Struct moka::dash::Cache

source · [−]
pub struct Cache<K, V, S = RandomState> { /* private fields */ }
This is supported on crate feature dash only.
Expand description

Experimental: A thread-safe concurrent in-memory cache built upon dashmap::DashMap.

The dash::Cache uses DashMap as the central key-value storage, while other sync and future caches are using a lock-free concurrent hash table. Since DashMap employs read-write locks on internal shards, it will have lower concurrency on retrievals and updates than other caches.

dash 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.

To use this cache, enable a crate feature called “dash” in your Cargo.toml. Please note that the API of dash cache will be changed very often in next few releases as this is yet an experimental component.

Examples

Cache entries are manually added using insert method, 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::dash::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)));
    }
}

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::dash::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 of 1. 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 or insert.
use moka::dash::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 bounds Send, Sync and 'static for K (key), V (value) and S (hasher state).
  • Cache<K, V, S> will implement Send and Sync.

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 or sync::Lazy from once_cell create, and set it to a static 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

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.

Returns a CacheBuilder, which can builds a Cache with various configuration knobs.

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.

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.

Inserts a key-value pair into the cache.

If the cache has this key present, the value is updated.

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.

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.

Returns a read-only cache policy of this cache.

At this time, cache policy cannot be modified after cache creation. A future version may support to modify it.

Creates an iterator visiting all key-value pairs in arbitrary order. The iterator element type is EntryRef<'a, K, V, S>.

Unlike the get method, visiting entries via an iterator do not update the historic popularity estimator or reset idle timers for keys.

Locking behavior

This iterator relies on the iterator of dashmap::DashMap, which employs read-write locks. May deadlock if the thread holding an iterator attempts to update the cache.

Examples
use moka::dash::Cache;

let cache = Cache::new(100);
cache.insert("Julia", 14);

let mut iter = cache.iter();
let entry_ref = iter.next().unwrap();
assert_eq!(entry_ref.pair(), (&"Julia", &14));
assert_eq!(entry_ref.key(), &"Julia");
assert_eq!(entry_ref.value(), &14);
assert_eq!(*entry_ref, 14);

assert!(iter.next().is_none());

Trait Implementations

Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

Performs any pending maintenance operations needed by the cache.

Formats the value using the given formatter. Read more

The type of the elements being iterated over.

Which kind of iterator are we turning this into?

Creates an iterator from a value. Read more

Auto Trait Implementations

Blanket Implementations

Gets the TypeId of self. Read more

Immutably borrows from an owned value. Read more

Mutably borrows from an owned value. Read more

Returns the argument unchanged.

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

The resulting type after obtaining ownership.

Creates owned data from borrowed data, usually by cloning. Read more

🔬 This is a nightly-only experimental API. (toowned_clone_into)

Uses borrowed data to replace owned data, usually by cloning. Read more

The type returned in the event of a conversion error.

Performs the conversion.

The type returned in the event of a conversion error.

Performs the conversion.