This is a library for servers based on worker process model (preforking).

Import System.Prefork in your Main module.

import ...
import System.Posix
import System.Prefork
import System.Console.CmdArgs

Define data type used for server configuration.

-- Application specific configuration
data Config = Config {
    cWarpSettings :: Warp.Settings
  }

Define workers as a data type that belongs to WorkerContext class. In this case, the field wId is a ID number for idenfitying single worker process and other fields are parameters for a worker process.

-- Worker context passed by the parent
data Worker = Worker {
    wId       :: Int
  , wPort     :: Int
  , wSocketFd :: CInt
  , wHost     :: String
  , wCap      :: Int
  } deriving (Show, Read)

instance WorkerContext Worker where
  rtsOptions w = ["-N" ++ show (wCap w)]

Define Eq and Ord instances for Worker. These are required for using relaunchSettings.

instance Eq Worker where
  (==) a b = wId a == wId b

instance Ord Worker where
  compare a b = compare (wId a) (wId b)

Call defaultMain with update and fork functions in your main function. relaunchSettings is a function that creates comvenient settings for a typical prefork server.

main :: IO ()
main = do
  option <- cmdArgs cmdLineOptions
  resource <- emptyPreforkResource
  mSoc <- newTVarIO Nothing
  let s = Server mSoc (port option) (workers option)
  defaultMain (relaunchSettings resource (update s) (fork s)) $ (Worker { wId = i, wSocketFd = fd, wHost = _host }) -> do
    -- worker action
    soc <- mkSocket fd AF_INET Stream defaultProtocol Listening
    mConfig <- updateConfig s
    case mConfig of
      Just config -> do
        a <- asyncOn i $ Warp.runSettingsSocket (cWarpSettings config) soc $ serverApp
        wait a
      Nothing -> return ()
  where
    ...

update function is used for modifying the worker process configuration. If you want to increase or decrease the number of workers, change worker parameters, and etc, you can use updateWorkerSet function here.

    update :: Server -> PreforkResource Worker -> IO (Maybe Config)
    update s resource = do
      mConfig <- updateConfig s
      updateWorkerSet resource $ flip map [0..(sWorkers s - 1)] $ i ->
        Worker { wId = i, wPort = (sPort s), wSocketFd = -1, wHost = "localhost", wCap = sWorkers s }
      return (mConfig)

fork function simply creates a worker process with the given parameters. You should call forkWorkerProcess or forkWorkerProcessWithArgs in this function to invoke a child process as a worker. In this case, the arguments of forkWorkerProcessWithArgs are just for displaying id number and not used.

    fork :: Server -> Worker -> IO (ProcessID)
    fork Server { sServerSoc = socVar } w = do
      msoc <- readTVarIO socVar
      soc <- case msoc of
        Just soc -> return (soc)
        Nothing -> do
          hentry <- getHostByName (wHost w)
          soc <- listenOnAddr (SockAddrInet (fromIntegral (wPort w)) (head $ hostAddresses hentry))
          atomically $ writeTVar socVar (Just soc)
          return (soc)
      let w' = w { wSocketFd = fdSocket soc }
      forkWorkerProcessWithArgs (w') ["id=" ++ show (wId w') ]

Import System.Prefork in your Main module.

import System.Prefork
import System.Posix
import System.Exit (exitSuccess)

Define data type used for server configuration.

data ServerConfig = ServerConfig

Define workers as a data type.

data Worker = Worker1 String deriving (Show, Read)

instance WorkerContext Worker

Call System.Prefork.defaultMain function with settings in your main function.

main :: IO ()
main = defaultMain defaultSettings {
    psUpdateConfig = updateConfig
  , psUpdateServer = updateServer
  , psOnStart      = _ -> do
      pid <- getProcessID
      putStrLn $ "Please send SIGHUP to " ++ show pid ++ " to relaunch a worker"
  } $ so -> case so of
  Worker1 msg -> print msg >> exitSuccess

updateConfig :: IO (Maybe ServerConfig)
updateConfig = do
  return (Just ServerConfig)

updateServer :: ServerConfig -> IO ([ProcessID])
updateServer ServerConfig = do
  pid <- forkWorkerProcess (Worker1 "Hello. I'm a worker.")
  return ([pid])