Using core.cache

note: see the page on including core.cache before you begin this section

Basic usage pattern

To use the cache implementations or extend the core.cache protocols you first need to require the proper namespace:

(require '[clojure.core.cache :as cache])

Next you should create an instance of a specific cache type, optionally seeded:

(def C (cache/fifo-cache-factory {:a 1, :b 2}))

To find a value in a map by its key you have a couple choices:

(cache/lookup C :a)
;=> 1

(:b C)
;=> 2

To ensure the proper cache policies are followed for each specific type, the following has?/hit/miss pattern should be used:

(if (cache/has? C :c)     ;; has? checks that the cache contains an item
  (cache/hit C :c)        ;; hit returns a cache with any relevant internal information updated
  (cache/miss C :c 42))   ;; miss returns a new cache with the new item and without evicted entries

;=> {:a 1, :b 2, :c 42}

Using the has?/hit/miss pattern ensures that the thresholding and eviction logic for each implementation works properly. It is built into the through and through-cache functions, as well as clojure.core.cache.wrapped/lookup-or-miss. Avoid this pattern at your own risk.

Finally, to explicitly evict an element in a cache, use the evict function:

(cache/evict C :b)

;=> {:a 1}

For specific information about eviction policies and thresholds, view the specific documentation for each cache type listed in the next section.

Sometimes you may wish to cache a function except when it returns certain values, such as a resource-loading function that may return nil if the resource is (temporarily) unavailable. Because of the edge cases that can arise with doing multiple lookups on a cache -- protections from which are baked into through, through-cache, and lookup-or-miss -- it is better to write your code to simply cache all results and then, post-retrieval, evict the key if the result is something you don't want cached:

(let [result (wrapped/lookup-or-miss C :c load-my-resource)]
  (when (nil? result) ; or seq or whatever condition should exclude this result
    (wrapped/evict C :c))
  result)

Builtin cache implementations

core.cache comes with a number of builtin immutable cache implementations, including (click through for specific information):

• FIFO cache
• LRU cache
• LU cache
• TTL cache
• LIRS cache
• Function-backed cache (work in progress)
• Soft-reference cache (work in progress)

The core.cache implementations are backed by any map-like object. Additionally, each cache implements the Clojure map behaviors and can therefore serve as special maps or even as backing stores for other cache implementations. For caches taking a limit argument, the eviction policies tend not to apply until the limit has been exceeded.

Extending core.cache

See the section on creating custom caches for more information.

Nesting cache types

See the section on composing caches for more information.