pub struct Cache<K, V, S = RandomState> { /* private fields */ }
Expand description
A thread-safe concurrent synchronous in-memory cache.
Cache
supports full concurrency of retrievals and a high expected concurrency
for updates.
Cache
utilizes a lock-free concurrent hash table as the central key-value
storage. Cache
performs a best-effort bounding of the map using an entry
replacement algorithm to determine which entries to evict when the capacity is
exceeded.
Examples
Cache entries are manually added using insert
or
get_with
methods, and are stored in
the cache until either evicted or manually invalidated.
Here’s an example of reading and updating a cache by using multiple threads:
use moka::sync::Cache;
use std::thread;
fn value(n: usize) -> String {
format!("value {}", n)
}
const NUM_THREADS: usize = 16;
const NUM_KEYS_PER_THREAD: usize = 64;
// Create a cache that can store up to 10,000 entries.
let cache = Cache::new(10_000);
// Spawn threads and read and update the cache simultaneously.
let threads: Vec<_> = (0..NUM_THREADS)
.map(|i| {
// To share the same cache across the threads, clone it.
// This is a cheap operation.
let my_cache = cache.clone();
let start = i * NUM_KEYS_PER_THREAD;
let end = (i + 1) * NUM_KEYS_PER_THREAD;
thread::spawn(move || {
// Insert 64 entries. (NUM_KEYS_PER_THREAD = 64)
for key in start..end {
my_cache.insert(key, value(key));
// get() returns Option<String>, a clone of the stored value.
assert_eq!(my_cache.get(&key), Some(value(key)));
}
// Invalidate every 4 element of the inserted entries.
for key in (start..end).step_by(4) {
my_cache.invalidate(&key);
}
})
})
.collect();
// Wait for all threads to complete.
threads.into_iter().for_each(|t| t.join().expect("Failed"));
// Verify the result.
for key in 0..(NUM_THREADS * NUM_KEYS_PER_THREAD) {
if key % 4 == 0 {
assert_eq!(cache.get(&key), None);
} else {
assert_eq!(cache.get(&key), Some(value(key)));
}
}
If you want to atomically initialize and insert a value when the key is not
present, you might want to check other insertion methods
get_with
and
try_get_with
.
Avoiding to clone the value at get
The return type of get
method is Option<V>
instead of Option<&V>
. Every
time get
is called for an existing key, it creates a clone of the stored value
V
and returns it. This is because the Cache
allows concurrent updates from
threads so a value stored in the cache can be dropped or replaced at any time by
any other thread. get
cannot return a reference &V
as it is impossible to
guarantee the value outlives the reference.
If you want to store values that will be expensive to clone, wrap them by
std::sync::Arc
before storing in a cache. Arc
is a
thread-safe reference-counted pointer and its clone()
method is cheap.
Size-based Eviction
use std::convert::TryInto;
use moka::sync::Cache;
// Evict based on the number of entries in the cache.
let cache = Cache::builder()
// Up to 10,000 entries.
.max_capacity(10_000)
// Create the cache.
.build();
cache.insert(1, "one".to_string());
// Evict based on the byte length of strings in the cache.
let cache = Cache::builder()
// A weigher closure takes &K and &V and returns a u32
// representing the relative size of the entry.
.weigher(|_key, value: &String| -> u32 {
value.len().try_into().unwrap_or(u32::MAX)
})
// This cache will hold up to 32MiB of values.
.max_capacity(32 * 1024 * 1024)
.build();
cache.insert(2, "two".to_string());
If your cache should not grow beyond a certain size, use the max_capacity
method of the CacheBuilder
to set the upper bound. The cache
will try to evict entries that have not been used recently or very often.
At the cache creation time, a weigher closure can be set by the weigher
method
of the CacheBuilder
. A weigher closure takes &K
and &V
as the arguments and
returns a u32
representing the relative size of the entry:
- If the
weigher
is not set, the cache will treat each entry has the same size of1
. This means the cache will be bounded by the number of entries. - If the
weigher
is set, the cache will call the weigher to calculate the weighted size (relative size) on an entry. This means the cache will be bounded by the total weighted size of entries.
Note that weighted sizes are not used when making eviction selections.
Time-based Expirations
Cache
supports the following expiration policies:
- Time to live: A cached entry will be expired after the specified duration
past from
insert
. - Time to idle: A cached entry will be expired after the specified duration
past from
get
orinsert
.
use moka::sync::Cache;
use std::time::Duration;
let cache = Cache::builder()
// Time to live (TTL): 30 minutes
.time_to_live(Duration::from_secs(30 * 60))
// Time to idle (TTI): 5 minutes
.time_to_idle(Duration::from_secs( 5 * 60))
// Create the cache.
.build();
// This entry will expire after 5 minutes (TTI) if there is no get().
cache.insert(0, "zero");
// This get() will extend the entry life for another 5 minutes.
cache.get(&0);
// Even though we keep calling get(), the entry will expire
// after 30 minutes (TTL) from the insert().
Thread Safety
All methods provided by the Cache
are considered thread-safe, and can be safely
accessed by multiple concurrent threads.
Cache<K, V, S>
requires trait boundsSend
,Sync
and'static
forK
(key),V
(value) andS
(hasher state).Cache<K, V, S>
will implementSend
andSync
.
Sharing a cache across threads
To share a cache across threads, do one of the followings:
- Create a clone of the cache by calling its
clone
method and pass it to other thread. - Wrap the cache by a
sync::OnceCell
orsync::Lazy
from once_cell create, and set it to astatic
variable.
Cloning is a cheap operation for Cache
as it only creates thread-safe
reference-counted pointers to the internal data structures.
Hashing Algorithm
By default, Cache
uses a hashing algorithm selected to provide resistance
against HashDoS attacks. It will be the same one used by
std::collections::HashMap
, which is currently SipHash 1-3.
While SipHash’s performance is very competitive for medium sized keys, other hashing algorithms will outperform it for small keys such as integers as well as large keys such as long strings. However those algorithms will typically not protect against attacks such as HashDoS.
The hashing algorithm can be replaced on a per-Cache
basis using the
build_with_hasher
method of the
CacheBuilder
. Many alternative algorithms are available on crates.io, such
as the aHash crate.
Implementations
sourceimpl<K, V> Cache<K, V, RandomState> where
K: Hash + Eq + Send + Sync + 'static,
V: Clone + Send + Sync + 'static,
impl<K, V> Cache<K, V, RandomState> where
K: Hash + Eq + Send + Sync + 'static,
V: Clone + Send + Sync + 'static,
sourcepub fn new(max_capacity: u64) -> Self
pub fn new(max_capacity: u64) -> Self
Constructs a new Cache<K, V>
that will store up to the max_capacity
.
To adjust various configuration knobs such as initial_capacity
or
time_to_live
, use the CacheBuilder
.
sourcepub fn builder() -> CacheBuilder<K, V, Cache<K, V, RandomState>>
pub fn builder() -> CacheBuilder<K, V, Cache<K, V, RandomState>>
Returns a CacheBuilder
, which can builds a Cache
or
SegmentedCache
with various configuration knobs.
sourceimpl<K, V, S> Cache<K, V, S> where
K: Hash + Eq + Send + Sync + 'static,
V: Clone + Send + Sync + 'static,
S: BuildHasher + Clone + Send + Sync + 'static,
impl<K, V, S> Cache<K, V, S> where
K: Hash + Eq + Send + Sync + 'static,
V: Clone + Send + Sync + 'static,
S: BuildHasher + Clone + Send + Sync + 'static,
sourcepub fn contains_key<Q>(&self, key: &Q) -> bool where
Arc<K>: Borrow<Q>,
Q: Hash + Eq + ?Sized,
pub fn contains_key<Q>(&self, key: &Q) -> bool where
Arc<K>: Borrow<Q>,
Q: Hash + Eq + ?Sized,
Returns true
if the cache contains a value for the key.
Unlike the get
method, this method is not considered a cache read operation,
so it does not update the historic popularity estimator or reset the idle
timer for the key.
The key may be any borrowed form of the cache’s key type, but Hash
and Eq
on the borrowed form must match those for the key type.
sourcepub fn get<Q>(&self, key: &Q) -> Option<V> where
Arc<K>: Borrow<Q>,
Q: Hash + Eq + ?Sized,
pub fn get<Q>(&self, key: &Q) -> Option<V> where
Arc<K>: Borrow<Q>,
Q: Hash + Eq + ?Sized,
Returns a clone of the value corresponding to the key.
If you want to store values that will be expensive to clone, wrap them by
std::sync::Arc
before storing in a cache. Arc
is a
thread-safe reference-counted pointer and its clone()
method is cheap.
The key may be any borrowed form of the cache’s key type, but Hash
and Eq
on the borrowed form must match those for the key type.
sourcepub fn get_or_insert_with(&self, key: K, init: impl FnOnce() -> V) -> V
👎 Deprecated since 0.8.0: Replaced with get_with
pub fn get_or_insert_with(&self, key: K, init: impl FnOnce() -> V) -> V
Replaced with get_with
Deprecated, replaced with get_with
sourcepub fn get_or_try_insert_with<F, E>(&self, key: K, init: F) -> Result<V, Arc<E>> where
F: FnOnce() -> Result<V, E>,
E: Send + Sync + 'static,
👎 Deprecated since 0.8.0: Replaced with try_get_with
pub fn get_or_try_insert_with<F, E>(&self, key: K, init: F) -> Result<V, Arc<E>> where
F: FnOnce() -> Result<V, E>,
E: Send + Sync + 'static,
Replaced with try_get_with
Deprecated, replaced with try_get_with
sourcepub fn get_with(&self, key: K, init: impl FnOnce() -> V) -> V
pub fn get_with(&self, key: K, init: impl FnOnce() -> V) -> V
Ensures the value of the key exists by inserting the result of the init function if not exist, and returns a clone of the value.
This method prevents to evaluate the init closure multiple times on the same key even if the method is concurrently called by many threads; only one of the calls evaluates its closure, and other calls wait for that closure to complete.
Example
use moka::sync::Cache;
use std::{sync::Arc, thread, time::Duration};
const TEN_MIB: usize = 10 * 1024 * 1024; // 10MiB
let cache = Cache::new(100);
// Spawn four threads.
let threads: Vec<_> = (0..4_u8)
.map(|task_id| {
let my_cache = cache.clone();
thread::spawn(move || {
println!("Thread {} started.", task_id);
// Try to insert and get the value for key1. Although all four
// threads will call `get_with` at the same time, the `init` closure
// must be evaluated only once.
let value = my_cache.get_with("key1", || {
println!("Thread {} inserting a value.", task_id);
Arc::new(vec![0u8; TEN_MIB])
});
// Ensure the value exists now.
assert_eq!(value.len(), TEN_MIB);
thread::sleep(Duration::from_millis(10));
assert!(my_cache.get(&"key1").is_some());
println!("Thread {} got the value. (len: {})", task_id, value.len());
})
})
.collect();
// Wait all threads to complete.
threads
.into_iter()
.for_each(|t| t.join().expect("Thread failed"));
Result
- The
init
closure was called exactly once by thread 1. - Other threads were blocked until thread 1 inserted the value.
Thread 1 started.
Thread 0 started.
Thread 3 started.
Thread 2 started.
Thread 1 inserting a value.
Thread 2 got the value. (len: 10485760)
Thread 1 got the value. (len: 10485760)
Thread 0 got the value. (len: 10485760)
Thread 3 got the value. (len: 10485760)
Panics
This method panics when the init
closure has been panicked. When it
happens, only the caller whose init
closure panicked will get the panic
(e.g. only thread 1 in the above sample). If there are other calls in
progress (e.g. thread 0, 2 and 3 above), this method will restart and resolve
one of the remaining init
closure.
sourcepub fn try_get_with<F, E>(&self, key: K, init: F) -> Result<V, Arc<E>> where
F: FnOnce() -> Result<V, E>,
E: Send + Sync + 'static,
pub fn try_get_with<F, E>(&self, key: K, init: F) -> Result<V, Arc<E>> where
F: FnOnce() -> Result<V, E>,
E: Send + Sync + 'static,
Try to ensure the value of the key exists by inserting an Ok
result of the
init closure if not exist, and returns a clone of the value or the Err
returned by the closure.
This method prevents to evaluate the init closure multiple times on the same key even if the method is concurrently called by many threads; only one of the calls evaluates its closure (as long as these closures return the same error type), and other calls wait for that closure to complete.
Example
use moka::sync::Cache;
use std::{path::Path, time::Duration, thread};
/// This function tries to get the file size in bytes.
fn get_file_size(thread_id: u8, path: impl AsRef<Path>) -> Result<u64, std::io::Error> {
println!("get_file_size() called by thread {}.", thread_id);
Ok(std::fs::metadata(path)?.len())
}
let cache = Cache::new(100);
// Spawn four threads.
let threads: Vec<_> = (0..4_u8)
.map(|thread_id| {
let my_cache = cache.clone();
thread::spawn(move || {
println!("Thread {} started.", thread_id);
// Try to insert and get the value for key1. Although all four
// threads will call `try_get_with` at the same time,
// get_file_size() must be called only once.
let value = my_cache.try_get_with(
"key1",
|| get_file_size(thread_id, "./Cargo.toml"),
);
// Ensure the value exists now.
assert!(value.is_ok());
thread::sleep(Duration::from_millis(10));
assert!(my_cache.get(&"key1").is_some());
println!(
"Thread {} got the value. (len: {})",
thread_id,
value.unwrap()
);
})
})
.collect();
// Wait all threads to complete.
threads
.into_iter()
.for_each(|t| t.join().expect("Thread failed"));
Result
get_file_size()
was called exactly once by thread 1.- Other threads were blocked until thread 1 inserted the value.
Thread 1 started.
Thread 2 started.
get_file_size() called by thread 1.
Thread 3 started.
Thread 0 started.
Thread 2 got the value. (len: 1466)
Thread 0 got the value. (len: 1466)
Thread 1 got the value. (len: 1466)
Thread 3 got the value. (len: 1466)
Panics
This method panics when the init
closure has been panicked. When it
happens, only the caller whose init
closure panicked will get the panic
(e.g. only thread 1 in the above sample). If there are other calls in
progress (e.g. thread 0, 2 and 3 above), this method will restart and resolve
one of the remaining init
closure.
sourcepub fn insert(&self, key: K, value: V)
pub fn insert(&self, key: K, value: V)
Inserts a key-value pair into the cache.
If the cache has this key present, the value is updated.
sourcepub fn invalidate<Q>(&self, key: &Q) where
Arc<K>: Borrow<Q>,
Q: Hash + Eq + ?Sized,
pub fn invalidate<Q>(&self, key: &Q) where
Arc<K>: Borrow<Q>,
Q: Hash + Eq + ?Sized,
Discards any cached value for the key.
The key may be any borrowed form of the cache’s key type, but Hash
and Eq
on the borrowed form must match those for the key type.
sourcepub fn invalidate_all(&self)
pub fn invalidate_all(&self)
Discards all cached values.
This method returns immediately and a background thread will evict all the
cached values inserted before the time when this method was called. It is
guaranteed that the get
method must not return these invalidated values
even if they have not been evicted.
Like the invalidate
method, this method does not clear the historic
popularity estimator of keys so that it retains the client activities of
trying to retrieve an item.
sourcepub fn invalidate_entries_if<F>(
&self,
predicate: F
) -> Result<PredicateId, PredicateError> where
F: Fn(&K, &V) -> bool + Send + Sync + 'static,
pub fn invalidate_entries_if<F>(
&self,
predicate: F
) -> Result<PredicateId, PredicateError> where
F: Fn(&K, &V) -> bool + Send + Sync + 'static,
Discards cached values that satisfy a predicate.
invalidate_entries_if
takes a closure that returns true
or false
. This
method returns immediately and a background thread will apply the closure to
each cached value inserted before the time when invalidate_entries_if
was
called. If the closure returns true
on a value, that value will be evicted
from the cache.
Also the get
method will apply the closure to a value to determine if it
should have been invalidated. Therefore, it is guaranteed that the get
method must not return invalidated values.
Note that you must call
CacheBuilder::support_invalidation_closures
at the cache creation time as the cache needs to maintain additional internal
data structures to support this method. Otherwise, calling this method will
fail with a
PredicateError::InvalidationClosuresDisabled
.
Like the invalidate
method, this method does not clear the historic
popularity estimator of keys so that it retains the client activities of
trying to retrieve an item.
sourcepub fn iter(&self) -> Iter<'_, K, V>ⓘNotable traits for Iter<'i, K, V>impl<'i, K, V> Iterator for Iter<'i, K, V> where
K: Eq + Hash + Send + Sync + 'static,
V: Clone + Send + Sync + 'static, type Item = (Arc<K>, V);
pub fn iter(&self) -> Iter<'_, K, V>ⓘNotable traits for Iter<'i, K, V>impl<'i, K, V> Iterator for Iter<'i, K, V> where
K: Eq + Hash + Send + Sync + 'static,
V: Clone + Send + Sync + 'static, type Item = (Arc<K>, V);
K: Eq + Hash + Send + Sync + 'static,
V: Clone + Send + Sync + 'static, type Item = (Arc<K>, V);
Creates an iterator visiting all key-value pairs in arbitrary order. The
iterator element type is (Arc<K>, V)
, where V
is a clone of a stored
value.
Iterators do not block concurrent reads and writes on the cache. An entry can be inserted to, invalidated or evicted from a cache while iterators are alive on the same cache.
Unlike the get
method, visiting entries via an iterator do not update the
historic popularity estimator or reset idle timers for keys.
Guarantees
In order to allow concurrent access to the cache, iterator’s next
method
does not guarantee the following:
- It does not guarantee to return a key-value pair (an entry) if its key has
been inserted to the cache after the iterator was created.
- Such an entry may or may not be returned depending on key’s hash and timing.
and the next
method guarantees the followings:
- It guarantees not to return the same entry more than once.
- It guarantees not to return an entry if it has been removed from the cache
after the iterator was created.
- Note: An entry can be removed by following reasons:
- Manually invalidated.
- Expired (e.g. time-to-live).
- Evicted as the cache capacity exceeded.
- Note: An entry can be removed by following reasons:
Examples
use moka::sync::Cache;
let cache = Cache::new(100);
cache.insert("Julia", 14);
let mut iter = cache.iter();
let (k, v) = iter.next().unwrap(); // (Arc<K>, V)
assert_eq!(*k, "Julia");
assert_eq!(v, 14);
assert!(iter.next().is_none());
Trait Implementations
sourceimpl<K, V, S> ConcurrentCacheExt<K, V> for Cache<K, V, S> where
K: Hash + Eq + Send + Sync + 'static,
V: Send + Sync + 'static,
S: BuildHasher + Clone + Send + Sync + 'static,
impl<K, V, S> ConcurrentCacheExt<K, V> for Cache<K, V, S> where
K: Hash + Eq + Send + Sync + 'static,
V: Send + Sync + 'static,
S: BuildHasher + Clone + Send + Sync + 'static,
sourceimpl<'a, K, V, S> IntoIterator for &'a Cache<K, V, S> where
K: Hash + Eq + Send + Sync + 'static,
V: Clone + Send + Sync + 'static,
S: BuildHasher + Clone + Send + Sync + 'static,
impl<'a, K, V, S> IntoIterator for &'a Cache<K, V, S> where
K: Hash + Eq + Send + Sync + 'static,
V: Clone + Send + Sync + 'static,
S: BuildHasher + Clone + Send + Sync + 'static,
impl<K, V, S> Send for Cache<K, V, S> where
K: Send + Sync,
V: Send + Sync,
S: Send,
impl<K, V, S> Sync for Cache<K, V, S> where
K: Send + Sync,
V: Send + Sync,
S: Sync,
Auto Trait Implementations
impl<K, V, S = RandomState> !RefUnwindSafe for Cache<K, V, S>
impl<K, V, S> Unpin for Cache<K, V, S>
impl<K, V, S = RandomState> !UnwindSafe for Cache<K, V, S>
Blanket Implementations
sourceimpl<T> BorrowMut<T> for T where
T: ?Sized,
impl<T> BorrowMut<T> for T where
T: ?Sized,
const: unstable · sourcefn borrow_mut(&mut self) -> &mut T
fn borrow_mut(&mut self) -> &mut T
Mutably borrows from an owned value. Read more
sourceimpl<T> ToOwned for T where
T: Clone,
impl<T> ToOwned for T where
T: Clone,
type Owned = T
type Owned = T
The resulting type after obtaining ownership.
sourcefn clone_into(&self, target: &mut T)
fn clone_into(&self, target: &mut T)
toowned_clone_into
)Uses borrowed data to replace owned data, usually by cloning. Read more