Clojure：定期輪詢數據庫 - 帶有超時通道的 core.async VS 帶有睡眠的普通遞歸線程？

Question

我有一個基於 Ring 的服務器，它有一個用於存儲應用程序 state 的原子，它每 10 秒定期從數據庫中獲取一次，用於頻繁更改信息，每 60 秒一次用於 rest。

(defn set-world-update-interval
  [f time-in-ms]
  (let [stop (async/chan)]
    (async/go-loop []
      (async/alt!
        (async/timeout time-in-ms) (do (async/<! (async/thread (f)))
                                       (recur))
        stop :stop))
    stop))

(mount/defstate world-listener
  :start (set-world-update-interval #(do (println "Checking data in db") (reset! world-atom (fetch-world-data)) ) 10000)
  :stop (async/close! world-listener))

它工作得很好。 RAM 使用率相當穩定。 但我想知道這是否是對 core.async 的不當使用？

也許它應該是這樣的常規線程？

(doto (Thread. (fn []
                 (loop []
                   (Thread/sleep 1000)
                   (println "Checking data in db")
                   (reset! world-atom (fetch-world-data))
                   (recur))))
  (.setUncaughtExceptionHandler
    (reify Thread$UncaughtExceptionHandler
      (uncaughtException [this thread exception]
        (println "Cleaning up!"))))
  (.start))

Answer 1

雖然此模式的core.async實現沒有任何問題，但我建議為此使用java.util.concurrent.ScheduledExecutorService 。 它使您可以精確控制線程池和調度。

嘗試這樣的事情：

(ns your-app.world
  (:require [clojure.tools.logging :as log]
            [mount.core :as mount])
  (:import
    (java.util.concurrent Executors ScheduledExecutorService ThreadFactory TimeUnit)))

(defn ^ThreadFactory create-thread-factory
  [thread-name-prefix]
  (let [thread-number (atom 0)]
    (reify ThreadFactory
      (newThread [_ runnable]
        (Thread. runnable (str thread-name-prefix "-" (swap! thread-number inc)))))))

(defn ^ScheduledExecutorService create-single-thread-scheduled-executor
  [thread-name-prefix]
  (let [thread-factory (create-thread-factory thread-name-prefix)]
    (Executors/newSingleThreadScheduledExecutor thread-factory)))

(defn schedule
  [executor runnable interval unit]
  (.scheduleWithFixedDelay executor runnable 0 interval unit))

(defn shutdown-executor
  "Industrial-strength executor shutdown, modify/simplify according to need."
  [^ScheduledExecutorService executor]
  (if (.isShutdown executor)
    (log/info "Executor already shut down")
    (do
      (log/info "Shutting down executor")
      (.shutdown executor)                                  ;; Disable new tasks from being scheduled
      (try
        ;; Wait a while for currently running tasks to finish
        (if-not (.awaitTermination executor 10 TimeUnit/SECONDS)
          (do
            (.shutdownNow executor)                         ;; Cancel currently running tasks
            (log/info "Still waiting to shut down executor. Sending interrupt to tasks.")
            ;; Wait a while for tasks to respond to being cancelled
            (when-not (.awaitTermination executor 10 TimeUnit/SECONDS)
              (throw (ex-info "Executor could not be shut down" {}))))
          (log/info "Executor shutdown completed"))
        (catch InterruptedException _
          (log/info "Interrupted while shutting down. Sending interrupt to tasks.")
          ;; Re-cancel if current thread also interrupted
          (.shutdownNow executor)
          ;; Preserve interrupt status
          (.interrupt (Thread/currentThread)))))))

(defn world-updating-fn
  []
  (log/info "Updating world atom")
  ;; Do your thing here
  )

(mount/defstate world-listener
  :start (doto (create-single-thread-scheduled-executor "world-listener")
           (schedule world-updating-fn 10 TimeUnit/MINUTES))
  :stop (shutdown-executor world-listener))

Answer 2

對我來說，使用go-loop創建一個在超時時停止的 goroutine 似乎有點愚蠢，當你真正想做的所有工作都是 IO 密集型的，因此（正確地）在一個單獨的線程中完成。 這樣做的結果是您在每個周期都經過線程。 這些線程由 core.async 池化，因此您無需執行從無到有創建新線程的昂貴工作，但是將它們從池中取出並與 core.async 交互以等待超時仍然存在一些開銷。 我只是通過將 core.async 排除在此操作之外來保持簡單。