Here (below) is a short program to shut down warp-3.3.15 based on a `TVar Bool` according to any policy you desire (catching a certain exception, receiving a request to a certain route, or receiving an OS signal). You need to route the TVar into places that you want to be able to initiate graceful shutdown. I believe that warp implements graceful shutdown by preventing new clients from connecting, but I haven't dug too far into its source code. In any case, the setInstallShutdownHandler https://hackage.haskell.org/package/warp-3.3.15/docs/Network-Wai-Handler-War... documentation indicates that you should also use setGracefulShutdownTimeout https://hackage.haskell.org/package/warp-3.3.15/docs/Network-Wai-Handler-War... to ensure the server eventually shuts down. I don't think that it's intended to throw exceptions in the setOnException handler or the setOnExceptionResponse handler. The former seems to be a hook for monitoring/logging and the latter a hook to give your users a less scary error page. --- --- --- {-# LANGUAGE OverloadedStrings #-} import qualified Control.Concurrent.Async as Async import qualified Control.Concurrent.STM as STM import qualified Network.HTTP.Types as HTTP import qualified Network.Wai as Wai import qualified Network.Wai.Handler.Warp as Warp app :: STM.TVar Bool -> Wai.Application app shutdownSignal req respond = do print ("Request from", Wai.remoteHost req) case Wai.rawPathInfo req of "/shutdown" -> do STM.atomically $ STM.writeTVar shutdownSignal True respond $ Wai.responseLBS HTTP.ok200 [] "shutting down" _ -> do respond $ Wai.responseLBS HTTP.ok200 [] "hello" -- | Spawn a thread to wait for the shutdown signal and initiate shutdown. installShutdownHandler :: STM.TVar Bool -> (IO ()) -> IO () installShutdownHandler shutdownSignal closeSocket = do _ <- Async.async $ do STM.atomically $ STM.check =<< STM.readTVar shutdownSignal closeSocket return () main :: IO () main = do shutdownSignal <- STM.newTVarIO False let settings = Warp.setPort 8080 . Warp.setInstallShutdownHandler (installShutdownHandler shutdownSignal) . Warp.setGracefulShutdownTimeout (Just 30) -- seconds $ Warp.defaultSettings print "warp is starting" Warp.runSettings settings $ app shutdownSignal print "warp is done" On Tue, Apr 26, 2022 at 1:00 PM Olaf Klinke wrote:

Dear Cafe,

this topic has been discussed [1,2] a decade ago. What is the current state of affairs in this matter? It still seems to be addressed neither in yesod nor warp [*]. We have two use cases: (1) A shutdown route that allows admins to cleanly shut down the web application. (2) We want certain exceptions to terminate the application and have the surrounding monitoring (systemd, docker, ...) re-start the application. For (2) we are safe, I think, because the worker threads are forked from the main thread [3] calling warp, so propagating the exceptions up to the top bypasses warp and yesod's very comprehensive exception handlers. But any exception thrown while answering a request (such as (1)) is caught by default.

Thanks Olaf

[1] https://groups.google.com/g/yesodweb/c/VoenrabRUBQ [2] https://stackoverflow.com/questions/7881327/how-do-i-implement-a-shutdown-co... [3] https://mail.haskell.org/pipermail/haskell-cafe/2022-March/135132.html [*] Warp offers setOnException and setOnExceptionResponse. But does re- throwing certain exceptions from there terminate warp?

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