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Haskell Prefork

haskell-prefork is a library for preforking daemons.

How to install

Execute the cabal install command and the library and example programs will be placed into your .cabal directory.

> git clone .../haskell-prefork.git
> cd haskell-prefork/
> cabal install --only-dependencies --extra-include-dirs=/usr/local/include/zookeeper # if you use brew on Mac OSX
> cabal install --fsample

How to use

Import System.Prefork in your Main module.

import System.Prefork

Define data type used for server 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 identifying a single worker process and other fields are parameters for a worker process.

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)

In your 'main' function, call 'defaultMain' with 'update' and 'fork' functions. 'relaunchSettings' is a function that creates convenient 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 }) -> 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 ()

The 'update' function is used to modify the worker process configuration. You can configure the number of workers and change other worker parameters by using the 'updateWorkerSet' function shown below.

    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)

The 'fork' function 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 the example below, 'forkWorkerProcessWithArgs' is used to create child processes that outputs its id number.

    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') ]

Please see warp.hs in sample/ for further information.



prefork-sample-simple is a very simple example that showcases the basic features of this library.

> prefork-sample-simple
Please send SIGHUP to 12345 to relaunch a worker <- parent process
"Hello. I'm a worker."                           <- child process

Open another terminal and send SIGHUP to the child process.

> kill -HUP 12345

The message will be output again.

Please send SIGHUP to 12345 to relaunch a worker <- parent process
"Hello. I'm a worker."                           <- child process
"Hello. I'm a worker."                           <- another child process


prefork-sample-warp is a slightly more complex and practical example that showcases the relaunch feature.

> prefork-sample-warp -p 3000 -w 5

Open another terminal and execute the ps command to see the parent and child processes.

> ps ax
17307 s017  S+     0:00.02 .cabal-sandbox/bin/prefork-sample-warp -p 3000 -w 5
17308 s017  S+     0:00.07 .cabal-sandbox/bin/prefork-sample-warp id=0 +RTS -N5 -RTS
17309 s017  S+     0:00.04 .cabal-sandbox/bin/prefork-sample-warp id=1 +RTS -N5 -RTS
17310 s017  S+     0:00.05 .cabal-sandbox/bin/prefork-sample-warp id=2 +RTS -N5 -RTS
17311 s017  S+     0:00.10 .cabal-sandbox/bin/prefork-sample-warp id=3 +RTS -N5 -RTS
17312 s017  S+     0:00.04 .cabal-sandbox/bin/prefork-sample-warp id=4 +RTS -N5 -RTS

Kill one of them.

> kill 17310
> ps ax
17307 s017  S+     0:00.03 .cabal-sandbox/bin/prefork-sample-warp -p 3000 -w 5
17308 s017  S+     0:00.27 .cabal-sandbox/bin/prefork-sample-warp id=0 +RTS -N5 -RTS
17309 s017  S+     0:00.21 .cabal-sandbox/bin/prefork-sample-warp id=1 +RTS -N5 -RTS
17311 s017  S+     0:00.51 .cabal-sandbox/bin/prefork-sample-warp id=3 +RTS -N5 -RTS
17312 s017  S+     0:00.48 .cabal-sandbox/bin/prefork-sample-warp id=4 +RTS -N5 -RTS
18697 s017  S+     0:00.03 .cabal-sandbox/bin/prefork-sample-warp id=2 +RTS -N5 -RTS

The process with id number 2 would have been relaunched by the parent process.