# 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: - [hspec](https://hspec.github.io/) - [hspec-wai](http://hackage.haskell.org/package/hspec-wai) - [QuickCheck](http://hackage.haskell.org/package/QuickCheck) - [servant-quickcheck](https://hackage.haskell.org/package/servant-quickcheck) ## Imports and Our Testing Module This recipe starts with the following ingredients: ```haskell {-# 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: ```haskell spec :: Spec spec = do businessLogicSpec thirdPartyResourcesSpec servantQuickcheckSpec ``` Often, codebases will use `hspec`'s [autodiscover pragma](http://hspec.github.io/hspec-discover.html) 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: ```haskell main :: IO () main = hspec spec ``` ## Testing Your Business Logic Let's say we have an API that looks something like this: ```haskell 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: ```haskell 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: ```haskell withUserApp :: IO () -> IO () withUserApp action = -- we can spin up a server in another thread and kill that thread when done -- in an exception-safe way bracket (liftIO $ C.forkIO $ Warp.run 8888 userApp) C.killThread (const 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:8888" manager <- runIO $ newManager defaultManagerSettings let clientEnv = mkClientEnv manager baseUrl -- testing scenarios start here describe "POST /user" $ do it "should create a user with a high enough ID" $ do result <- runClientM (createUser 50001) clientEnv result `shouldBe` (Right $ User { name = "some user", user_id = 50001}) it "will it fail with a too-small ID?" $ do result <- runClientM (createUser 4999) clientEnv 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: ```haskell 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 ServantError 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: ```haskell 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 ServantError 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: ```haskell -- | 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: ```haskell 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`](https://github.com/haskell-servant/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](https://github.com/haskell-servant/servant-quickcheck/blob/master/src/Servant/QuickCheck/Internal/Predicates.hs). 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`: ```haskell 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: ```haskell -- 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](https://github.com/haskell-servant/servant/tree/master/doc/cookbook/testing).