Skip to content


Switch branches/tags

Name already in use

A tag already exists with the provided branch name. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. Are you sure you want to create this branch?


Build Status docs

Caching structures and simplified function memoization

cached provides implementations of several caching structures as well as a handy macros for defining memoized functions.

Memoized functions defined using #[cached]/#[once]/#[io_cached]/cached! macros are thread-safe with the backing function-cache wrapped in a mutex/rwlock, or externally synchronized in the case of #[io_cached]. By default, the function-cache is not locked for the duration of the function's execution, so initial (on an empty cache) concurrent calls of long-running functions with the same arguments will each execute fully and each overwrite the memoized value as they complete. This mirrors the behavior of Python's functools.lru_cache. To synchronize the execution and caching of un-cached arguments, specify #[cached(sync_writes = true)] / #[once(sync_writes = true)] (not supported by #[io_cached].


  • default: Include proc_macro and async features
  • proc_macro: Include proc macros
  • async: Include support for async functions and async cache stores
  • async_tokio_rt_multi_thread: Enable tokio's optional rt-multi-thread feature.
  • redis_store: Include Redis cache store
  • redis_async_std: Include async Redis support using async-std and async-std tls support, implies redis_store and async
  • redis_tokio: Include async Redis support using tokio and tokio tls support, implies redis_store and async
  • redis_ahash: Enable the optional ahash feature of redis
  • wasm: Enable WASM support. Note that this feature is incompatible with tokio's multi-thread runtime (async_tokio_rt_multi_thread) and all Redis features (redis_store, redis_async_std, redis_tokio, redis_ahash)

The procedural macros (#[cached], #[once], #[io_cached]) offer more features, including async support. See the proc_macro and macros modules for more samples, and the examples directory for runnable snippets.`

Any custom cache that implements cached::Cached/cached::CachedAsync can be used with the #[cached]/#[once]/cached! macros in place of the built-ins. Any custom cache that implements cached::IOCached/cached::IOCachedAsync can be used with the #[io_cached] macro.

The basic usage looks like:

use cached::proc_macro::cached;

/// Defines a function named `fib` that uses a cache implicitly named `FIB`.
/// By default, the cache will be the function's name in all caps.
/// The following line is equivalent to #[cached(name = "FIB", unbound)]
fn fib(n: u64) -> u64 {
    if n == 0 || n == 1 { return n }
    fib(n-1) + fib(n-2)

use std::thread::sleep;
use std::time::Duration;
use cached::proc_macro::cached;
use cached::SizedCache;

/// Use an explicit cache-type with a custom creation block and custom cache-key generating block
    type = "SizedCache<String, usize>",
    create = "{ SizedCache::with_size(100) }",
    convert = r#"{ format!("{}{}", a, b) }"#
fn keyed(a: &str, b: &str) -> usize {
    let size = a.len() + b.len();
    sleep(Duration::new(size as u64, 0));

use cached::proc_macro::once;

/// Only cache the initial function call.
/// Function will be re-executed after the cache
/// expires (according to `time` seconds).
/// When no (or expired) cache, concurrent calls
/// will synchronize (`sync_writes`) so the function
/// is only executed once.
#[once(time=10, option = true, sync_writes = true)]
fn keyed(a: String) -> Option<usize> {
    if a == "a" {
    } else {

use cached::proc_macro::io_cached;
use cached::AsyncRedisCache;
use thiserror::Error;

#[derive(Error, Debug, PartialEq, Clone)]
enum ExampleError {
    #[error("error with redis cache `{0}`")]

/// Cache the results of an async function in redis. Cache
/// keys will be prefixed with `cache_redis_prefix`.
/// A `map_error` closure must be specified to convert any
/// redis cache errors into the same type of error returned
/// by your function. All `io_cached` functions must return `Result`s.
    map_error = r##"|e| ExampleError::RedisError(format!("{:?}", e))"##,
    type = "AsyncRedisCache<u64, String>",
    create = r##" {
        AsyncRedisCache::new("cached_redis_prefix", 1)
            .expect("error building example redis cache")
    } "##
async fn async_cached_sleep_secs(secs: u64) -> Result<String, ExampleError> {

Functions defined via macros will have their results cached using the function's arguments as a key, a convert expression specified on a procedural macros, or a Key block specified on a cached_key! declarative macro.

When a macro-defined function is called, the function's cache is first checked for an already computed (and still valid) value before evaluating the function body.

Due to the requirements of storing arguments and return values in a global cache:

  • Function return types:
    • For all store types, except Redis, must be owned and implement Clone
    • For the Redis store type, must be owned and implement serde::Serialize + serde::DeserializeOwned
  • Function arguments:
    • For all store types, except Redis, must either be owned and implement Hash + Eq + Clone, the cached_key! macro is used with a Key block specifying key construction, or a convert expression is specified on a procedural macro to specify how to construct a key of a Hash + Eq + Clone type.
    • For the Redis store type, must either be owned and implement Display, or the cached_key! & Key or procedural macro & convert expression used to specify how to construct a key of a Display type.
  • Arguments and return values will be cloned in the process of insertion and retrieval. Except for Redis where arguments are formatted into Strings and values are de/serialized.
  • Macro-defined functions should not be used to produce side-effectual results!
  • Macro-defined functions cannot live directly under impl blocks since macros expand to a once_cell/lazy_static initialization and one or more function definitions.
  • Macro-defined functions cannot accept Self types as a parameter.

License: MIT