(require '[clojure.core.memoize :as memo])

;; suppose this is a real DB
(def db (atom {}))

(defn my-get [k]
  ;; expensive database call
  (Thread/sleep 5000)
  (get @db k))

(def my-get-cached
  (memo/memo my-get))

(defn my-put
  [k val]
  (swap! db assoc k val)
  (memo/memo-clear! my-get-cached [k]))

(comment
  (my-put :foo "the value")
  (my-get-cached :foo) ;; wait 5 seconds, "the value"
  (my-get-cached :foo) ;; "the value", instantly
  (my-put :bar "other-value")
  (my-get-cached :foo) ;; "the value", still instantly
  (my-get-cached :bar) ;; wait 5 seconds, "other value"
  (my-put :foo "changed")
  (my-get-cached :foo) ;; wait 5 seconds, "changed"
  )