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Make tests run and include failings for illustrative purposes

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This commit is contained in:
Erik Aker 2018-10-12 08:48:25 -07:00
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doc/cookbook/testing/Testing.lhs
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@ -1,7 +1,7 @@
# 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.
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
@ -13,17 +13,27 @@ This recipe uses the following ingredients:
{-# LANGUAGE OverloadedStrings, TypeFamilies, DataKinds,
DeriveGeneric, TypeOperators #-}
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
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
import Test.Hspec.Wai.Matcher
```
@ -33,13 +43,13 @@ aspects of our application, and we'll ask `hspec` to run all of our different
```haskell
spec :: Spec
spec =
spec = do
businessLogicSpec
thirdPartyResourcesSpec
servantQuickcheckSpec
```
Often, codebases will use `hspec`'s
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
@ -55,15 +65,15 @@ main = hspec spec
Let's say we have an API that looks something like this:
```haskell
data User = User {
name :: Text
, user_id :: Integer
data User = User {
name :: Text
, user_id :: Integer
} deriving (Eq, Show, Generic)
instance FromJSON User
instance ToJSON User
type UserApi =
type UserApi =
-- One endpoint: create a user
"user" :> Capture "userId" Integer :> Post '[JSON] User
```
@ -74,11 +84,14 @@ to test. With that said, here's a sample handler for the endpoint described
above:
```haskell
userServer :: Server UserApi Handler
userServer :: Server UserApi
userServer = createUser
where creatUser userId
| userId > 5000 = pure $ User { name = "some user", user_id = userId }
| otherwise = throwError $ err400 { errBody = "userId is too small" }
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
@ -97,23 +110,23 @@ Let's write some tests:
businessLogicSpec :: Spec
businessLogicSpec = do
-- create a test client function
createUser = client (Proxy :: Proxy UserApi)
let createUser = client (Proxy :: Proxy UserApi)
-- create a servant-client ClientEnv
baseUrl <- parseBaseUrl "http://localhost:8888"
manager <- newManager defaultManagerSettings
baseUrl <- runIO $ parseBaseUrl "http://localhost:8888"
manager <- runIO $ newManager defaultManagerSettings
let clientEnv = mkClientEnv manager baseUrl
-- Run the server in another thread.
liftIO $ C.forkIO $ Warp.run 8888 (server userServer)
-- Run the server in another thread (`runIO` is from `hspec`)
runIO $ C.forkIO $ Warp.run 8888 (serve (Proxy :: Proxy UserApi) userServer)
-- testing scenarios start here
describe "POST /user" $ do
it "should create a user with a high enough ID" $
result <- runClientM clientEnv (createUser 50001)
result `shouldEqual` Right $ User { name = "some_user", user_id = "5001 "}
it "should fail with a too-small ID" $
result <- runClientM clientEnv (createUser 4999)
result `shouldEqual` Right $ User { name = "some_user", user_id = "5001 "}
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
@ -122,13 +135,26 @@ 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/"
```
Great: we passed! Servers obviously get more complex, though, and we may not
wish to create a whole suite of clients for our server every time. In our next
scenario we're going to mock out a 3rd-party resource that our server talks to
and we're going to be using `hspec-wai` to run our `Application` instance and
to make requests.
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.
## *Mocking* 3rd Party Resources
@ -147,7 +173,7 @@ to get data out of Elasticsearch. Let's first define the Elasticsearch server
and client using Servant API descriptions:
```haskell
type SearchAPI =
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
@ -156,11 +182,11 @@ type SearchAPI =
-- Here's our Servant Client function
getDocument = client (Proxy :: Proxy SearchAPI)
-- We can use these helpers when we want to make requests
-- 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 $ T.unpack $ esHost <> ":" <> esPort
baseUrl <- parseBaseUrl $ unpack $ esHost <> ":" <> esPort
manager <- newManager defaultManagerSettings
pure $ mkClientEnv manager baseUrl
@ -170,22 +196,21 @@ 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 with one
function that retrieves a document by its id. 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.
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 `aeson-lens` here, which may
look a bit foreign. The gist of it is that we're going to traverse a JSON
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.
```haskell
type DocApi =
type DocApi =
"docs" :> Capture "docId" Integer :> Get '[JSON] Value
docServer :: Text -> Text -> Server DocApi Handler
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
@ -206,9 +231,11 @@ getDocById esHost esPort docId = do
### Testing Our Backend
So the above represents our application. How 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
So the above represents our application and is close to a server we may
actually deploy. How 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
@ -234,30 +261,34 @@ withElasticsearch action =
esTestApp :: Application
esTestApp = serve (Proxy :: Proxy SearchAPI) esTestServer
esTestServer :: Server SearchAPI Handler
esTestServer :: Server SearchAPI
esTestServer = getESDocument
getESDocument :: Integer -> Handler Value
getESDocument docId
-- arbitrary things we can trigger in our tests to check for failure
-- We want to try to trigger different code paths
| docId > 1000 = pure . Left $ ConnectionError "Bad connection!"
| docId > 1000 = throwError err500
| docId > 500 = pure . Object $ HM.fromList [("bad", String "data")]
| otherwise = pure $ Object $ HM.fromList [("_source", defaultDocument)]
| otherwise = pure $ Object $ HM.fromList [("_source", Object $ HM.fromList [("a", String "b")])]
```
Now, we should be ready to write some tests. As mentioned above we're going
to use `hspec-wai` in this example to make test requests and to run our own
application. This should simplify our testing code somewhat:
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` and pass our servant-server `Application`
with (pure $ serve $ docServer "localhost" "9999") $ do
with (pure $ serve (Proxy :: Proxy DocApi) $ docServer "localhost" "9999") $ do
describe "GET /docs" $ do
it "should be able to get a document" $
-- `get` is a function from hspec-wai`.
-- `get` is a function from hspec-wai`.
get "/docs/1" `shouldRespondWith` 200
it "should be able to handle connection failures" $
-- We can also make custom HTTP requests with the `request` function
@ -267,11 +298,36 @@ thirdPartyResourcesSpec = around_ withElasticsearch $ do
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]
-- request :: Method -> ByteString -> [Header]
-- -> LB.ByteString -> WaiSession SResponse
request methodPost "/docs/501" [] "{" `shouldRespondWith` 415
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"
```
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!
## Servant Quickcheck
@ -284,6 +340,8 @@ 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).
Thus, it's one way to assert that your APIs conform to specs and best
practices.
### Quickcheckable API
@ -301,31 +359,94 @@ server :: IO (Server API)
server = do
mvar <- newMVar ""
return $ (\x -> liftIO $ swapMVar mvar x)
:<|> (liftIO $ readMVar mvar >>= return . length)
:<|> (liftIO $ readMVar mvar >>= return . Prelude.length)
:<|> (const $ return ())
```
### Using `servant-quickcheck`
`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:
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" $
withServantServer api server $ \burl ->
-- `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
<%> mempty)
it "API doesn't have these things implemented yet" $
withServantServer api server $ \burl -> do
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 return 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 testing 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. Lastly, if you want a broad
overview of where your application fits in with regard to best practices,
consider using `servant-quickcheck`.

4
doc/cookbook/testing/testing.cabal
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@ -17,14 +17,18 @@ executable cookbook-testing
build-depends: base == 4.*
, text >= 1.2
, aeson >= 1.2
, lens-aeson
, lens
, servant
, servant-client
, servant-server
, servant-quickcheck
, http-client
, http-types >= 0.12
, hspec
, hspec-wai
, QuickCheck
, unordered-containers
, warp >= 3.2
, wai >= 3.2
, wai-extra