A while back, I made a post about deploying yesod apps to heroku. The method used back then is no longer required (thank God!) and deploying to heroku is super simple these days. So simple, in fact, that I won’t reiterate those instructions here, this post is about something a bit more specific.

Chances are, your app is using a database. And you probably don’t want to hard-code those database credentials in your (probably shared) source code. What you’d rather do is parse them out of the DATABASE_URL environment variable provided by heroku.

Well, here is how you do that:


Eventually, I might wrap this up in a cabal package you can install, but for now just create a helper like this:


module Helpers.Heroku (herokuConf) where

import Prelude
import Data.ByteString (ByteString)
import Data.Text (Text)
import Data.Text.Encoding (encodeUtf8)
import Database.Persist.Postgresql (PostgresConf(..))
import Web.Heroku (dbConnParams)

import qualified Data.Text as T

herokuConf :: IO PostgresConf
herokuConf = do
    params <- dbConnParams

    return PostgresConf
        { pgConnStr  = formatParams params
        , pgPoolSize = 10 -- Adjust this as you see fit!

        formatParams :: [(Text, Text)] -> ByteString
        formatParams = encodeUtf8 . T.unwords . map toKeyValue

toKeyValue :: (Text, Text) -> Text
toKeyValue (k, v) = k `T.append` "=" `T.append` v

This relies on the heroku package, so be sure you add that to the build-depends in your cabal file.


Now, modify your application loading like so:


import Helpers.Heroku

makeFoundation :: AppConfig DefaultEnv Extra -> IO App
makeFoundation conf = do
    -- ...

    dbconf <- if development
                -- default behavior when in development
                then withYamlEnvironment "config/postgresql.yml" (appEnv conf)
                    Database.Persist.loadConfig >>=

                -- but parse DATABASE_URL in non-development
                else herokuConf

    -- ...

    return foundation

That’s it. Commit, push, enjoy!

07 Jun 2013, tagged with haskell