Clojure：定期轮询数据库 - 带有超时通道的 core.async VS 带有睡眠的普通递归线程？

Question

我有一个基于 Ring 的服务器，它有一个用于存储应用程序 state 的原子，它每 10 秒定期从数据库中获取一次，用于频繁更改信息，每 60 秒一次用于 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 使用率相当稳定。 但我想知道这是否是对 core.async 的不当使用？

也许它应该是这样的常规线程？

(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

虽然此模式的core.async实现没有任何问题，但我建议为此使用java.util.concurrent.ScheduledExecutorService 。 它使您可以精确控制线程池和调度。

尝试这样的事情：

(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

对我来说，使用go-loop创建一个在超时时停止的 goroutine 似乎有点愚蠢，当你真正想做的所有工作都是 IO 密集型的，因此（正确地）在一个单独的线程中完成。 这样做的结果是您在每个周期都经过线程。 这些线程由 core.async 池化，因此您无需执行从无到有创建新线程的昂贵工作，但是将它们从池中取出并与 core.async 交互以等待超时仍然存在一些开销。 我只是通过将 core.async 排除在此操作之外来保持简单。