# sqlite-cache

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SQLite-based on-disk cache for Rust.

## Usage

```rust
let cache = Cache::new(
    CacheConfig::default(),
    rusqlite::Connection::open_in_memory().unwrap(),
).unwrap();
let topic = cache.topic("test-topic").unwrap();
assert!(topic.get("hello").unwrap().is_none());
topic.set("hello", b"world", Duration::from_secs(60))
assert!(&topic.get("hello").unwrap().unwrap().data[..] == b"world");
```

### Locked updates

This library supports *locked updates* to prevent the [thundering herd problem](https://en.wikipedia.org/wiki/Thundering_herd_problem) on cache misses. The `get_for_update` API acquires a per-key lock and returns a `KeyUpdater`; subsequent `get_for_update` calls on the same key will block until the previous `KeyUpdater` is dropped.

```rust
let (updater, current_value) = topic.get_for_update("hello").await.unwrap();
let new_value = expensive_computation(current_value).await;
updater.write(new_value, Duration::from_secs(60)).unwrap();
```

## Benchmark

These results are from running `benches/cache_benchmark.rs` on an Apple M1 processor.

- `mt(4)`: Per-thread operation latency when running the same task on 4 threads.

```
lookup - cache size 10000
                        time:   [1.5978 us 1.6051 us 1.6130 us]
lookup mt(4) - cache size 10000
                        time:   [9.7801 us 9.8464 us 9.9329 us]
insert - cache size 10000
                        time:   [4.6316 us 4.6785 us 4.7169 us]
insert mt(4) - cache size 10000
                        time:   [21.195 us 21.420 us 21.614 us]
```