Shared data in Yesod

3
votes

I want to share some data across requests in Yesod. In my case that data is a MVar (Data.Map Text ReadWriteLock), but I don't think the format of the data being shared matters too much here.

In Foundation.hs, there is a comment that says I can add fields to App, and every handler will have access to the data there. This seems like an approach I could use to share data between different handlers. I have been looking through the Yesod book, but I could not find any examples of getting data from App.

  • How would I access the newly created field from within a handler?

I think this might be a good use case for STM. I could share a TVar (Data.Map Text ReadWriteLock). But creating a TVar wraps the TVar in the STM monad. I might be mistaken, but to me that seems like the entire Yesod "main loop" would need to be run in the STM monad.

  • Is using STM a viable option here? Could anyone elaborate on how this might be achieved?
2
haskellyesod
Do you need this data persisted? - The Internet
I do not need it persisted. The reason I'm not persisting the data is so I can use RWLocks that the threads will block on aquiring. This approach seemed easier to me than polling the DB to try and get locks. It would be nice if it was persisted, but that seems like a significant amount more work to me. I could be wrong about that though. - Kevin
getYesod - Samvel Truzyan

2 Answers

2
votes

This tutorial for building a file server with Yesod shows quite nicely how you can use STM to access shared data. The relevant part starts from part 2.

1
votes

To elaborate on pxqr's comment, you want to do something like this.

In your Foundation.hs file (assuming you started your project with yesod init).

data App = App
    { ... other fields
    , shared :: TVar Int -- New shared TVar field
    }

Then in your Application.hs file where you create the App instance.

makeFoundation conf = do
    .... snip .....
    tv <- newTVarIO 0  -- Initialize your TVar

    let logger = Yesod.Core.Types.Logger loggerSet' getter
        foundation = App conf s manager logger tv  -- Add TVar here

    return foundation

Then in your Handler use the TVar

getHomeR :: Handler Html
getHomeR = do
    app <- getYesod  -- Get the instance of App
    x <- liftIO $ atomically $ do -- Read and update the TVar value.
      val <- readTVar (shared app)
      writeTVar (shared app) (val + 1)
      return val
    (formWidget, formEnctype) <- generateFormPost sampleForm
    let submission = Nothing :: Maybe (FileInfo, Text)
        handlerName = "getHomeR" :: Text
    defaultLayout $ do
        aDomId <- newIdent
         -- Use the TVar value (increments on each page refresh).
        setTitle $  fromString (show x)
        $(widgetFile "homepage")