atom cache 缓存

00 [clojure atom实现缓存].md

clojure atom实现缓存

1_atom_cache.clj

;;
;; 定制缓存，可以通过meta清理缓存
;;
(defn manipulable-memoize [function]
  (let [cache (atom {})]
    (with-meta (fn [& args]
                 (or (second (find @cache args))
                     (let [ret (apply function args)]
                       (swap! cache assoc args ret) ret)))
               {:cache cache})))


(def slowly (fn [x] (Thread/sleep 3000) x))
;; 调用两次 slowly方法
(time [(slowly 9) (slowly 9)])


(def sometimes-slowly (manipulable-memoize slowly))
;;调用两次 sometimes-slowly 方法， 第二次会使用到缓存
(time [(sometimes-slowly 108)  (sometimes-slowly 108) ] )


;; 操纵缓存的meta data
(meta sometimes-slowly)
;; => {:cache #object[clojure.lang.Atom 0x6a45de93 {:status :ready, :val {(108) 108, (100) 100}}]}

(let [cache (:cache (meta sometimes-slowly))]
  (swap! cache dissoc '(108)))

2_memoize.clj

;;
;; clojure.core
;;
(comment
  (defn memoize
    "Returns a memoized version of a referentially transparent function. The
    memoized version of the function keeps a cache of the mapping from arguments
    to results and, when calls with the same arguments are repeated often, has
    higher performance at the expense of higher memory use."
    {:added "1.0"
     :static true}
    [f]
    (let [mem (atom {})]
      (fn [& args]
        (if-let [e (find @mem args)]
          (val e)
          (let [ret (apply f args)]
            (swap! mem assoc args ret)
            ret))))))

3_rotary.client.clj

(ns atom_cache.rotary)

;;
;; from: rotary.client
;; 官方memoize用法之一举例
;; 原来的方法加星号， 需要缓存的方法通过def定义， 借助memoize缓存
;;
(defn- db-client*
  "Get a AmazonDynamoDBClient instance for the supplied credentials."
  [cred]
  (let [aws-creds (BasicAWSCredentials. (:access-key cred) (:secret-key cred))
        client (AmazonDynamoDBClient. aws-creds)]
    (when-let [endpoint (:endpoint cred)]
      (.setEndpoint client endpoint))
    client))

(def db-client
  (memoize db-client*))