Ṛta (ऋत)

Ṛta (ऋत) is a minimal metadata store for durable system state.

The type T

T must satisfy following layout and safety constraints:

• Implements RTA
• Uses #[repr(C)]
• 8 bytes alignment
• Does not implements Drop
• Size must be >0 and multiple of 8
• Implements Default + Clone + Send + Sync + 'static

Limitations of T

• No non-deterministic feilds
• No interior pointers or self-references
• Changes made in struct layout break compatibility

Example

use rta::{Rta, RTA};

#[repr(C)]
#[derive(Default, Clone, Copy, RTA)]
struct TestType {
    a: u64,
    b: u64,
}

const MOD_ID: u8 = 0;

let path = tempfile::NamedTempFile::new().unwrap().into_temp_path().to_path_buf();
let rta = Rta::<TestType, MOD_ID>::new(&path).unwrap();

rta.write(|t| {
  t.a = 0x20;
  t.b = 0x40;
}).unwrap();

let val = rta.read().unwrap();
assert_eq!(val.a, 0x20);
assert_eq!(val.b, 0x40);

Writes

The [Rta::write()] operation in Rta is designed as a lightweight fire-and-forget metadata update primitive. This call itself does not wait for disk synchronization on the fast path, allowing extremely low write latency while still preserving crash-safe durability semantics by handing off durability responsibility to a background thread.

Guarantees

Following guarantees are provided,

• serialized metadata updates
• crash-safe durability via multiple on-disk copies
• automatic recovery from torn writes using latest valid version

Benchmarks

Notes

• Most writes complete in ~100ns on the uncontended fast path.
• The benchmark primarily measures,
  • lock acquisition
  • in-memory mutation
  • dirty state propagation
• Disk durability is handled asynchronously by a dedicated background thread.
• Tail latency may increase if writes overlap with active durability synchronization.

Reads

The [Rta::read()] operation returns the latest available in-memory metadata state.

The read path is optimistic and does not provide durability guarantees at read time. Instead, reads are served directly from the synchronized in-memory cache, avoiding any disk IO overhead.

Guarantees

Following guarantees are provided,

• lock-safe concurrent access
• latest available in-memory metadata view
• no torn or partially visible updates
• parallel read operations

Notes

• Reads are performed entirely from the in-memory cache.
• No mmap scan or disk synchronization occurs during the read path.
• Read latency generally remains stable even during background durability synchronization.

Concurrency Model

Etymology

ऋत (transliterated as Ṛta) is a Vedic concept of cosmic order, truth, and invariance that inspired the design of Ṛta crate.