# Snapshot Lock | [snaplock](https://crates.io/crates/snaplock)
*Harness MVCC-lock performance in-memory*
[crates.io](https://crates.io/crates/snaplock)
This library provides `ReadWriteLock` - `SnapshotLock`, which has lock-free reads powered by Reference Counted clone
that is stored near the locked value.
Choose `SnapshotLock` if:
- Reads vastly outnumber writes (10:1 ratio or higher)
- You need predictable read latency (scales linearly with minimal memory footprint)
- Application is read-performance critical
- You can tolerate slightly slower writes
Writes are protected by `std::sync::RwLock`. It also provides `tokio::sync::RwLock` implementation under the `async`
feature.
Features:
- `async` : Adds `new_async` constructor. `.write()` method becomes `async`
Example:
```rust
use snaplock::SnapshotLock;
fn main() {
let lock = SnapshotLock::new(String::from("Hello"));
// Multiple readers can access the data concurrently
let reader1 = lock.read();
let reader2 = lock.read();
assert_eq!(*reader1, "Hello");
assert_eq!(*reader2, "Hello");
// Writers have exclusive access
{
let mut writer = lock.write();
writer.push_str(", World!");
// During write readers see old values
let reader = lock.read();
assert_eq!(*reader, "Hello");
}
// When write is finished, new readers see the updated value
let reader3 = lock.read();
assert_eq!(*reader3, "Hello, World!");
// Old readers still see their old value
assert_eq!(*reader1, "Hello");
assert_eq!(*reader2, "Hello");
}
```
### MSRV Policy:
There is none. Current Minimum Supported Rust Version is 1.85.0
### Alternatives:
- [`crossbeam::sync::SharedLock`](https://docs.rs/crossbeam/latest/crossbeam/sync/struct.ShardedLock.html) - A sharded
reader-writer lock.
- [vlock](https://crates.io/crates/vlock) - A fast and scalable multi-version shared state lock with wait-free read
access.
- [parking_lot](https://crates.io/crates/parking_lot) - More compact and efficient implementations of the standard
synchronization primitives.
### Benchmark:
Benchmark data was collected on:
- Windows 10 Version 10.0.19045 Build 19045
- AMD Ryzen 9 7900X3D 12-Core Processor
Results may vary based on hardware architecture and workload characteristics.
To run benchmarks use `cargo run --release --example rwlock`
<details>
<summary>Dev Profile</summary>
Write-Heavy Workload (50 writers, 5 readers)
| parking_lot::RwLock | 2646 - 2655 | 2583 - 2615 | 1.02:1 |
| std::sync::RwLock | 2860 - 2888 | 755 - 765 | 3.78:1 |
| seqlock::SeqLock | 1313 - 1315 | 33621 - 33690 | 1:25.61 |
| snaplock::SnapshotLock | 2532 - 2635 | 100910 - 102094 | 1:39.29 |
| crossbeam::sync::ShardedLock | 2822 - 2854 | 1287 - 1318 | 2.18:1 |
| SrwLock | 2331 - 2672 | 1448 - 1544 | 1.67:1 |
Balanced Workload (10 writers, 10 readers)
| parking_lot::RwLock | 8721 - 8728 | 8543 - 8588 | 1.02:1 |
| std::sync::RwLock | 14329 - 14415 | 966 - 984 | 14.74:1 |
| seqlock::SeqLock | 3056 - 3058 | 20560 - 21255 | 1:6.84 |
| snaplock::SnapshotLock | 12794 - 12928 | 89615 - 93864 | 1:7.13 |
| crossbeam::sync::ShardedLock | 6102 - 6132 | 1419 - 66331 | 1:5.54 |
| SrwLock | 9384 - 10271 | 5427 - 6365 | 1.67:1 |
Read-Heavy Workload (5 writers, 50 readers)
| parking_lot::RwLock | 11175 - 11186 | 11127 - 11157 | 1:1 |
| std::sync::RwLock | 16772 - 16902 | 8715 - 12010 | 1.62:1 |
| seqlock::SeqLock | 5505 - 5516 | 13228 - 29256 | 1:3.85 |
| snaplock::SnapshotLock | 10821 - 13468 | 17427 - 67536 | 1:3.5 |
| crossbeam::sync::ShardedLock | 1 - 1202 | 237 - 63322 | 1:52.83 |
| SrwLock | 293 - 435 | 25271 - 31891 | 1:78.52 |
</details>
<details>
<summary>Release Profile</summary>
Write-Heavy Workload (50 writers, 5 readers)
| parking_lot::RwLock | 36971 - 37016 | 37212 - 42922 | 1:1.08 |
| std::sync::RwLock | 169 - 233 | 101781 - 164053 | 1:661.28 |
| seqlock::SeqLock | 125082 - 125938 | 101491 - 268350 | 1:1.47 |
| snaplock::SnapshotLock | 49484 - 68635 | 54255 - 277519 | 1:2.81 |
| crossbeam::sync::ShardedLock | 1 - 784 | 2115 - 253801 | 1:326.01 |
| SrwLock | 1805 - 2032 | 172679 - 247496 | 1:109.51 |
Balanced Workload (10 writers, 10 readers)
| parking_lot::RwLock | 35748 - 35892 | 34653 - 34855 | 1.03:1 |
| std::sync::RwLock | 7899 - 8752 | 148495 - 148745 | 1:17.85 |
| seqlock::SeqLock | 53776 - 53913 | 511459 - 515983 | 1:9.54 |
| snaplock::SnapshotLock | 53785 - 54012 | 272401 - 276799 | 1:5.09 |
| crossbeam::sync::ShardedLock | 4050 - 4244 | 9438 - 258410 | 1:32.29 |
| SrwLock | 30869 - 31200 | 48836 - 49830 | 1:1.59 |
Read-Heavy Workload (5 writers, 50 readers)
| parking_lot::RwLock | 10693 - 10801 | 10223 - 10261 | 1.05:1 |
| std::sync::RwLock | 9467 - 9982 | 47042 - 47212 | 1:4.85 |
| seqlock::SeqLock | 10639 - 10738 | 532480 - 532875 | 1:49.84 |
| snaplock::SnapshotLock | 10964 - 11185 | 280155 - 280390 | 1:25.31 |
| crossbeam::sync::ShardedLock | 9035 - 9446 | 50122 - 50711 | 1:5.46 |
| SrwLock | 11366 - 12109 | 12864 - 13396 | 1:1.12 |
</details>