Clojure Q&A - Recent questions tagged core.memoize

clojure.core.cache caches delays that hold exceptions as of version 1.2.254

Mon, 23 Feb 2026 19:43:34 +0000

As of version 1.2.254 of org.clojure/core.cache, using a value-fn that throws will cause an entry to be added to the cache associated with a Delay with :status :failed and the exception as a value.

The code below can be used to compare the behavior between versions 1.1.234 and 1.2.254.

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

(def counter (atom 0))

(defn value-fn [v]
  (prn :value-fn-called)
  (swap! counter inc)
  (if (= @counter 1)
    (throw (ex-info "thrown" {}))
    v))

(def test-cache (cw/lu-cache-factory {} :threshold 10))

(reset! counter 0)
; this will throw
(cw/lookup-or-miss test-cache "throw" value-fn)
; in 1.1.234 this will call value-fn again and not throw; in 1.2.254, it won't call and return
; the cached delay result, which is an exception
(cw/lookup-or-miss test-cache "throw" value-fn)

; evict to force call to value-fn
(cw/evict test-cache "throw")

This is related to work done in CCACHE-65 to avoid cache stampede in multi-threaded applications, and could be related to what has been reported in CMEMOIZE-31.

This was initially reported in Clojurians Slack.

Multi-threaded cache stampede in core.cache

Wed, 18 Jan 2023 18:06:18 +0000

We recently deployed core.cache in a web context, and we found out that there was a cache stampede for multi-threaded access. It was obvious in retrospect, but the references to stampede in the docs made me think that this was handled.

With the help of Slack, I have a reproduction here:

(let [thread-count 20
      cache-atom (-> {}
                   (cache/ttl-cache-factory :ttl 120000)
                   (cache/lu-cache-factory :threshold 100)
                   (atom))
      latch (java.util.concurrent.CountDownLatch. thread-count)
      invocations-counter (atom 0)]
 (doseq [i (range thread-count)]
   (println "starting thread" i)
   (.start (Thread. (fn []
                     (cache-wrapped/lookup-or-miss cache-atom "my-key"
                                                   (fn [k]
                                                     (swap! invocations-counter inc)
                                                     (Thread/sleep 3000)
                                                     "some value"))
                     (.countDown latch)))))

 (.await latch)
 (deref invocations-counter))

I would expect that invocations counter would be 1 but it's 20 (once per thread).

Rogue backtick in core.memoize's Including.md impairs formatting

Fri, 16 Jul 2021 16:34:18 +0000

core.memoize's docs/Including.md contains a rogue "`" that messes up presentation a bit. I instinctively went to create a PR, but then was reminded that patches go in Jira. A bit of hoop-jumping later, and here is my suggested fix, having signed the CLA:

https://github.com/clojure/core.memoize/commit/56305f04e9b5a351bf760df663e0884013b43210