[Haskell-cafe] Application Level server state in Haskell Web server

Donald Bruce Stewart dons at cse.unsw.edu.au
Sat Feb 17 21:26:45 EST 2007

> Hello,
> I'm trying to write a simple module for the haskell web server (hws- 
> cgi).
> And I would like to write a simple module that maintains some kind of  
> state for a session.
> But I'm I write I cannot do this in pure Haskell ? Without adopting  
> the sources of the Haskell web server ?
> I'll examplify to make it more concrete :
> The requestHandler function a module has to implement has this  
> signature.
> requestHandler :: ServerState -> ServerRequest  -> IO (Maybe Response)
> Let 's assume I have this implementation
> requestHandler st _ = return $ Just  $ mkRequest
>     where mkRequest =
>               okResponse (serverConfig st) mkBody hs True
>           mkBody = HereItIs " This is a test"
>           hs = mkHeaders [contentTypeHeader "text/html"]
> And I would like the response to include, for example,  a number  
> indicating the number of calls that has been handled by the module.
> I would concider using an Mvar but I can't "remember"  the mvar  
> between requests.
> Am I right to assume  that the interface of the requestHandler method  
> has to be adapted ?  Or that serverstate has to be adopted so that it  
> can act as a datastore ?

I don't think so.

You could, for example store the count on disk, and read it back in. Or
you could simulate a disk store by using a mutable variable, hidden in
your module:

    module M (requestHandler) where

    import Control.Concurrent.MVar
    import System.IO.Unsafe

    -- A threadsafe mutable variable, internal to this module. Rather
    -- than use, say, a disk file as storage, we can keep the count here.
    countRef :: MVar Int
    countRef = unsafePerformIO $ newMVar 0
    {-# NOINLINE countRef #-}

    -- And a quick example:

    type Response = Int

    requestHandler :: IO (Maybe Response)
    requestHandler = do
        n <- modifyMVar countRef $ \c -> return (c+1, c)
        print $ "received " ++ show n ++ " requests."
        return $ case n of
            0 -> Nothing
            _ -> Just n

*Main> requestHandler 
"received 0 requests."

*Main> requestHandler
"received 1 requests."
Just 1

*Main> requestHandler
"received 2 requests."
Just 2

*Main> requestHandler
"received 3 requests."
Just 3

This seems simpler than writing the count to disk.  And as long as you
stay in IO, perfectly safe.

In the longer term, you might want to look at state-parameterised
plugins for the HWS. We do this in lambdabot, associating a private
state type with each plugin, which is managed by the main server. The
plugins can then get or set internal state, without resorting to local
mutable variables.

-- Don

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