[][src]Struct arc_swap::cache::Cache

pub struct Cache<A, T> { /* fields omitted */ }

Caching handle for ArcSwapAny.

Instead of loading the Arc on every request from the shared storage, this keeps another copy inside itself. Upon request it only cheaply revalidates it is up to date. If it is, access is significantly faster. If it is stale, the load_full is done and the cache value is replaced. Under a read-heavy loads, the measured speedup are 10-25 times, depending on the architecture.

There are, however, downsides:


use std::sync::Arc;

use arc_swap::{ArcSwap, Cache};

let shared = Arc::new(ArcSwap::from_pointee(42));
// Start 10 worker threads...
for _ in 0..10 {
    let mut cache = Cache::new(Arc::clone(&shared));
    std::thread::spawn(move || {
        // Keep loading it like mad..
        loop {
            let value = cache.load();


impl<A, T, S> Cache<A, T> where
    A: Deref<Target = ArcSwapAny<T, S>>,
    T: RefCnt,
    S: LockStorage

pub fn new(arc_swap: A) -> Self[src]

Creates a new caching handle.

The parameter is something dereferencing into an ArcSwapAny (eg. either to ArcSwap or ArcSwapOption). That can be ArcSwapAny itself, but that's not very useful. But it also can be a reference to it or Arc, which makes it possible to share the ArcSwapAny with multiple caches or access it in non-cached way too.

pub fn arc_swap(&self) -> &A::Target[src]

Gives access to the (possibly shared) cached ArcSwapAny.

pub fn load(&mut self) -> &T[src]

Loads the currently held value.

This first checks if the cached value is up to date. This check is very cheap.

If it is up to date, the cached value is simply returned without additional costs. If it is outdated, a load is done on the underlying shared storage. The newly loaded value is then stored in the cache and returned.

pub fn map<F, U>(self, f: F) -> MapCache<A, T, F> where
    F: FnMut(&T) -> &U, 

Turns this cache into a cache with a projection inside the cached value.

You'd use this in case when some part of code needs access to fresh values of U, however a bigger structure containing U is provided by this cache. The possibility of giving the whole structure to the part of the code falls short in terms of reusability (the part of the code could be used within multiple contexts, each with a bigger different structure containing U) and code separation (the code shouldn't needs to know about the big structure).


As the provided f is called inside every load, this one should be cheap. Most often it is expected to be just a closure taking reference of some inner field.

For the same reasons, it should not have side effects and should never panic (these will not break Rust's safety rules, but might produce behaviour you don't expect).


extern crate arc_swap;

use arc_swap::ArcSwap;
use arc_swap::cache::{Access, Cache};

struct InnerCfg {
    answer: usize,

struct FullCfg {
    inner: InnerCfg,

fn use_inner<A: Access<InnerCfg>>(cache: &mut A) {
    let value = cache.load();
    println!("The answer is: {}", value.answer);

let full_cfg = ArcSwap::from_pointee(FullCfg {
    inner: InnerCfg {
        answer: 42,
let cache = Cache::new(&full_cfg);
use_inner(&mut cache.map(|full| &full.inner));

let inner_cfg = ArcSwap::from_pointee(InnerCfg { answer: 24 });
let mut inner_cache = Cache::new(&inner_cfg);
use_inner(&mut inner_cache);

Trait Implementations

impl<A, T, S> Access<<T as Deref>::Target> for Cache<A, T> where
    A: Deref<Target = ArcSwapAny<T, S>>,
    T: Deref<Target = <T as RefCnt>::Base> + RefCnt,
    S: LockStorage

impl<A: Clone, T: Clone> Clone for Cache<A, T>[src]

impl<A: Debug, T: Debug> Debug for Cache<A, T>[src]

impl<A, T, S> From<A> for Cache<A, T> where
    A: Deref<Target = ArcSwapAny<T, S>>,
    T: RefCnt,
    S: LockStorage

Auto Trait Implementations

impl<A, T> RefUnwindSafe for Cache<A, T> where
    A: RefUnwindSafe,
    T: RefUnwindSafe

impl<A, T> Send for Cache<A, T> where
    A: Send,
    T: Send

impl<A, T> Sync for Cache<A, T> where
    A: Sync,
    T: Sync

impl<A, T> Unpin for Cache<A, T> where
    A: Unpin,
    T: Unpin

impl<A, T> UnwindSafe for Cache<A, T> where
    A: UnwindSafe,
    T: UnwindSafe

Blanket Implementations

impl<T> Any for T where
    T: 'static + ?Sized

impl<T> Borrow<T> for T where
    T: ?Sized

impl<T> BorrowMut<T> for T where
    T: ?Sized

impl<T> From<T> for T[src]

impl<T, U> Into<U> for T where
    U: From<T>, 

impl<T> ToOwned for T where
    T: Clone

type Owned = T

The resulting type after obtaining ownership.

impl<T, U> TryFrom<U> for T where
    U: Into<T>, 

type Error = Infallible

The type returned in the event of a conversion error.

impl<T, U> TryInto<U> for T where
    U: TryFrom<T>, 

type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.