clojure sendoff println 函数

Question

我正在尝试在事务中send-off 函数println。

(ns com.lapots.functional.clojure.transact
    (:gen-class))

(defn transfer [from to amount]
    (alter
        (.balance from) - amount)
     (alter
        (.balance from) + amount))

(defrecord Account [balance])

(defn -main [& args]
    (def account1 (Account. (ref 100)))
    (def account2 (Account. (ref 100)))
    (def trx-agent (agent 0))

    (future
        (dosync
            (send-off trx-agent println "T2 transfer")
            (Thread/sleep 5000)
            (transfer account1 account2 10)))

    (dosync
        (println "T1 transfer")
        (transfer account1 account2 10))

    (shutdown-agents)
)

如果我喜欢这个

(println "T2 transfer")
(Thread/sleep 5000)

它在事务重试时显示两次消息。所以我决定使用agents让副作用操作println只运行一次。

但是当我这样做时

(send-off trx-agent println "T2 transfer")

它根本不打印T2 transfer 消息。有什么问题？

Answer 1

您使用shutdown-agents 太早了。

(defn transfer [from to amount]
  (println :transfer-enter amount)
  (alter
    (.balance from) - amount)
  (alter
    (.balance from) + amount)
  (println :transfer-exit  amount)
)

(defrecord Account [balance])

(def account1 (Account. (ref 100)))
(def account2 (Account. (ref 100)))
(def trx-agent (agent 0))

(defn -main [& args]
  (println :main-enter )

  (future
    (dosync
      (println :t2-enter)
      (send-off trx-agent println "agent: T2 transfer")
      (Thread/sleep 500)
      (transfer account1 account2 20)
      (println :t2-exit)
    ))

  (dosync
    (println :t1-enter)
    (send-off trx-agent println "agent: T1 transfer")
    (transfer account1 account2 10)
    (println :t1-exit))

  (Thread/sleep 2000)
  (shutdown-agents)
  (println :main-exit )
)

结果：

:main-enter
:t2-enter
:t1-enter
:transfer-enter 10
:transfer-exit 10
:t1-exit
0 agent: T1 transfer
:transfer-enter 20
:t2-enter
:transfer-enter 20
:transfer-exit 20
:t2-exit
nil agent: T2 transfer
:main-exit

所以 T2 现在只等待 500 毫秒，而 T1 立即运行。我们在调用 shutdown-agents 之前等待 2000 毫秒，这将杀死所有代理线程，阻止代理运行。

Answer 2

我发现您的代码片段至少有三处错误：

1. 你的transfer函数

   (defn transfer [from to amount]
     (alter
       (.balance from) - amount)   ;; <======== from - amount
     (alter
       (.balance from) + amount))  ;; <======== from + amount
   

   看起来你真的想说alter from然后alter to

2. 来自send-off 文档：

   向代理分派潜在的阻塞操作。返回代理 立即地。随后，在一个单独的线程中， agent 将设置为以下值：（应用 action-fn state-of-agent 参数）

   所以我认为你不需要那个未来。还有一行

   (send-off trx-agent println "T2 transfer")
   

   表示您正在将代理的状态设置为

   的结果

   (println state-of-agent "T2 transfer")`
   

   这是nil，因为println 总是返回nil。我不认为 这就是你想要的。

3. transfer 函数中存在竞争条件。由于您没有以原子方式修改 from 和 to，因此可能会出现竞争。