116 lines
5.3 KiB
Text
116 lines
5.3 KiB
Text
# Error logging with Sentry
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In this recipe we will use [Sentry](https://sentry.io) to collect the runtime exceptions generated by our application. We will use the [raven-haskell](https://hackage.haskell.org/package/raven-haskell) package, which is a client for a Sentry event server. Mind that this package is not present on [Stackage](https://www.stackage.org/), so if we are using [Stack](https://docs.haskellstack.org) we’ll need to add it to our `extra-deps` section in the `stack.yaml` file.
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To exemplify this we will need the following imports
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```haskell
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{-# LANGUAGE DataKinds #-}
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{-# LANGUAGE TypeOperators #-}
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import Control.Exception (Exception,
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SomeException, throw)
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import Data.ByteString.Char8 (unpack)
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import Network.Wai (Request, rawPathInfo,
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requestHeaderHost)
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import Network.Wai.Handler.Warp (defaultOnException,
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defaultSettings,
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runSettings,
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setOnException,
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setPort)
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import Servant
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import System.Log.Raven (initRaven, register,
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silentFallback)
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import System.Log.Raven.Transport.HttpConduit (sendRecord)
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import System.Log.Raven.Types (SentryLevel (Error),
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SentryRecord (..))
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```
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Just for the sake of the example we will use the following API which will throw an exception
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```haskell
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type API = "break" :> Get '[JSON] ()
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data MyException = MyException deriving (Show)
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instance Exception MyException
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server = breakHandler
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where breakHandler :: Handler ()
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breakHandler = do
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throw MyException
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return ()
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```
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First thing we need to do if we want to intercept and log this exception, we need to look in the section of our code where we run the `warp` application, and instead of using the simple `run` function from `warp`, we use the `runSettings` functions which allows to customise the handling of requests
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```haskell
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main :: IO ()
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main =
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let
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settings =
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setPort 8080 $
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setOnException sentryOnException $
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defaultSettings
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in
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runSettings settings $ serve (Proxy :: Proxy API) server
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```
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The definition of the `sentryOnException` function could look as follows
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```haskell
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sentryOnException :: Maybe Request -> SomeException -> IO ()
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sentryOnException mRequest exception = do
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sentryService <- initRaven
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"https://username:password@senty.host/id"
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id
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sendRecord
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silentFallback
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register
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sentryService
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"myLogger"
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Error
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(formatMessage mRequest exception)
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(recordUpdate mRequest exception)
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defaultOnException mRequest exception
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```
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It does three things. First it initializes the service which will communicate with Sentry. The parameters it receives are:
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- the Sentry `DSN`, which is obtained when creating a new project on Sentry
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- a default way to update sentry fields, where we use the identity function
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- an event trasport, which generally would be `sendRecord`, an HTTPS capable trasport which uses http-conduit
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- a fallback handler, which we choose to be `silentFallback` since later we are logging to the console anyway.
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In the second step it actually sends our message to Sentry with the `register` function. Its arguments are:
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- the configured Sentry service which we just created
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- the name of the logger
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- the error level (see [SentryLevel](https://hackage.haskell.org/package/raven-haskell-0.1.2.0/docs/System-Log-Raven-Types.html#t:SentryLevel) for the possible options)
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- the message we want to send
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- an update function to handle the specific `SentryRecord`
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Eventually it just delegates the error handling to the default warp mechanism.
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The function `formatMessage` simply uses the request and the exception to return a string with the error message.
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```haskell
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formatMessage :: Maybe Request -> SomeException -> String
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formatMessage Nothing exception = "Exception before request could be parsed: " ++ show exception
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formatMessage (Just request) exception = "Exception " ++ show exception ++ " while handling request " ++ show request
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```
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The only piece left now is the `recordUpdate` function which allows to decorate with other [attributes](https://docs.sentry.io/clientdev/attributes/) the default `SentryRecord`.
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```haskell
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recordUpdate :: Maybe Request -> SomeException -> SentryRecord -> SentryRecord
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recordUpdate Nothing exception record = record
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recordUpdate (Just request) exception record = record
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{ srCulprit = Just $ unpack $ rawPathInfo request
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, srServerName = fmap unpack $ requestHeaderHost request
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}
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```
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In this examples we set the raw path as the culprit and we use the `Host` header to populate the server name field.
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You can try to run this code using the `cookbook-sentry` executable. You should obtain a `MyException` error in the console and, if you provided a valid Sentry DSN, you should also find your error in the Sentry interface.
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