Fix Load Balancer Error in Haskell

Haskell Web Apps Behind Load Balancers

Haskell web frameworks like Servant and Yesod run on Warp under the hood. When placed behind a load balancer, you need to handle proxy headers and connection management properly.

Warp Proxy Configuration

Warp needs to trust forwarded headers from your load balancer:

import Network.Wai.Handler.Warp
import Network.Wai.Middleware.RequestLogger
import Network.Wai.Middleware.ForceSSL

main :: IO ()
main = do
  let settings = setPort 8080
                $ setHost "0.0.0.0"
                $ setTimeout 120
                $ defaultSettings
  runSettings settings app

For SSL detection behind a proxy, check X-Forwarded-Proto:

import Network.Wai

isSecure :: Request -> Bool
isSecure req =
  case lookup "X-Forwarded-Proto" (requestHeaders req) of
    Just "https" -> True
    _            -> False

Health Check Endpoint

With Servant:

type HealthAPI = "healthz" :> Get '[PlainText] Text

healthHandler :: Handler Text
healthHandler = return "ok"

Connection Draining

Haskell's lightweight threads mean Warp can handle thousands of connections, but graceful shutdown requires explicit handling:

import Control.Concurrent (threadDelay)
import System.Posix.Signals

main = do
  installHandler sigTERM (Catch $ gracefulShutdown) Nothing
  runSettings settings app

gracefulShutdown :: IO ()
gracefulShutdown = do
  putStrLn "Shutting down gracefully..."
  threadDelay 5000000 -- 5 seconds for in-flight requests

Bugsly can receive error reports from Haskell backends via its HTTP API, capturing exceptions that might otherwise vanish in container restarts behind the balancer.