haskell - 保持后台进程在 heroku 上运行

Question

我正在尝试在 heroku 上运行 rabbitmq 后台进程以从队列中挑选任务并处理它们。我正在使用 AMQP haskell 库，他们提供以下 example (部分省略或简洁。

main = do
    --setup connection omitted
    --connect to queue, wait for messages
    consumeMsgs chan "myQueue" Ack myCallback
    --halts process so messages are taken off the queue until a key is presseed
    getLine -- wait for keypress
    closeConnection conn -- close connection after key
    putStrLn "connection closed"

这在本地工作正常，因为 getLine保持进程运行，直到你按下一个键。但是，当我将其部署到 heroku 时，进程退出并显示

2016-04-19T08:37:23.373087+00:00 app[worker.1]: worker: <stdin>: hGetChar: end of file

我从 the accepted answer to this question 中得出结论这是因为为了通过 ssh 部署后台进程，您需要重定向 /dev/null/至 stdin它发送一个 EOF向进程发出信号。

在我们的例子中 getLine函数因为这个信号而退出，整个过程停止，防止我们的工作人员熬夜。

我如何在部署时让这个 worker 保持运行？

编辑:最终解决方案使用@carstons 评论我最终得到了以下有效的实现:

main :: IO ()
main = do
    mvar <- newEmptyMVar
    conn <- setupConnection
    queueName <- pack <$> getEnv "QUEUE_NAME"
    chan <- openChannel conn
    consumeMsgs chan queueName Ack processMessage
    installHandler sigINT (Catch (cleanupConnection conn mvar)) Nothing
    putStrLn "Running forever, press ctrl c to exit"
    -- this blocks until sigint is recieved and the handler for SIGINT
    -- "fills" the mvar. once that is filled the process exits
    run <- takeMVar mvar
    case run of
      _ -> return ()

mixpanelConfig :: IO Config
mixpanelConfig = liftM2 Config (ApiToken . pack <$> getEnv "MIXPANEL_API_TOKEN") (newManager tlsManagerSettings)

cleanupConnection :: Connection -> MVar () -> IO ()
cleanupConnection conn mvar = do
  closeConnection conn
  putStrLn "SIGINT received.. closing rabbitmq connection"
  putMVar mvar ()

processMessage :: (Message, Envelope) -> IO ()

Answer 1

正如我在评论中指出的那样，如果你只是想让它永远运行，你可以使用 forever与 - 例如 - threadDelay :

import Control.Concurrent (threadDelay)
import Control.Monad (forever)

main = do
    --setup connection omitted
    --connect to queue, wait for messages
    consumeMsgs chan "myQueue" Ack myCallback
    --halts process so messages are taken off the queue forever
    forever $ threadDelay 10000
    -- so this will never happen and you could remove it
    closeConnection conn -- close connection after key
    putStrLn "connection closed"

请注意，这当然不会真正关闭连接或退出应用程序 - 您必须终止该进程。

---

替代方案会涉及更多，因为您需要一些消息/方式来向您的程序发送终止信号。

一个简单的方法是使用 MVar当您的队列收到某个停止消息时，您可以在myCallback 中设置它:

import Control.Concurrent.MVar

main = do
    -- MVar to receve the quit-signal
    quitSignal <- newEmptyMVar
    --setup connection omitted
    --connect to queue, wait for messages - the callback should
    --set the quitSignal with putMVar quitSignal ()
    consumeMsgs chan "myQueue" Ack (myCallback quitSignal)
    --halts process so messages are taken off the queue till quitSignal
    takeMVar quitSignal
    -- so this will happen after quitSignal was set
    closeConnection conn -- close connection after key
    putStrLn "connection closed"