HydraCache

HydraCache is a Rust-native local async cache that is designed to grow toward database result caching and distributed synchronization later.

Status

HydraCache is in early development. The current implementation targets the first local-cache release.

Why HydraCache?

HydraCache is not trying to replace low-level cache engines, databases, or query processors. It is an application-facing cache layer for Rust services.

Compared with using Moka directly, HydraCache adds a smaller product-shaped API: loader helpers, TTLs, tag invalidation, local single-flight, codec-backed storage, and lightweight stats in one place.

Compared with ORM-level caches, HydraCache keeps freshness explicit. Keys, tags, and invalidation are application-controlled instead of hidden behind a large persistence framework.

Compared with Redis-style caches, HydraCache is embedded and local-first. The first version needs no server, proxy, daemon, or network hop.

Compared with ReadySet or Noria-style query engines, HydraCache deliberately does not try to incrementally maintain SQL result graphs. It is a lightweight cache library first, with database-result caching planned as an adapter layer.

The long-term direction is:

simple local cache -> database result-cache adapter -> optional distributed synchronization

v0 Scope

The first version includes:

local async cache runtime
HydraCache::local() builder
get
put
get_or_load
get_or_insert_with
try_get_or_insert_with
local single-flight miss deduplication
contains_key
per-entry TTL and default TTL
tag-aware invalidation
key invalidation
remove as a local-cache alias for key invalidation
flush
postcard codec over Bytes
lightweight stats
single-flight join stats
tag-generation invalidation safety
Moka-backed local storage

Out of scope for v0:

SQLx adapter
proc macros
distributed invalidation
cluster roles
generation counters
persistence

Example

use std::time::Duration;

use hydracache::{CacheOptions, HydraCache};
use serde::{Deserialize, Serialize};

#[derive(Debug, Clone, Serialize, Deserialize)]
struct User {
    id: u64,
    name: String,
}

async fn load_user(id: u64) -> Result<User, std::io::Error> {
    Ok(User {
        id,
        name: format!("user-{id}"),
    })
}

# async fn example() -> hydracache::CacheResult<()> {
let cache = HydraCache::local()
    .default_ttl(Duration::from_secs(300))
    .max_capacity(10_000)
    .build();

let user = cache
    .try_get_or_insert_with(
        "user:42",
        CacheOptions::new()
            .ttl(Duration::from_secs(60))
            .tags(["user:42", "users"]),
        || async { load_user(42).await },
    )
    .await?;

cache.invalidate_tag("user:42").await?;
# Ok(())
# }

API Notes

get returns Ok(None) when the key is missing or expired.

get_or_load runs the loader on a miss and stores the loaded value with the provided CacheOptions.

get_or_insert_with is the short local-cache spelling for infallible async loaders.

try_get_or_insert_with is the fallible-loader spelling. It behaves the same as get_or_load.

Concurrent get_or_load calls for the same missing key share one loader execution. Cache hits bypass single-flight entirely.

If a tag is invalidated while a tagged loader is still running, HydraCache skips storing that stale loader result. Callers after the invalidation start or join a fresh in-flight load instead of joining the stale one.

contains_key checks whether a key currently maps to a usable value. Expired entries are removed and reported as absent.

remove and invalidate_key both remove one key. remove is the shorter local-cache spelling; invalidate_key is kept for consistency with tag invalidation.

invalidate_tag removes all entries currently associated with the tag.

Use CacheOptions::tag("users") for one tag and CacheOptions::tags(["users", "user:42"]) for multiple tags.

stats returns lightweight counters for hits, misses, loads, single-flight joins, stale load discards, invalidations, and evictions. v0 does not wire backend eviction listeners yet, so evictions remains zero.

Release Plan

The v0 release plan is maintained here:

Workspace

crates/hydracache-core - core public types, codec, options, errors, stats
crates/hydracache - user-facing local cache runtime
crates/hydracache-macros - future macro ergonomics
crates/hydracache-sqlx - future SQLx-first adapter layer

hydracache-core 0.4.0