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
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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