][__link11]|Decides whether a value should be inserted into a tier.|
|[`TimeToRefresh`][__link12]|Configures background refresh: how stale an entry must be before a background task refreshes it.|
|[`Error`][__link13]|The error type returned by all fallible cache operations.|
## How Tiers Compose
Tiers are composed at build time using the builder:
```text
Cache::builder::<K, V>(clock)
.memory() // L1: fast in-process store
.ttl(Duration::from_secs(30)) // entries expire from L1 after 30 s
.fallback( // on L1 miss, consult L2
Cache::builder::<K, V>(clock)
.memory() // L2: a second in-process store (or a remote service)
.ttl(Duration::from_secs(300))
)
.insert_policy(InsertPolicy::always()) // control when values are inserted into L1
.time_to_refresh(TimeToRefresh::new(Duration::from_secs(20), spawner)) // refresh L1 in background
.build()
```
On a `get`:
1. Check L1. If hit and not stale, return immediately.
1. If hit but stale (TTR elapsed), return the stale value *and* spawn a background
task to fetch from L2 and repopulate L1.
1. If miss or expired (TTL elapsed), check L2. If found, optionally promote to L1,
then return.
1. If both miss, return `Ok(None)`.
**Note:** expired entries are not automatically removed from storage. The wrapper
uses lazy expiration - it returns `None` but leaves cleanup to the storage
backend (e.g. moka built-in eviction).
TO-DO add an `ExpirationPolicy` that would make this configurable.
Invalidation and clear are sent to **all** tiers concurrently.
## Companion Crates
`cachet` is the main entry point. The ecosystem is split into focused crates:
|[`cachet_tier`][__link14]|Core `CacheTier` trait, `CacheEntry`, `Error`, and `MockCache` for testing.|
|[`cachet_memory`][__link15]|In-process memory cache backed by [moka][__link16] (`TinyLFU` eviction).|
|[`cachet_service`][__link17]|Adapters between the `CacheTier` trait and the `layered::Service` / Tower service patterns.|
You rarely need to depend on companion crates directly - `cachet` re-exports the
most commonly used types from all of them.
## Cargo Features
|`memory`|✅|Enables `InMemoryCache` and the `.memory()` builder method via `cachet_memory`.|
|`logs`|❌|Enables structured `tracing` log events for every cache operation. Subscribe via [`telemetry::attributes`][__link18] constants.|
|`service`|❌|Enables `ServiceAdapter`, `CacheServiceExt`, and `CacheOperation`/`CacheResponse` types for service middleware integration.|
|`serialize`|❌|Enables `.serialize()` on builders for automatic postcard serialization of keys and values to `BytesView`.|
|`test-util`|❌|Enables `MockCache`, frozen-clock utilities, and other test helpers.|
## Examples
### Basic In-Memory Cache
```rust
use cachet::{Cache, CacheEntry};
use tick::Clock;
let clock = Clock::new_tokio();
let cache: Cache<String, i32> = Cache::builder(clock).memory().build();
cache.insert("key".to_string(), CacheEntry::new(42)).await?;
let value = cache.get("key").await?;
assert_eq!(*value.unwrap().value(), 42);
```
### Multi-Tier Cache with Fallback
```rust
use std::time::Duration;
use cachet::Cache;
use tick::Clock;
let clock = Clock::new_tokio();
let l2 = Cache::builder::<String, String>(clock.clone()).memory();
let cache = Cache::builder::<String, String>(clock)
.memory()
.ttl(Duration::from_secs(60))
.fallback(l2)
.build();
```
### Serialization Boundary
When a fallback tier operates on serialized bytes (e.g., Redis), use `.serialize()`
to add a postcard serialization boundary. Keys and values are automatically serialized
to [`BytesView`][__link19] before reaching the fallback tier, and
deserialized on the way back.
```rust
use cachet::{Cache, FallbackPromotionPolicy};
use tick::Clock;
let clock = Clock::new_tokio();
let remote = Cache::builder::<bytesbuf::BytesView, bytesbuf::BytesView>(clock.clone()).memory();
let cache = Cache::builder::<String, String>(clock)
.memory()
.serialize()
.fallback(remote)
.promotion_policy(FallbackPromotionPolicy::always())
.build();
// Keys and values are String on the outside, BytesView in the fallback tier.
cache.insert("key".to_string(), "value".to_string()).await?;
```
## Telemetry
Cachet provides two complementary telemetry channels:
### Tracing events
Enable with the `logs` feature and `.enable_logs()` on the cache builder.
Each tier outcome and operation completion emits a structured [`tracing`][__link20] event.
**Tier events** carry `cache.name`, `cache.event`, and `cache.duration_ns`.
**Operation-complete events** carry `cache.name`, `cache.operation`,
`cache.duration_ns`, and `cache.coalesced`.
Use [`telemetry::attributes`][__link21] constants to filter and match events in a
custom `tracing_subscriber::Layer`:
```rust
use cachet::telemetry::attributes;
// Filter by tracing target prefix
let filter = tracing_subscriber::filter::Targets::new()
.with_target(attributes::TARGET, tracing::Level::DEBUG);
// Match specific events in a Visit impl
if event_value == attributes::EVENT_HIT { /* cache hit */ }
```
See the `telemetry_subscriber` example for a complete demonstration.
#### Event types
|ERROR|`cache.get_error`, `cache.insert_error`, `cache.invalidate_error`, `cache.clear_error`|
|INFO|`cache.expired`, `cache.refresh_miss`, `cache.inserted`, `cache.insert_rejected`, `cache.invalidated`, `cache.eviction`|
|DEBUG|`cache.hit`, `cache.miss`, `cache.refresh_hit`, `cache.cleared`|
### Event handler callback API
Register a [`CacheEventHandler`][__link22] via
`.event_handler(handler)` on the cache builder to receive typed
[`CacheTierEvent`][__link23] and
[`CacheOperationEvent`][__link24] callbacks.
Events carry a `request_id` for correlating tier outcomes with their parent
operation. Works independently of the `logs` feature.
See the `telemetry_accumulator` example for a DashMap-based accumulation pattern.
<hr/>
<sub>
This crate was developed as part of <a href="../..">The Oxidizer Project</a>. Browse this crate's <a href="https://github.com/microsoft/oxidizer/tree/main/crates/cachet">source code</a>.
</sub>
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[__link0]: https://docs.rs/cachet/0.7.4/cachet/?search=TimeToRefresh
[__link1]: https://crates.io/crates/uniflight/0.2.5
[__link10]: https://docs.rs/cachet_tier/0.2.6/cachet_tier/?search=CacheTier
[__link11]: https://docs.rs/cachet/0.7.4/cachet/?search=InsertPolicy
[__link12]: https://docs.rs/cachet/0.7.4/cachet/?search=TimeToRefresh
[__link13]: https://docs.rs/cachet_tier/0.2.6/cachet_tier/?search=Error
[__link14]: https://crates.io/crates/cachet_tier/0.2.6
[__link15]: https://crates.io/crates/cachet_memory/0.3.7
[__link16]: https://docs.rs/moka
[__link17]: https://crates.io/crates/cachet_service/0.2.8
[__link18]: https://docs.rs/cachet/0.7.4/cachet/?search=telemetry::attributes
[__link19]: https://docs.rs/bytesbuf/0.5.6/bytesbuf/?search=BytesView
[__link2]: https://docs.rs/cachet/0.7.4/cachet/?search=CacheBuilder::stampede_protection
[__link20]: https://crates.io/crates/tracing/0.1.44
[__link21]: https://docs.rs/cachet/0.7.4/cachet/?search=telemetry::attributes
[__link22]: https://docs.rs/cachet/0.7.4/cachet/?search=telemetry::handler::CacheEventHandler
[__link23]: https://docs.rs/cachet/0.7.4/cachet/?search=telemetry::handler::CacheTierEvent
[__link24]: https://docs.rs/cachet/0.7.4/cachet/?search=telemetry::handler::CacheOperationEvent
[__link3]: https://docs.rs/cachet_tier/0.2.6/cachet_tier/?search=CacheTier
[__link4]: https://docs.rs/cachet_tier/0.2.6/cachet_tier/?search=DynamicCache
[__link5]: https://docs.rs/cachet/0.7.4/cachet/?search=InsertPolicy
[__link6]: https://docs.rs/tick/0.3.6/tick/?search=Clock
[__link7]: https://docs.rs/cachet/0.7.4/cachet/?search=Cache
[__link8]: https://docs.rs/cachet/0.7.4/cachet/?search=CacheBuilder
[__link9]: https://docs.rs/cachet_tier/0.2.6/cachet_tier/?search=CacheEntry