mirror of
https://github.com/ilyakooo0/servant-quickcheck.git
synced 2024-11-22 23:27:08 +03:00
126 lines
4.0 KiB
Plaintext
126 lines
4.0 KiB
Plaintext
# Testing that servers behave identically
|
|
|
|
## Rewriting an application
|
|
|
|
If you are rewriting, or significantly refactoring, an application, you often
|
|
want to ensure that the behaviour of the rewritten application is the same as
|
|
that of the old one. Sometimes what the behaviour of the old application is is
|
|
not always clear, making the process a difficult and error-prone one.
|
|
|
|
**servant-quickcheck** can help. It provides a `serversEqual` function that,
|
|
given a **servant** API type and two URLs, generates arbitrary requests of the
|
|
right type and checks that, for the same request *history*, the two servers
|
|
respond identically.
|
|
|
|
To see how this works, let's re-implement the [Django
|
|
Todo-Backend](https://github.com/mihirk/todo-backend-django) application
|
|
in **servant**. (`serversEqual` works for non-**servant** applications, though
|
|
it's somewhat nicer to use when one of them is written with **servant**.) You
|
|
don't need to know anything about Django or Python to follow along; indeed,
|
|
part of the fun of it is using `serversEqual` to guide you through
|
|
re-implementing code you may not entirely understand.
|
|
|
|
Looking around the codebase (`urls.py`, `models.py` and `views.py` in
|
|
particular) may lead us to a first attempt at a rewrite:
|
|
|
|
``` haskell
|
|
{-# LANGUAGE DataKinds #-}
|
|
{-# LANGUAGE DeriveAnyClass #-}
|
|
{-# LANGUAGE DeriveGeneric #-}
|
|
{-# LANGUAGE TypeOperators #-}
|
|
|
|
module ServersEqual where
|
|
|
|
import Control.Concurrent (forkIO, killThread)
|
|
import Control.Concurrent.STM as STM
|
|
import Control.Monad.IO.Class
|
|
import Data.Aeson (ToJSON, FromJSON)
|
|
import Data.IntMap as M
|
|
import GHC.Generics (Generic)
|
|
import Network.Wai.Handler.Warp (run)
|
|
import Servant
|
|
import Servant.QuickCheck (serversEqual, BaseUrl(..), Scheme(..))
|
|
import Test.QuickCheck (Arbitrary(..))
|
|
|
|
data Todo = Todo
|
|
{ title :: String
|
|
, completed :: Bool
|
|
, url :: String
|
|
, order :: Int
|
|
} deriving (Eq, Show, Read, Generic, ToJSON, FromJSON)
|
|
|
|
type API = TodosAPI :<|> TodoAPI
|
|
|
|
type TodosAPI
|
|
= "todos" :>
|
|
( Get '[JSON] [Todo]
|
|
:<|> ReqBody '[JSON] Todo :> Post '[JSON] ()
|
|
:<|> Delete '[JSON] ())
|
|
|
|
type TodoAPI
|
|
= "todo" :> Capture "id " Int :>
|
|
( Get '[JSON] Todo
|
|
:<|> ReqBody '[JSON] Todo :> Patch '[JSON] ()
|
|
:<|> Delete '[JSON] ())
|
|
|
|
serverTodos :: DB -> Server TodosAPI
|
|
serverTodos tvar = getTodos tvar
|
|
:<|> postTodos tvar
|
|
:<|> deleteAllTodos tvar
|
|
|
|
{-
|
|
serverTodo :: Server TodoAPI
|
|
serverTodo id' = getTodo tvar id'
|
|
:<|> patchTodo tvar id'
|
|
:<|> deleteTodo tvar id'
|
|
-}
|
|
|
|
type DB = TVar (M.IntMap Todo)
|
|
|
|
getTodos :: DB -> Handler [Todo]
|
|
getTodos m = liftIO $ M.elems <$> STM.readTVarIO m
|
|
|
|
postTodos :: DB -> Todo -> Handler ()
|
|
postTodos m t = liftIO . STM.atomically $ STM.modifyTVar' m (M.insert (order t) t)
|
|
|
|
deleteAllTodos :: DB -> Handler ()
|
|
deleteAllTodos m = liftIO . STM.atomically $ STM.writeTVar m M.empty
|
|
|
|
main1 :: IO ()
|
|
main1 = do
|
|
db <- STM.newTVarIO M.empty
|
|
tId <- forkIO $ run 8000 $ serve (Proxy :: Proxy TodosAPI) $ serverTodos db
|
|
_ <- serversEqual (Proxy :: Proxy TodosAPI)
|
|
(BaseUrl Http "localhost" 8000 "")
|
|
(BaseUrl Http "localhost" 8001 "")
|
|
1000
|
|
killThread tId
|
|
```
|
|
|
|
(We're keeping the `Todo`s in an `MVar` for simplicity. If this were a
|
|
production application, we'd likely want to use a database.)
|
|
|
|
Notice that we split up the API into two sub-APIs. Partly this makes things
|
|
cleaner and more readable, but there's also a more concrete benefit: we can
|
|
start testing that **parts** of the API have been correctly rewritten without
|
|
implementing the entire server.
|
|
|
|
In order to check how we're doing, we need to add an `Arbitrary` instance for
|
|
`Todo`:
|
|
|
|
``` haskell
|
|
instance Arbitrary Todo where
|
|
arbitrary = Todo <$> arbitrary <*> arbitrary <*> arbitrary <*> arbitrary
|
|
```
|
|
|
|
Let's try it out. First we need to get the old application running:
|
|
|
|
``` bash
|
|
git clone https://github.com/mihirk/todo-backend-django.git
|
|
cd todo-backend-django
|
|
virtualenv venv
|
|
. ./venv/bin/activate
|
|
python manage.py runserver localhost:8001
|
|
```
|
|
|