persist.clj

(ns file-backed-cache.core
  (:require [clojure.core.cache :as cache :refer [defcache]]
            [clojure.edn :as edn]
            [clojure.java.io :as io]
            [clojure.pprint :as pp]
            [clojure.test :refer :all]
            [hasch.core :as hash])
  (:import [clojure.lang IDeref]
           [java.io PushbackReader]
           [java.nio.file Files]
           [java.nio.file.attribute FileAttribute]))
           
(defrecord Persisted [file]
  IDeref
  (deref [_]
    (let [v (-> file io/reader PushbackReader. edn/read)]
      (io/delete-file file)
      v)))
(defmethod pp/simple-dispatch Persisted [{:keys [file]}] (pr file))

(defn persist
  [v]
  (let [f (io/file store-location (str (hash/uuid v)))]
    (pr spit f v)
    (->Persisted f)))

;; Not only does this test fail (likely due to confounding variables),
;; it also made me realize that because the file is never
;; dereferenced, it is never deleted -- it is effectively leaked.
(deftest mem-usage
  (let [mem-0 (.totalMemory (Runtime/getRuntime))
        large-object-holder (->> rand
                                 repeatedly
                                 (take (* 1024 1024 4))
                                 vec
                                 atom)
        mem-1 (.totalMemory (Runtime/getRuntime))
        _ (swap! large-object-holder file-backed-cache.core/persist)
        _ (System/gc)
        mem-2 (.totalMemory (Runtime/getRuntime))]
    (is (< mem-0 mem-1))
    (is (< mem-2 mem-1))))