/node-lru-cache

Primary LanguageJavaScriptISC LicenseISC

lru-cache

A cache object that deletes the least-recently-used items.

Specify a max number of the most recently used items that you want to keep, and this cache will keep that many of the most recently accessed items.

This is not primarily a TTL cache, and does not make strong TTL guarantees. There is no preemptive pruning of expired items, but you may set a TTL on the cache or on a single set. If you do so, it will treat expired items as missing, and delete them when fetched.

As of version 7, this is one of the most performant LRU implementations available in JavaScript, and supports a wide diversity of use cases. However, note that using some of the features will necessarily impact performance, by causing the cache to have to do more work. See the "Performance" section below.

Installation

npm install lru-cache --save

Usage

const LRU = require('lru-cache')

// only 'max' is required, the others are optional, but MAY be
// required if certain other fields are set.
const options = {
  // the number of most recently used items to keep.
  // note that we may store fewer items than this if maxSize is hit.
  max: 500,

  // if you wish to track item size, you must provide a maxSize
  // note that we still will only keep up to max *actual items*,
  // so size tracking may cause fewer than max items to be stored.
  // At the extreme, a single item of maxSize size will cause everything
  // else in the cache to be dropped when it is added.  Use with caution!
  // Note also that size tracking can negatively impact performance,
  // though for most cases, only minimally.
  maxSize: 5000,

  // function to calculate size of items.  useful if storing strings or
  // buffers or other items where memory size depends on the object itself.
  // also note that oversized items do NOT immediately get dropped from
  // the cache, though they will cause faster turnover in the storage.
  sizeCalculation: (value, key) => {
    // return an positive integer which is the size of the item,
    // if a positive integer is not returned, will use 0 as the size.
    return 1
  },

  // function to call when the item is removed from the cache
  // Note that using this can negatively impact performance.
  dispose: (value, key) => {
    freeFromMemoryOrWhatever(value)
  },

  // max time to live for items before they are considered stale
  // note that stale items are NOT preemptively removed, and MAY
  // live in the cache, contributing to its LRU max, long after they
  // have expired.
  // Also, as this cache is optimized for LRU/MRU operations, some of
  // the staleness/TTL checks will reduce performance, as they will incur
  // overhead by deleting items.
  // Must be a positive integer in ms, defaults to 0, which means "no TTL"
  ttl: 1000 * 60 * 5,

  // return stale items from cache.get() before disposing of them
  // boolean, default false
  allowStale: false,

  // update the age of items on cache.get(), renewing their TTL
  // boolean, default false
  updateAgeOnGet: false,

  // update the age of items on cache.has(), renewing their TTL
  // boolean, default false
  updateAgeOnHas: false,

  // update the "recently-used"-ness of items on cache.has()
  // boolean, default false
  updateRecencyOnHas: false,
}

const cache = new LRU(options)

cache.set("key", "value")
cache.get("key") // "value"

// non-string keys ARE fully supported
// but note that it must be THE SAME object, not
// just a JSON-equivalent object.
var someObject = { a: 1 }
cache.set(someObject, 'a value')
// Object keys are not toString()-ed
cache.set('[object Object]', 'a different value')
assert.equal(cache.get(someObject), 'a value')
// A similar object with same keys/values won't work,
// because it's a different object identity
assert.equal(cache.get({ a: 1 }), undefined)

cache.clear()    // empty the cache

If you put more stuff in it, then items will fall out.

Options

  • max - The maximum number (or size) of items that remain in the cache (assuming no TTL pruning or explicit deletions). Note that fewer items may be stored if size calculation is used, and maxSize is exceeded. This must be a positive finite intger.

  • maxSize - Set to a positive integer to track the sizes of items added to the cache, and automatically evict items in order to stay below this size. Note that this may result in fewer than max items being stored.

  • sizeCalculation - Function used to calculate the size of stored items. If you're storing strings or buffers, then you probably want to do something like n => n.length. The item is passed as the first argument, and the key is passed as the second argumnet.

    This may be overridden by passing an options object to cache.set().

    Requires maxSize to be set.

    Deprecated alias: length

  • dispose Function that is called on items when they are dropped from the cache. This can be handy if you want to close file descriptors or do other cleanup tasks when items are no longer stored in the cache.

    It is called after the item has been fully removed from the cache, so if you want to put it right back in, that is safe to do.

    Unlike several other options, this may not be overridden by passing an option to set(), for performance reasons. If disposal functions may vary between cache entries, then the entire list must be scanned on every cache swap, even if no disposal function is in use.

    Optional, must be a function.

  • noDisposeOnSet Set to true to suppress calling the dispose() function if the entry key is still accessible within the cache.

    This may be overridden by passing an options object to cache.set().

    Boolean, default false. Only relevant if dispose option is set.

  • ttl - max time to live for items before they are considered stale. Note that stale items are NOT preemptively removed, and MAY live in the cache, contributing to its LRU max, long after they have expired.

    Also, as this cache is optimized for LRU/MRU operations, some of the staleness/TTL checks will reduce performance, as they will incur overhead by deleting from Map objects rather than simply throwing old Map objects away.

    This is not primarily a TTL cache, and does not make strong TTL guarantees. There is no pre-emptive pruning of expired items, but you may set a TTL on the cache, and it will treat expired items as missing when they are fetched, and delete them.

    Optional, but must be a positive integer in ms if specified.

    This may be overridden by passing an options object to cache.set().

    Deprecated alias: maxAge

  • allowStale - By default, if you set ttl, it'll only delete stale items from the cache when you get(key). That is, it's not pre-emptively pruning items.

    If you set allowStale:true, it'll return the stale value as well as deleting it. If you don't set this, then it'll return undefined when you try to get a stale entry.

    Note that when a stale entry is fetched, even if it is returned due to allowStale being set, it is removed from the cache immediately. You can immediately put it back in the cache if you wish, thus resetting the TTL.

    This may be overridden by passing an options object to cache.get(). The cache.has() method will always return false for stale items.

    Boolean, default false, only relevant if ttl is set.

    Deprecated alias: stale

  • updateAgeOnGet - When using time-expiring entries with ttl, setting this to true will make each item's age reset to 0 whenever it is retrieved from cache with get(), causing it to not expire. (It can still fall out of cache based on recency of use, of course.)

    This may be overridden by passing an options object to cache.get().

    Boolean, default false, only relevant if ttl is set.

API

  • new LRUCache(options)

    Create a new LRUCache. All options are documented above, and are on the cache as public members.

  • cache.max, cache.ttl, cache.allowStale, etc.

    All option names are exposed as public members on the cache object.

    These are intended for read access only. Changing them during program operation can cause undefined behavior.

  • cache.size

    The total number of items held in the cache at the current moment.

  • cache.calculatedSize

    The total size of items in cache when using size tracking.

  • set(key, value, [{ size, sizeCalculation, ttl, noDisposeOnSet }])

    Add a value to the cache.

    Optional options object may contain ttl and sizeCalculation as described above, which default to the settings on the cache object.

    Options object my also include size, which will prevent calling the sizeCalculation function and just use the specified number if it is a positive integer, and noDisposeOnSet which will prevent calling a dispose function in the case of overwrites.

    Will update the recency of the entry.

  • get(key, { updateAgeOnGet, allowStale } = {}) => value

    Return a value from the cache.

    Will update the recency of the cache entry found.

    If the key is not found, get() will return undefined. This can be confusing when setting values specifically to undefined, as in cache.set(key, undefined). Use cache.has() to determine whether a key is present in the cache at all.

  • peek(key, { allowStale } = {}) => value

    Like get() but doesn't update recency or delete stale items.

    Returns undefined if the item is stale, unless allowStale is set either on the cache or in the options object.

  • has(key)

    Check if a key is in the cache, without updating the recency or age.

    Will return false if the item is stale, even though it is technically in the cache.

  • delete(key)

    Deletes a key out of the cache.

  • clear()

    Clear the cache entirely, throwing away all values.

    Deprecated alias: reset()

  • keys()

    Return a generator yielding the keys in the cache.

  • values()

    Return a generator yielding the values in the cache.

  • entries()

    Return a generator yielding [key, value] pairs.

  • find(fn, [getOptions])

    Find a value for which the supplied fn method returns a truthy value, similar to Array.find().

    fn is called as fn(value, key, cache).

    The optional getOptions are applied to the resulting get() of the item found.

  • dump()

    Return an array of [key, entry] objects which can be passed to cache.load()

    Note: this returns an actual array, not a generator, so it can be more easily passed around.

  • load(entries)

    Reset the cache and load in the items in entries in the order listed. Note that the shape of the resulting cache may be different if the same options are not used in both caches.

  • purgeStale()

    Delete any stale entries. Returns true if anything was removed, false otherwise.

Internal Methods and Properties

Do not use or rely on these. They will change or be removed without notice. They will cause undefined behavior if used inappropriately.

This documentation is here so that it is especially clear that this not "undocumented" because someone forgot; it is documented, and the documentation is telling you not to do it.

Do not report bugs that stem from using these properties. They will be ignored.

  • setKeyIndex() Assign an index to a given key.
  • getKeyIndex() Get the index for a given key.
  • deleteKeyIndex() Remove the index for a given key.
  • getDisposeData() Get the data to pass to a dispose() call.
  • callDispose() Actually call the dispose() function.
  • onSet() Called to assign data when set() is called.
  • evict() Delete the least recently used item.
  • onDelete() Perform actions required for deleting an entry.
  • isStale() Check if an item is stale, by index.
  • list The internal linked list of indexes defining recency.

Performance

As of January 2022, version 7 of this library is one of the most performant LRU cache implementations in JavaScript.

Benchmarks can be extremely difficult to get right. In particular, the performance of set/get/delete operations on objects will vary wildly depending on the type of key used. V8 is highly optimized for objects with keys that are short strings, especially integer numeric strings. Thus any benchmark which tests solely using numbers as keys will tend to find that an object-based approach performs the best.

Note that coercing anything to strings to use as object keys is unsafe, unless you can be 100% certain that no other type of value will be used. For example:

const myCache = {}
const set = (k, v) => myCache[k] = v
const get = (k) => myCache[k]

set({}, 'please hang onto this for me')
set('[object Object]', 'oopsie')

Also beware of "Just So" stories regarding performance. Garbage collection of large (especially: deep) object graphs can be incredibly costly, with several "tipping points" where it increases exponentially. As a result, putting that off until later can make it much worse, and less predictable. If a library performs well, but only in a scenario where the object graph is kept shallow, then that won't help you if you are using large objects as keys.

In general, when attempting to use a library to improve performance (such as a cache like this one), it's best to choose an option that will perform well in the sorts of scenarios where you'll actually use it.

This library is optimized for repeated gets and minimizing eviction time, since that is the expected need of a LRU. Set operations are somewhat slower on average than a few other options, in part because of that optimization. It is assumed that you'll be caching some costly operation, ideally as rarely as possible, so optimizing set over get would be unwise.

If performance matters to you:

  1. If it's at all possible to use small integer values as keys, and you can guarantee that no other types of values will be used as keys, then do that, and use a cache such as lru-fast which uses an Object as its data store.
  2. Failing that, if at all possible, use short non-numeric strings (ie, less than 256 characters) as your keys.
  3. If you know that the types of your keys will be long strings, strings that look like floats, null, objects, or some mix of types, then this library will work well for you.
  4. Do not use a dispose function, size tracking, or ttl behavior, unless absolutely needed. These features are convenient, and necessary in some use cases, and every attempt has been made to make the performance impact minimal, but it isn't nothing.

Breaking Changes in Version 7

This library changed to a different algorithm and internal data structure in version 7, yielding significantly better performance, albeit with some subtle changes as a result.

If you were relying on the internals of LRUCache in version 6 or before, it probably will not work in version 7 and above.

Specific API Changes

For the most part, the feature set has been maintained as much as possible.

However, some other cleanup and refactoring changes were made in v7 as well.

  • The set(), get(), and has() functions take options objects instead of positional booleans/integers for optional parameters.
  • size can be set explicitly on set().
  • cache.length was renamed to the more fitting cache.size.
  • Option name deprecations:
    • stale -> allowStale
    • length -> sizeCalculation
    • maxAge -> ttl
  • The objects used by cache.load() and cache.dump() are incompatible with previous versions.