An implementation of an LRU Cache in Julia
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February 2015


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Provides a thread-safe implementation of a Least Recently Used (LRU) Cache for Julia.

An LRU Cache is a useful associative data structure (AbstractDict in Julia) that has a set maximum size (as measured by number of elements or a custom size measure for items). Once that size is reached, the least recently used items are removed first. A lock ensures that data access does not lead to race conditions.

A particular use case of this package is to implement function memoization for functions that can simultaneously be called from different threads.


Install with the package manager via ]add LRUCache or

using Pkg


LRU supports the standard AbstractDict interface. Some examples of common operations are shown below:


lru = LRU{K, V}(, maxsize = size [, by = ...])

Create an LRU Cache with a maximum size (number of items) specified by the required keyword argument maxsize. Here, the size can be the number of elements (default), or the maximal total size of the values in the dictionary, as counted by an arbitrary user function (which should return a single value of type Int) specified with the keyword argument by. Sensible choices would for example be by = sizeof for e.g. values which are Arrays of bitstypes, or by = Base.summarysize for values of some arbitrary user type.

Add an item to the cache

setindex!(lru, value, key)
lru[key] = value

Lookup an item in the cache

getindex(lru, key)

Change the maxsize

resize!(lru; maxsize = size)

Here, the maximal size is specified via a required keyword argument. Remember that the maximal size is not necessarily the same as the maximal length, if a custom function was specified using the keyword argument by in the construction of the LRU cache.

Empty the cache


Caching Use

To effectively use LRU as a cache, several functions from the AbstractDict interface can be used for easy checking if an item is present, and if not quickly calculating a default.

get!(lru::LRU, key, default)

Returns the value stored in lru for key if present. If not, stores key => default, and returns default.

get!(default::Callable, lru::LRU, key)

Like above, except if key is not present, stores key => default(), and returns the result. This is intended to be used in do block syntax:

get!(lru, key) do

get(lru::LRU, key, default)

Returns the value stored in lru for key if present. If not, returns default without storing this value in lru. Also comes in the following form:

get(default::Callable, lru::LRU, key)


Commonly, you may have some long running function that sometimes gets called with the same parameters more than once. As such, it may benefit from caching the results.

Here's our example, long running calculation:

function foo(a::Float64, b::Float64)
    result = a * b

As this function requires more than one parameter, we need a cache from (Float64, Float64) to Float64. A cached version is then:

const lru = LRU{Tuple{Float64, Float64}, Float64}()

function cached_foo(a::Float64, b::Float64)
    get!(lru, (a, b)) do

Required Packages

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