How To Test Servant Applications¶
Even with a nicely structured API that passes Haskell’s strict type checker, it’s a good idea to write some tests for your application.
In this recipe we’ll work through some common testing strategies and provide examples of utlizing these testing strategies in order to test Servant applications.
Testing strategies¶
There are many testing strategies you may wish to employ when testing your Servant application, but included below are three common testing patterns:
- We’ll use
servant-client
to derive client functions and then send valid requests to our API, running in another thread. This is great for testing that our business logic is correctly implemented with only valid HTTP requests. - We’ll also use
hspec-wai
to make arbitrary HTTP requests, in order to test how our application may respond to invalid or otherwise unexpected requests. - Finally, we can also use
servant-quickcheck
for whole-API tests, in order to assert that our entire application conforms to best practices.
Useful Libraries¶
The following libraries will often come in handy when we decide to test our Servant applications:
Imports and Our Testing Module¶
This recipe starts with the following ingredients:
{-# LANGUAGE OverloadedStrings, TypeFamilies, DataKinds,
DeriveGeneric, TypeOperators #-}
import Prelude ()
import Prelude.Compat
import qualified Control.Concurrent as C
import Control.Concurrent.MVar
import Control.Exception (bracket)
import Control.Lens hiding (Context)
import Data.Aeson
import Data.Aeson.Lens
import qualified Data.HashMap.Strict as HM
import Data.Text (Text, unpack)
import GHC.Generics
import Network.HTTP.Client hiding (Proxy)
import Network.HTTP.Types
import Network.Wai
import qualified Network.Wai.Handler.Warp as Warp
import Servant
import Servant.Client
import Servant.Server
import Servant.QuickCheck
import Servant.QuickCheck.Internal (serverDoesntSatisfy)
import Test.Hspec
import Test.Hspec.Wai
import Test.Hspec.Wai.Matcher
We’re going to produce different Spec
s that represent different
aspects of our application, and we’ll ask hspec
to run all of our different
Spec
s. This is a common organizational method for testing modules:
spec :: Spec
spec = do
businessLogicSpec
thirdPartyResourcesSpec
servantQuickcheckSpec
Often, codebases will use hspec
’s
autodiscover pragma
to find all testing modules and Spec
s inside, but we’re going to
explicitly make a main
function to run our tests because we have only one
spec
defined above:
main :: IO ()
main = hspec spec
Testing Your Business Logic¶
Let’s say we have an API that looks something like this:
data User = User {
name :: Text
, user_id :: Integer
} deriving (Eq, Show, Generic)
instance FromJSON User
instance ToJSON User
type UserApi =
-- One endpoint: create a user
"user" :> Capture "userId" Integer :> Post '[JSON] User
A real server would likely use a database to store, retrieve, and validate
users, but we’re going to do something really simple merely to have something
to test. With that said, here’s a sample handler, server, and Application
for the endpoint described above:
userApp :: Application
userApp = serve (Proxy :: Proxy UserApi) userServer
userServer :: Server UserApi
userServer = createUser
createUser :: Integer -> Handler User
createUser userId = do
if userId > 5000
then pure $ User { name = "some user", user_id = userId }
else throwError $ err400 { errBody = "userId is too small" }
Strategy 1: Spin Up a Server, Create a Client, Make Some Requests¶
One of the benefits of Servant’s type-level DSL for describing APIs is that once you have provided a type-level description of your API, you can create clients, documentation, or other tools for it somewhat magically.
In this case, we’d like to test our server, so we can use servant-client
to create a client, after which we’ll run our server, and then make requests
of it and see how it responds.
Let’s write some tests:
withUserApp :: (Warp.Port -> IO ()) -> IO ()
withUserApp action =
-- testWithApplication makes sure the action is executed after the server has
-- started and is being properly shutdown.
Warp.testWithApplication (pure userApp) action
businessLogicSpec :: Spec
businessLogicSpec =
-- `around` will start our Server before the tests and turn it off after
around withUserApp $ do
-- create a test client function
let createUser = client (Proxy :: Proxy UserApi)
-- create a servant-client ClientEnv
baseUrl <- runIO $ parseBaseUrl "http://localhost"
manager <- runIO $ newManager defaultManagerSettings
let clientEnv port = mkClientEnv manager (baseUrl { baseUrlPort = port })
-- testing scenarios start here
describe "POST /user" $ do
it "should create a user with a high enough ID" $ \port -> do
result <- runClientM (createUser 50001) (clientEnv port)
result `shouldBe` (Right $ User { name = "some user", user_id = 50001})
it "will it fail with a too-small ID?" $ \port -> do
result <- runClientM (createUser 4999) (clientEnv port)
result `shouldBe` (Right $ User { name = "some user", user_id = 50001})
Running These Tests¶
Let’s run our tests and see what happens:
$ cabal new-test all
POST /user
should create a user with a high enough ID
should fail with a too-small ID FAILED [1]
Failures:
Testing.lhs:129:7:
1) POST /user should fail with a too-small ID
expected: Right (User {name = "some user", user_id = 50001})
but got: Left (FailureResponse (Response {responseStatusCode = Status {statusCode = 400, statusMessage = "Bad Request"}, responseHeaders = fromList [("Transfer-Encoding","chunked"),("Date","Fri, 12 Oct 2018 04:36:22 GMT"),("Server","Warp/3.2.25")], responseHttpVersion = HTTP/1.1, responseBody = "userId is too small"}))
To rerun use: --match "/POST /user/should fail with a too-small ID/"
Hmm. One passed and one failed! It looks like I was expecting a success
response in the second test, but I actually got a failure. We should fix that,
but first I’d like to introduce hspec-wai
, which will give us different
mechanisms for making requests of our application and validating the responses
we get. We’re also going to spin up a fake Elasticsearch server, so that our
server can think it’s talking to a real database.
Mocking 3rd Party Resources¶
Often our web applications will need to make their own web requests to other 3rd-party applications. These requests provide a lot of opportunity for failure and so we’d like to test that the right messages and failure values (in addition to success values) are returned from our application.
Define the 3rd-Party Resource¶
With Servant’s type-level API definitions, assuming you’ve already defined the API you want to mock, it’s relatively trivial to create a simple server for the purposes of running tests. For instance, consider an API server that needs to get data out of Elasticsearch. Let’s first define the Elasticsearch server and client using Servant API descriptions:
type SearchAPI =
-- We're using Aeson's Generic JSON `Value` to make things easier on
-- ourselves. We're also representing only one Elasticsearch endpoint:
-- get item by id
"myIndex" :> "myDocType" :> Capture "docId" Integer :> Get '[JSON] Value
-- Here's our Servant Client function
getDocument = client (Proxy :: Proxy SearchAPI)
-- We can use these helpers when we want to make requests
-- using our client function
clientEnv :: Text -> Text -> IO ClientEnv
clientEnv esHost esPort = do
baseUrl <- parseBaseUrl $ unpack $ esHost <> ":" <> esPort
manager <- newManager defaultManagerSettings
pure $ mkClientEnv manager baseUrl
runSearchClient :: Text -> Text -> ClientM a -> IO (Either ClientError a)
runSearchClient esHost esPort = (clientEnv esHost esPort >>=) . runClientM
Servant Server Example Using this 3rd-Party Resource¶
So we’ve got an Elasticsearch server and a client to talk to it. Let’s now build a simple app server that uses this client to retrieve documents. This is somewhat contrived, but hopefully it illustrates the typical three-tier application architecture.
One note: we’re also going to take advantage of lens-aeson
here, which may
look a bit foreign. The gist of it is that we’re going to traverse a JSON
Value
from Elasticsearch and try to extract some kind of document to
return.
Imagine, then, that this is our real server implementation:
type DocApi =
"docs" :> Capture "docId" Integer :> Get '[JSON] Value
docsApp :: Text -> Text -> Application
docsApp esHost esPort = serve (Proxy :: Proxy DocApi) $ docServer esHost esPort
docServer :: Text -> Text -> Server DocApi
docServer esHost esPort = getDocById esHost esPort
-- Our Handler tries to get a doc from Elasticsearch and then tries to parse
-- it. Unfortunately, there's a lot of opportunity for failure in these
-- actions
getDocById :: Text -> Text -> Integer -> Handler Value
getDocById esHost esPort docId = do
-- Our Servant Client function returns Either ClientError Value here:
docRes <- liftIO $ runSearchClient esHost esPort (getDocument docId)
case docRes of
Left err -> throwError $ err404 { errBody = "Failed looking up content" }
Right value -> do
-- we'll either fail to parse our document or we'll return it
case value ^? _Object . ix "_source" of
Nothing -> throwError $ err400 { errBody = "Failed parsing content" }
Just obj -> pure obj
Testing Our Backend¶
So the above represents our application and is close to a server we may actually deploy. How then shall we test this application?
Ideally, we’d like it to make requests of a real Elasticsearch server, but we certainly don’t want our tests to trigger requests to a live, production database. In addition, we don’t want to depend on our real Elasticsearch server having specific, consistent results for us to test against, because that would make our tests flaky (and flaky tests are sometimes described as worse than not having tests at all).
One solution to this is to create a trivial Elasticsearch server as part of our testing code. We can do this relatively easily because we already have an API definition for it above. With a real server, we can then let our own application make requests of it and we’ll simulate different scenarios in order to make sure our application responds the way we expect it to.
Let’s start with some helpers which will allow us to run a testing version of our Elasticsearch server in another thread:
-- | We'll run the Elasticsearch server so we can test behaviors
withElasticsearch :: IO () -> IO ()
withElasticsearch action =
bracket (liftIO $ C.forkIO $ Warp.run 9999 esTestApp)
C.killThread
(const action)
esTestApp :: Application
esTestApp = serve (Proxy :: Proxy SearchAPI) esTestServer
esTestServer :: Server SearchAPI
esTestServer = getESDocument
-- This is the *mock* handler we're going to use. We create it
-- here specifically to trigger different behavior in our tests.
getESDocument :: Integer -> Handler Value
getESDocument docId
-- arbitrary things we can use in our tests to simulate failure:
-- we want to trigger different code paths.
| docId > 1000 = throwError err500
| docId > 500 = pure . Object $ HM.fromList [("bad", String "data")]
| otherwise = pure $ Object $ HM.fromList [("_source", Object $ HM.fromList [("a", String "b")])]
Now, we should be ready to write some tests.
In this case, we’re going to use hspec-wai
, which will give us a simple way
to run our application, make requests, and make assertions against the
responses we receive.
Hopefully, this will simplify our testing code:
thirdPartyResourcesSpec :: Spec
thirdPartyResourcesSpec = around_ withElasticsearch $ do
-- we call `with` from `hspec-wai` and pass *real* `Application`
with (pure $ docsApp "localhost" "9999") $ do
describe "GET /docs" $ do
it "should be able to get a document" $
-- `get` is a function from hspec-wai`.
get "/docs/1" `shouldRespondWith` 200
it "should be able to handle connection failures" $
get "/docs/1001" `shouldRespondWith` 404
it "should be able to handle parsing failures" $
get "/docs/501" `shouldRespondWith` 400
it "should be able to handle odd HTTP requests" $
-- we can also make all kinds of arbitrary custom requests to see how
-- our server responds using the `request` function:
-- request :: Method -> ByteString -> [Header]
-- -> LB.ByteString -> WaiSession SResponse
request methodPost "/docs/501" [] "{" `shouldRespondWith` 405
it "we can also do more with the Response using hspec-wai's matchers" $
-- see also `MatchHeader` and JSON-matching tools as well...
get "/docs/1" `shouldRespondWith` 200 { matchBody = MatchBody bodyMatcher }
bodyMatcher :: [Network.HTTP.Types.Header] -> Body -> Maybe String
bodyMatcher _ body = case (decode body :: Maybe Value) of
-- success in this case means we return `Nothing`
Just val | val == (Object $ HM.fromList [("a", String "b")]) -> Nothing
_ -> Just "This is how we represent failure: this message will be printed"
Out of the box, hspec-wai
provides a lot of useful tools for us to run tests
against our application. What happens when we run these tests?
$ cabal new-test all
...
GET /docs
should be able to get a document
should be able to handle connection failures
should be able to handle parsing failures
should be able to handle odd HTTP requests
we can also do more with the Response using hspec-wai's matchers
Fortunately, they all passed! Let’s move to another strategy: whole-API testing.
Servant Quickcheck¶
servant-quickcheck
is a project that allows users to write tests for whole Servant APIs using
quickcheck-style property-checking mechanisms.
servant-quickcheck
is great for asserting API-wide rules, such as “no
endpoint throws a 500” or “all 301 status codes also come with a Location
header”. The project even comes with a number of predicates that reference
the RFCs they originate from.
In other words, it’s one way to assert that your APIs conform to specs and best practices.
Quickcheckable API¶
Let’s make an API and a server to demonstrate how to use servant-quickcheck
:
type API = ReqBody '[JSON] String :> Post '[JSON] String
:<|> Get '[JSON] Int
:<|> BasicAuth "some-realm" () :> Get '[JSON] ()
api :: Proxy API
api = Proxy
server :: IO (Server API)
server = do
mvar <- newMVar ""
return $ (\x -> liftIO $ swapMVar mvar x)
:<|> (liftIO $ readMVar mvar >>= return . length)
:<|> (const $ return ())
Using servant-quickcheck
¶
Let’s build some tests for our API using servant-quickcheck
.
Similar to the above examples, we’re going to create Spec
s, but in this
case, we’ll rely on a number of predicates available from servant-quickcheck
to see if our API server conforms to best practices:
-- Let's set some QuickCheck values
args :: Args
args = defaultArgs { maxSuccess = 500 }
-- Here's a Servant Context object we'll use
ctx :: Context '[BasicAuthCheck ()]
ctx = BasicAuthCheck (const . return $ NoSuchUser) :. EmptyContext
servantQuickcheckSpec :: Spec
servantQuickcheckSpec = describe "" $ do
it "API demonstrates best practices" $
-- `withServerServer` and `withServantServerAndContext` come from `servant-quickcheck`
withServantServerAndContext api ctx server $ \burl ->
-- `serverSatisfies` and the predicates also come from `servant-quickcheck`
serverSatisfies api burl args (unauthorizedContainsWWWAuthenticate
<%> not500
<%> onlyJsonObjects -- this one isn't true!
<%> mempty)
it "API doesn't have these things implemented yet" $
withServantServerAndContext api ctx server $ \burl -> do
serverDoesntSatisfy api burl args (getsHaveCacheControlHeader
<%> notAllowedContainsAllowHeader
<%> mempty)
Let’s see what happens when we run these tests:
API demonstrates best practices FAILED [2]
+++ OK, passed 500 tests.
API doesn't have these things implemented yet
src/Servant/QuickCheck/Internal/QuickCheck.hs:143:11:
2) Main[339:25] API demonstrates best practices
Failed:
Just Predicate failed
Predicate: onlyJsonObjects
Response:
Status code: 200
Headers: "Transfer-Encoding": "chunked"
"Date": "Fri, 12 Oct 2018 04:36:22 GMT"
"Server": "Warp/3.2.25"
"Content-Type": "application/json;charset=utf-8"
Body: ""
To rerun use: --match "/Main[339:25]/API demonstrates best practices/"
Randomized with seed 1046277487
Finished in 0.4306 seconds
Hmm. It looks like we thought our API only returned JSON objects, which is a
best practice, but in fact, we did have an endpoint that returned an empty
body, which you can see in the printed response above: Body: ""
. We should
consider revising our API to only return top-level JSON Objects in the future!
Other Cool Things¶
servant-quickcheck
also has a cool mechanism where you can compare two API
servers to demonstrate that they respond identically to requests. This may be
useful if you are planning to rewrite one API in another language or with
another web framework. You have to specify whether you’re looking for
jsonEquality
vs regular ByteString
equality, though.
Conclusion¶
There are lots of techniques for testing and we only covered a few here.
Useful libraries such as hspec-wai
have ways of running Wai Application
s
and sending requests to them, while Servant’s type-level DSL for defining APIs
allows us to more easily mock out servers and to derive clients, which will
only craft valid requests.
Lastly, if you want a broad overview of where your application fits in with
regard to best practices, consider using servant-quickcheck
.
This program is available as a cabal project here.